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Thinking in Java
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public void run() {

while (true) {

try {

sleep(100);

synchronized(this) {

while(suspended)

wait();

}

} catch(InterruptedException e) {

System.err.println("Interrupted");

}

t.setText(Integer.toString(count++));

}

public void init() {

Container cp = getContentPane();

cp.setLayout(new FlowLayout());

cp.add(t);

suspend.addActionListener(

new ActionListener() {

public

void actionPerformed(ActionEvent e) {

ss.fauxSuspend();

}

});

cp.add(suspend);

resume.addActionListener(

new ActionListener() {

public

void actionPerformed(ActionEvent e) {

ss.fauxResume();

}

});

cp.add(resume);

}

public static void main(String[] args) {

Console.run(new Suspend(), 300, 100);

}

} ///:~

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Thinking in Java

The flag suspended inside Suspendable is used to turn suspension on

and off. To suspend, the flag is set to true by calling fauxSuspend( )

and this is detected inside run( ). The wait( ), as described earlier in this

chapter, must be synchronized so that it has the object lock. In

fauxResume( ), the suspended flag is set to false and notify( ) is

called--since this wakes up wait( ) inside a synchronized clause the

fauxResume( ) method must also be synchronized so that it acquires

the lock before calling notify( ) (thus the lock is available for the wait( )

to wake up with). If you follow the style shown in this program you can

avoid using suspend( ) and resume( ).

The destroy( ) method of Thread has never been implemented; it's like

a suspend( ) that cannot resume, so it has the same deadlock issues as

suspend( ). However, this is not a deprecated method and it might be

implemented in a future version of Java (after 2) for special situations in

which the risk of a deadlock is acceptable.

You might wonder why these methods, now deprecated, were included in

Java in the first place. It seems an admission of a rather significant

mistake to simply remove them outright (and pokes yet another hole in

the arguments for Java's exceptional design and infallibility touted by Sun

marketing people). The heartening part about the change is that it clearly

indicates that the technical people and not the marketing people are

running the show--they discovered a problem and they are fixing it. I find

this much more promising and hopeful than leaving the problem in

because "fixing it would admit an error." It means that Java will continue

to improve, even if it means a little discomfort on the part of Java

programmers. I'd rather deal with the discomfort than watch the language

stagnate.

Priorities

The priority of a thread tells the scheduler how important this thread is.

If there are a number of threads blocked and waiting to be run, the

scheduler will run the one with the highest priority first. However, this

doesn't mean that threads with lower priority don't get run (that is, you

can't get deadlocked because of priorities). Lower priority threads just

tend to run less often.

Chapter 14: Multiple Threads

877

Although priorities are interesting to know about and to play with, in

practice you almost never need to set priorities yourself. So feel free to

skip the rest of this section if priorities aren't interesting to you.

Reading and setting priorities

You can read the priority of a thread with getPriority( ) and change it

with setPriority( ). The form of the prior "counter" examples can be

used to show the effect of changing the priorities. In this applet you'll see

that the counters slow down as the associated threads have their priorities

lowered:

//: c14:Counter5.java

// Adjusting the priorities of threads.

// <applet code=Counter5 width=450 height=600>

// </applet>

import javax.swing.*;

import java.awt.*;

import java.awt.event.*;

import com.bruceeckel.swing.*;

class Ticker2 extends Thread {

private JButton

b = new JButton("Toggle"),

incPriority = new JButton("up"),

decPriority = new JButton("down");

private JTextField

t = new JTextField(10),

pr = new JTextField(3); // Display priority

private int count = 0;

private boolean runFlag = true;

public Ticker2(Container c) {

b.addActionListener(new ToggleL());

incPriority.addActionListener(new UpL());

decPriority.addActionListener(new DownL());

JPanel p = new JPanel();

p.add(t);

p.add(pr);

p.add(b);

p.add(incPriority);

p.add(decPriority);

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Thinking in Java

c.add(p);

}

class ToggleL implements ActionListener {

public void actionPerformed(ActionEvent e) {

runFlag = !runFlag;

}

class UpL implements ActionListener {

public void actionPerformed(ActionEvent e) {

int newPriority = getPriority() + 1;

if(newPriority > Thread.MAX_PRIORITY)

newPriority = Thread.MAX_PRIORITY;

setPriority(newPriority);

}

class DownL implements ActionListener {

public void actionPerformed(ActionEvent e) {

int newPriority = getPriority() - 1;

if(newPriority < Thread.MIN_PRIORITY)

newPriority = Thread.MIN_PRIORITY;

setPriority(newPriority);

}

public void run() {

while (true) {

if(runFlag) {

t.setText(Integer.toString(count++));

pr.setText(

Integer.toString(getPriority()));

}

yield();

}

public class Counter5 extends JApplet {

private JButton

start = new JButton("Start"),

upMax = new JButton("Inc Max Priority"),

downMax = new JButton("Dec Max Priority");

private boolean started = false;

Chapter 14: Multiple Threads

879

private static final int SIZE = 10;

private Ticker2[] s = new Ticker2[SIZE];

private JTextField mp = new JTextField(3);

public void init() {

Container cp = getContentPane();

cp.setLayout(new FlowLayout());

for(int i = 0; i < s.length; i++)

s[i] = new Ticker2(cp);

cp.add(new JLabel(

"MAX_PRIORITY = " + Thread.MAX_PRIORITY));

cp.add(new JLabel("MIN_PRIORITY = "

+ Thread.MIN_PRIORITY));

cp.add(new JLabel("Group Max Priority = "));

cp.add(mp);

cp.add(start);

cp.add(upMax);

cp.add(downMax);

start.addActionListener(new StartL());

upMax.addActionListener(new UpMaxL());

downMax.addActionListener(new DownMaxL());

showMaxPriority();

// Recursively display parent thread groups:

ThreadGroup parent =

s[0].getThreadGroup().getParent();

while(parent != null) {

cp.add(new Label(

"Parent threadgroup max priority = "

+ parent.getMaxPriority()));

parent = parent.getParent();

}

public void showMaxPriority() {

mp.setText(Integer.toString(

s[0].getThreadGroup().getMaxPriority()));

}

class StartL implements ActionListener {

public void actionPerformed(ActionEvent e) {

if(!started) {

started = true;

for(int i = 0; i < s.length; i++)

s[i].start();

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Thinking in Java

}

class UpMaxL implements ActionListener {

public void actionPerformed(ActionEvent e) {

int maxp =

s[0].getThreadGroup().getMaxPriority();

if(++maxp > Thread.MAX_PRIORITY)

maxp = Thread.MAX_PRIORITY;

s[0].getThreadGroup().setMaxPriority(maxp);

showMaxPriority();

}

class DownMaxL implements ActionListener {

public void actionPerformed(ActionEvent e) {

int maxp =

s[0].getThreadGroup().getMaxPriority();

if(--maxp < Thread.MIN_PRIORITY)

maxp = Thread.MIN_PRIORITY;

s[0].getThreadGroup().setMaxPriority(maxp);

showMaxPriority();

}

public static void main(String[] args) {

Console.run(new Counter5(), 450, 600);

}

} ///:~

Ticker2 follows the form established earlier in this chapter, but there's

an extra JTextField for displaying the priority of the thread and two

more buttons for incrementing and decrementing the priority.

Also notice the use of yield( ), which voluntarily hands control back to

the scheduler. Without this the multithreading mechanism still works, but

you'll notice it runs slowly (try removing the call to yield( ) to see this).

You could also call sleep( ), but then the rate of counting would be

controlled by the sleep( ) duration instead of the priority.

The init( ) in Counter5 creates an array of ten Ticker2s; their buttons

and fields are placed on the form by the Ticker2 constructor. Counter5

adds buttons to start everything up as well as increment and decrement

Chapter 14: Multiple Threads

881

the maximum priority of the thread group. In addition, there are labels

that display the maximum and minimum priorities possible for a thread

and a JTextField to show the thread group's maximum priority. (The

next section will describe thread groups.) Finally, the priorities of the

parent thread groups are also displayed as labels.

When you press an "up" or "down" button, that Ticker2's priority is

fetched and incremented or decremented accordingly.

When you run this program, you'll notice several things. First of all, the

thread group's default priority is five. Even if you decrement the

maximum priority below five before starting the threads (or before

creating the threads, which requires a code change), each thread will have

a default priority of five.

The simple test is to take one counter and decrement its priority to one,

and observe that it counts much slower. But now try to increment it again.

You can get it back up to the thread group's priority, but no higher. Now

decrement the thread group's priority a couple of times. The thread

priorities are unchanged, but if you try to modify them either up or down

you'll see that they'll automatically pop to the priority of the thread group.

Also, new threads will still be given a default priority, even if that's higher

than the group priority. (Thus the group priority is not a way to prevent

new threads from having higher priorities than existing ones.)

Finally, try to increment the group maximum priority. It can't be done.

You can only reduce thread group maximum priorities, not increase them.

Thread groups

All threads belong to a thread group. This can be either the default thread

group or a group you explicitly specify when you create the thread. At

creation, the thread is bound to a group and cannot change to a different

group. Each application has at least one thread that belongs to the system

thread group. If you create more threads without specifying a group, they

will also belong to the system thread group.

Thread groups must also belong to other thread groups. The thread group

that a new one belongs to must be specified in the constructor. If you

create a thread group without specifying a thread group for it to belong to,

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Thinking in Java

it will be placed under the system thread group. Thus, all thread groups in

your application will ultimately have the system thread group as the

parent.

The reason for the existence of thread groups is hard to determine from

the literature, which tends to be confusing on this subject. It's often cited

as "security reasons." According to Arnold & Gosling,2 "Threads within a

thread group can modify the other threads in the group, including any

farther down the hierarchy. A thread cannot modify threads outside of its

own group or contained groups." It's hard to know what "modify" is

supposed to mean here. The following example shows a thread in a "leaf"

subgroup modifying the priorities of all the threads in its tree of thread

groups as well as calling a method for all the threads in its tree.

//: c14:TestAccess.java

// How threads can access other threads

// in a parent thread group.

public class TestAccess {

public static void main(String[] args) {

ThreadGroup

x = new ThreadGroup("x"),

y = new ThreadGroup(x, "y"),

z = new ThreadGroup(y, "z");

Thread

one = new TestThread1(x, "one"),

two = new TestThread2(z, "two");

}

class TestThread1 extends Thread {

private int i;

TestThread1(ThreadGroup g, String name) {

super(g, name);

}

void f() {

2 The Java Programming Language, by Ken Arnold and James Gosling, Addison-Wesley

1996 pp 179.

Chapter 14: Multiple Threads

883

i++; // modify this thread

System.out.println(getName() + " f()");

}

class TestThread2 extends TestThread1 {

TestThread2(ThreadGroup g, String name) {

super(g, name);

start();

}

public void run() {

ThreadGroup g =

getThreadGroup().getParent().getParent();

g.list();

Thread[] gAll = new Thread[g.activeCount()];

g.enumerate(gAll);

for(int i = 0; i < gAll.length; i++) {

gAll[i].setPriority(Thread.MIN_PRIORITY);

((TestThread1)gAll[i]).f();

}

g.list();

}

} ///:~

In main( ), several ThreadGroups are created, leafing off from each

other: x has no argument but its name (a String), so it is automatically

placed in the "system" thread group, while y is under x and z is under y.

Note that initialization happens in textual order so this code is legal.

Two threads are created and placed in different thread groups.

TestThread1 doesn't have a run( ) method but it does have an f( ) that

modifies the thread and prints something so you can see it was called.

TestThread2 is a subclass of TestThread1 and its run( ) is fairly

elaborate. It first gets the thread group of the current thread, then moves

up the heritage tree by two levels using getParent( ). (This is contrived

since I purposely place the TestThread2 object two levels down in the

hierarchy.) At this point, an array of references to Threads is created

using the method activeCount( ) to ask how many threads are in this

thread group and all the child thread groups. The enumerate( ) method

places references to all of these threads in the array gAll, then I simply

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Thinking in Java

move through the entire array calling the f( ) method for each thread, as

well as modifying the priority. Thus, a thread in a "leaf" thread group

modifies threads in parent thread groups.

The debugging method list( ) prints all the information about a thread

group to standard output and is helpful when investigating thread group

behavior. Here's the output of the program:

java.lang.ThreadGroup[name=x,maxpri=10]

Thread[one,5,x]

java.lang.ThreadGroup[name=y,maxpri=10]

java.lang.ThreadGroup[name=z,maxpri=10]

Thread[two,5,z]

one f()

two f()

java.lang.ThreadGroup[name=x,maxpri=10]

Thread[one,1,x]

java.lang.ThreadGroup[name=y,maxpri=10]

java.lang.ThreadGroup[name=z,maxpri=10]

Thread[two,1,z]

Not only does list( ) print the class name of ThreadGroup or Thread,

but it also prints the thread group name and its maximum priority. For

threads, the thread name is printed, followed by the thread priority and

the group that it belongs to. Note that list( ) indents the threads and

thread groups to indicate that they are children of the unindented thread

group.

You can see that f( ) is called by the TestThread2 run( ) method, so it's

obvious that all threads in a group are vulnerable. However, you can

access only the threads that branch off from your own system thread

group tree, and perhaps this is what is meant by "safety." You cannot

access anyone else's system thread group tree.

Controlling thread groups

Putting aside the safety issue, one thing thread groups seem to be useful

for is control: you can perform certain operations on an entire thread

group with a single command. The following example demonstrates this,

and the restrictions on priorities within thread groups. The commented

numbers in parentheses provide a reference to compare to the output.

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//: c14:ThreadGroup1.java

// How thread groups control priorities

// of the threads inside them.

public class ThreadGroup1 {

public static void main(String[] args) {

// Get the system thread & print its Info:

ThreadGroup sys =

Thread.currentThread().getThreadGroup();

sys.list(); // (1)

// Reduce the system thread group priority:

sys.setMaxPriority(Thread.MAX_PRIORITY - 1);

// Increase the main thread priority:

Thread curr = Thread.currentThread();

curr.setPriority(curr.getPriority() + 1);

sys.list(); // (2)

// Attempt to set a new group to the max:

ThreadGroup g1 = new ThreadGroup("g1");

g1.setMaxPriority(Thread.MAX_PRIORITY);

// Attempt to set a new thread to the max:

Thread t = new Thread(g1, "A");

t.setPriority(Thread.MAX_PRIORITY);

g1.list(); // (3)

// Reduce g1's max priority, then attempt

// to increase it:

g1.setMaxPriority(Thread.MAX_PRIORITY - 2);

g1.setMaxPriority(Thread.MAX_PRIORITY);

g1.list(); // (4)

// Attempt to set a new thread to the max:

t = new Thread(g1, "B");

t.setPriority(Thread.MAX_PRIORITY);

g1.list(); // (5)

// Lower the max priority below the default

// thread priority:

g1.setMaxPriority(Thread.MIN_PRIORITY + 2);

// Look at a new thread's priority before

// and after changing it:

t = new Thread(g1, "C");

g1.list(); // (6)

t.setPriority(t.getPriority() -1);

g1.list(); // (7)

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Thinking in Java

// Make g2 a child Threadgroup of g1 and

// try to increase its priority:

ThreadGroup g2 = new ThreadGroup(g1, "g2");

g2.list(); // (8)

g2.setMaxPriority(Thread.MAX_PRIORITY);

g2.list(); // (9)

// Add a bunch of new threads to g2:

for (int i = 0; i < 5; i++)

new Thread(g2, Integer.toString(i));

// Show information about all threadgroups

// and threads:

sys.list(); // (10)

System.out.println("Starting all threads:");

Thread[] all = new Thread[sys.activeCount()];

sys.enumerate(all);

for(int i = 0; i < all.length; i++)

if(!all[i].isAlive())

all[i].start();

// Suspends & Stops all threads in

// this group and its subgroups:

System.out.println("All threads started");

sys.suspend(); // Deprecated in Java 2

// Never gets here...

System.out.println("All threads suspended");

sys.stop(); // Deprecated in Java 2

System.out.println("All threads stopped");

}

} ///:~

The output that follows has been edited to allow it to fit on the page (the

java.lang. has been removed) and to add numbers to correspond to the

commented numbers in the listing above.

(1) ThreadGroup[name=system,maxpri=10]

Thread[main,5,system]

(2) ThreadGroup[name=system,maxpri=9]

Thread[main,6,system]

(3) ThreadGroup[name=g1,maxpri=9]

Thread[A,9,g1]

(4) ThreadGroup[name=g1,maxpri=8]

Thread[A,9,g1]

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887

(5) ThreadGroup[name=g1,maxpri=8]

Thread[A,9,g1]

Thread[B,8,g1]

(6) ThreadGroup[name=g1,maxpri=3]

Thread[A,9,g1]

Thread[B,8,g1]

Thread[C,6,g1]

(7) ThreadGroup[name=g1,maxpri=3]

Thread[A,9,g1]

Thread[B,8,g1]

Thread[C,3,g1]

(8) ThreadGroup[name=g2,maxpri=3]

(9) ThreadGroup[name=g2,maxpri=3]

(10)ThreadGroup[name=system,maxpri=9]

Thread[main,6,system]

ThreadGroup[name=g1,maxpri=3]

Thread[A,9,g1]

Thread[B,8,g1]

Thread[C,3,g1]

ThreadGroup[name=g2,maxpri=3]

Thread[0,6,g2]

Thread[1,6,g2]

Thread[2,6,g2]

Thread[3,6,g2]

Thread[4,6,g2]

Starting all threads:

All threads started

All programs have at least one thread running, and the first action in

main( ) is to call the static method of Thread called

currentThread( ). From this thread, the thread group is produced and

list( ) is called for the result. The output is:

(1) ThreadGroup[name=system,maxpri=10]

Thread[main,5,system]

You can see that the name of the main thread group is system, and the

name of the main thread is main, and it belongs to the system thread

group.

The second exercise shows that the system group's maximum priority

can be reduced and the main thread can have its priority increased:

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Thinking in Java

(2) ThreadGroup[name=system,maxpri=9]

Thread[main,6,system]

The third exercise creates a new thread group, g1, which automatically

belongs to the system thread group since it isn't otherwise specified. A

new thread A is placed in g1. After attempting to set this group's

maximum priority to the highest level and A's priority to the highest level,

the result is:

(3) ThreadGroup[name=g1,maxpri=9]

Thread[A,9,g1]

Thus, it's not possible to change the thread group's maximum priority to

be higher than its parent thread group.

The fourth exercise reduces g1's maximum priority by two and then tries

to increase it up to Thread.MAX_PRIORITY. The result is:

(4) ThreadGroup[name=g1,maxpri=8]

Thread[A,9,g1]

You can see that the increase in maximum priority didn't work. You can

only decrease a thread group's maximum priority, not increase it. Also,

notice that thread A's priority didn't change, and now it is higher than the

thread group's maximum priority. Changing a thread group's maximum

priority doesn't affect existing threads.

The fifth exercise attempts to set a new thread to maximum priority:

(5) ThreadGroup[name=g1,maxpri=8]

Thread[A,9,g1]

Thread[B,8,g1]

The new thread cannot be changed to anything higher than the maximum

thread group priority.

The default thread priority for this program is six; that's the priority a new

thread will be created at and where it will stay if you don't manipulate the

priority. Exercise 6 lowers the maximum thread group priority below the

default thread priority to see what happens when you create a new thread

under this condition:

(6) ThreadGroup[name=g1,maxpri=3]

Chapter 14: Multiple Threads

889

Thread[A,9,g1]

Thread[B,8,g1]

Thread[C,6,g1]

Even though the maximum priority of the thread group is three, the new

thread is still created using the default priority of six. Thus, maximum

thread group priority does not affect default priority. (In fact, there

appears to be no way to set the default priority for new threads.)

After changing the priority, attempting to decrement it by one, the result

is:

(7) ThreadGroup[name=g1,maxpri=3]

Thread[A,9,g1]

Thread[B,8,g1]

Thread[C,3,g1]

Only when you attempt to change the priority is the thread group's

maximum priority enforced.

A similar experiment is performed in (8) and (9), in which a new thread

group g2 is created as a child of g1 and its maximum priority is changed.

You can see that it's impossible for g2's maximum to go higher than g1's:

(8) ThreadGroup[name=g2,maxpri=3]

(9) ThreadGroup[name=g2,maxpri=3]

Also notice that g2 is automatically set to the thread group maximum

priority of g1 as g2 is created.

After all of these experiments, the entire system of thread groups and

threads is printed:

(10)ThreadGroup[name=system,maxpri=9]

Thread[main,6,system]

ThreadGroup[name=g1,maxpri=3]

Thread[A,9,g1]

Thread[B,8,g1]

Thread[C,3,g1]

ThreadGroup[name=g2,maxpri=3]

Thread[0,6,g2]

Thread[1,6,g2]

Thread[2,6,g2]

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Thinking in Java

Thread[3,6,g2]

Thread[4,6,g2]

So because of the rules of thread groups, a child group must always have a

maximum priority that's less than or equal to its parent's maximum

priority.

The last part of this program demonstrates methods for an entire group of

threads. First the program moves through the entire tree of threads and

starts each one that hasn't been started. For drama, the system group is

then suspended and finally stopped. (Although it's interesting to see that

suspend( ) and stop( ) work on entire thread groups, you should keep

in mind that these methods are deprecated in Java 2.) But when you

suspend the system group you also suspend the main thread and the

whole program shuts down, so it never gets to the point where the threads

are stopped. Actually, if you do stop the main thread it throws a

ThreadDeath exception, so this is not a typical thing to do. Since

ThreadGroup is inherited from Object, which contains the wait( )

method, you can also choose to suspend the program for any number of

seconds by calling wait(seconds * 1000). This must acquire the lock

inside a synchronized block, of course.

The ThreadGroup class also has suspend( ) and resume( ) methods

so you can stop and start an entire thread group and all of its threads and

subgroups with a single command. (Again, suspend( ) and resume( )

are deprecated in Java 2.)

Thread groups can seem a bit mysterious at first, but keep in mind that

you probably won't be using them directly very often.

Runnable revisited

Earlier in this chapter, I suggested that you think carefully before making

an applet or main Frame as an implementation of Runnable. Of course,

if you must inherit from a class and you want to add threading behavior to

the class, Runnable is the correct solution. The final example in this

chapter exploits this by making a Runnable JPanel class that paints

different colors on itself. This application is set up to take values from the

command line to determine how big the grid of colors is and how long to

Chapter 14: Multiple Threads

891

sleep( ) between color changes. By playing with these values you'll

discover some interesting and possibly inexplicable features of threads:

//: c14:ColorBoxes.java

// Using the Runnable interface.

// <applet code=ColorBoxes width=500 height=400>

// <param name=grid value="12">

// <param name=pause value="50">

// </applet>

import javax.swing.*;

import java.awt.*;

import java.awt.event.*;

import com.bruceeckel.swing.*;

class CBox extends JPanel implements Runnable {

private Thread t;

private int pause;

private static final Color[] colors = {

Color.black, Color.blue, Color.cyan,

Color.darkGray, Color.gray, Color.green,

Color.lightGray, Color.magenta,

Color.orange, Color.pink, Color.red,

Color.white, Color.yellow

};

private Color cColor = newColor();

private static final Color newColor() {

return colors[

(int)(Math.random() * colors.length)

];

}

public void paintComponent(Graphics g) {

super.paintComponent(g);

g.setColor(cColor);

Dimension s = getSize();

g.fillRect(0, 0, s.width, s.height);

}

public CBox(int pause) {

this.pause = pause;

t = new Thread(this);

t.start();

}

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Thinking in Java

public void run() {

while(true) {

cColor = newColor();

repaint();

try {

t.sleep(pause);

} catch(InterruptedException e) {

System.err.println("Interrupted");

}

public class ColorBoxes extends JApplet {

private boolean isApplet = true;

private int grid = 12;

private int pause = 50;

public void init() {

// Get parameters from Web page:

if (isApplet) {

String gsize = getParameter("grid");

if(gsize != null)

grid = Integer.parseInt(gsize);

String pse = getParameter("pause");

if(pse != null)

pause = Integer.parseInt(pse);

}

Container cp = getContentPane();

cp.setLayout(new GridLayout(grid, grid));

for (int i = 0; i < grid * grid; i++)

cp.add(new CBox(pause));

}

public static void main(String[] args) {

ColorBoxes applet = new ColorBoxes();

applet.isApplet = false;

if(args.length > 0)

applet.grid = Integer.parseInt(args[0]);

if(args.length > 1)

applet.pause = Integer.parseInt(args[1]);

Console.run(applet, 500, 400);

}

Chapter 14: Multiple Threads

893

} ///:~

ColorBoxes is the usual applet/application with an init( ) that sets up

the GUI. This sets up the GridLayout so that it has grid cells in each

dimension. Then it adds the appropriate number of CBox objects to fill

the grid, passing the pause value to each one. In main( ) you can see

how pause and grid have default values that can be changed if you pass

in command-line arguments, or by using applet parameters.

CBox is where all the work takes place. This is inherited from JPanel

and it implements the Runnable interface so each JPanel can also be a

Thread. Remember that when you implement Runnable, you don't

make a Thread object, just a class that has a run( ) method. Thus, you

must explicitly create a Thread object and hand the Runnable object to

the constructor, then call start( ) (this happens in the constructor). In

CBox this thread is called t.

Notice the array colors, which is an enumeration of all the colors in class

Color. This is used in newColor( ) to produce a randomly selected

color. The current cell color is cColor.

paintComponent( ) is quite simple--it just sets the color to cColor and

fills the entire JPanel with that color.

In run( ), you see the infinite loop that sets the cColor to a new random

color and then calls repaint( ) to show it. Then the thread goes to

sleep( ) for the amount of time specified on the command line.

Precisely because this design is flexible and threading is tied to each

JPanel element, you can experiment by making as many threads as you

want. (In reality, there is a restriction imposed by the number of threads

your JVM can comfortably handle.)

This program also makes an interesting benchmark, since it can show

dramatic performance differences between one JVM threading

implementation and another.

Too many threads

At some point, you'll find that ColorBoxes bogs down. On my machine,

this occurred somewhere after a 10 x 10 grid. Why does this happen?

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Thinking in Java

You're naturally suspicious that Swing might have something to do with

it, so here's an example that tests that premise by making fewer threads.

The following code is reorganized so that an ArrayList implements

Runnable and that ArrayList holds a number of color blocks and

randomly chooses ones to update. Then a number of these ArrayList

objects are created, depending roughly on the grid dimension you choose.

As a result, you have far fewer threads than color blocks, so if there's a

speedup we'll know it was because there were too many threads in the

previous example:

//: c14:ColorBoxes2.java

// Balancing thread use.

// <applet code=ColorBoxes2 width=600 height=500>

// <param name=grid value="12">

// <param name=pause value="50">

// </applet>

import javax.swing.*;

import java.awt.*;

import java.awt.event.*;

import java.util.*;

import com.bruceeckel.swing.*;

class CBox2 extends JPanel {

private static final Color[] colors = {

Color.black, Color.blue, Color.cyan,

Color.darkGray, Color.gray, Color.green,

Color.lightGray, Color.magenta,

Color.orange, Color.pink, Color.red,

Color.white, Color.yellow

};

private Color cColor = newColor();

private static final Color newColor() {

return colors[

(int)(Math.random() * colors.length)

];

}

void nextColor() {

cColor = newColor();

repaint();

}

public void paintComponent(Graphics g) {

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super.paintComponent(g);

g.setColor(cColor);

Dimension s = getSize();

g.fillRect(0, 0, s.width, s.height);

}

class CBoxList

extends ArrayList implements Runnable {

private Thread t;

private int pause;

public CBoxList(int pause) {

this.pause = pause;

t = new Thread(this);

}

public void go() { t.start(); }

public void run() {

while(true) {

int i = (int)(Math.random() * size());

((CBox2)get(i)).nextColor();

try {

t.sleep(pause);

} catch(InterruptedException e) {

System.err.println("Interrupted");

}

public Object last() { return get(size() - 1);}

}

public class ColorBoxes2 extends JApplet {

private boolean isApplet = true;

private int grid = 12;

// Shorter default pause than ColorBoxes:

private int pause = 50;

private CBoxList[] v;

public void init() {

// Get parameters from Web page:

if (isApplet) {

String gsize = getParameter("grid");

if(gsize != null)

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grid = Integer.parseInt(gsize);

String pse = getParameter("pause");

if(pse != null)

pause = Integer.parseInt(pse);

}

Container cp = getContentPane();

cp.setLayout(new GridLayout(grid, grid));

v = new CBoxList[grid];

for(int i = 0; i < grid; i++)

v[i] = new CBoxList(pause);

for (int i = 0; i < grid * grid; i++) {

v[i % grid].add(new CBox2());

cp.add((CBox2)v[i % grid].last());

}

for(int i = 0; i < grid; i++)

v[i].go();

}

public static void main(String[] args) {

ColorBoxes2 applet = new ColorBoxes2();

applet.isApplet = false;

if(args.length > 0)

applet.grid = Integer.parseInt(args[0]);

if(args.length > 1)

applet.pause = Integer.parseInt(args[1]);

Console.run(applet, 500, 400);

}

} ///:~

In ColorBoxes2 an array of CBoxList is created and initialized to hold

grid CBoxLists, each of which knows how long to sleep. An equal

number of CBox2 objects is then added to each CBoxList, and each list

is told to go( ), which starts its thread.

CBox2 is similar to CBox: it paints itself with a randomly chosen color.

But that's all a CBox2 does. All of the threading has been moved into

CBoxList.

The CBoxList could also have inherited Thread and had a member

object of type ArrayList. That design has the advantage that the add( )

and get( ) methods could then be given specific argument and return

value types instead of generic Objects. (Their names could also be

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changed to something shorter.) However, the design used here seemed at

first glance to require less code. In addition, it automatically retains all

the other behaviors of an ArrayList. With all the casting and parentheses

necessary for get( ), this might not be the case as your body of code

grows.

As before, when you implement Runnable you don't get all of the

equipment that comes with Thread, so you have to create a new Thread

and hand yourself to its constructor in order to have something to

start( ), as you can see in the CBoxList constructor and in go( ). The

run( ) method simply chooses a random element number within the list

and calls nextColor( ) for that element to cause it to choose a new

randomly selected color.

Upon running this program, you see that it does indeed run faster and

respond more quickly (for instance, when you interrupt it, it stops more

quickly), and it doesn't seem to bog down as much at higher grid sizes.

Thus, a new factor is added into the threading equation: you must watch

to see that you don't have "too many threads" (whatever that turns out to

mean for your particular program and platform--here, the slowdown in

ColorBoxes appears to be caused by the fact that there's only one thread

that is responsible for all painting, and it gets bogged down by too many

requests). If you have too many threads, you must try to use techniques

like the one above to "balance" the number of threads in your program. If

you see performance problems in a multithreaded program you now have

a number of issues to examine:

Do you have enough calls to sleep( ), yield( ), and/or wait( )?

Are calls to sleep( ) long enough?

Are you running too many threads?

Have you tried different platforms and JVMs?

Issues like this are one reason that multithreaded programming is often

considered an art.

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Summary

It is vital to learn when to use multithreading and when to avoid it. The

main reason to use it is to manage a number of tasks whose intermingling

will make more efficient use of the computer (including the ability to

transparently distribute the tasks across multiple CPUs) or be more

convenient for the user. The classic example of resource balancing is using

the CPU during I/O waits. The classic example of user convenience is

monitoring a "stop" button during long downloads.

The main drawbacks to multithreading are:

Slowdown while waiting for shared resources

Additional CPU overhead required to manage threads

Unrewarded complexity, such as the silly idea of having a separate

thread to update each element of an array

Pathologies including starving, racing, and deadlock

An additional advantage to threads is that they substitute "light"

execution context switches (of the order of 100 instructions) for "heavy"

process context switches (of the order of 1000s of instructions). Since all

threads in a given process share the same memory space, a light context

switch changes only program execution and local variables. On the other

hand, a process change--the heavy context switch--must exchange the full

memory space.

Threading is like stepping into an entirely new world and learning a whole

new programming language, or at least a new set of language concepts.

With the appearance of thread support in most microcomputer operating

systems, extensions for threads have also been appearing in programming

languages or libraries. In all cases, thread programming (1) seems

mysterious and requires a shift in the way you think about programming;

and (2) looks similar to thread support in other languages, so when you

understand threads, you understand a common tongue. And although

support for threads can make Java seem like a more complicated

language, don't blame Java. Threads are tricky.

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One of the biggest difficulties with threads occurs because more than one

thread might be sharing a resource--such as the memory in an object--

and you must make sure that multiple threads don't try to read and

change that resource at the same time. This requires judicious use of the

synchronized keyword, which is a helpful tool but must be understood

thoroughly because it can quietly introduce deadlock situations.

In addition, there's a certain art to the application of threads. Java is

designed to allow you to create as many objects as you need to solve your

problem--at least in theory. (Creating millions of objects for an

engineering finite-element analysis, for example, might not be practical in

Java.) However, it seems that there is an upper bound to the number of

threads you'll want to create, because at some point a large number of

threads seems to become unwieldy. This critical point is not in the many

thousands as it might be with objects, but rather in the low hundreds,

sometimes less than 100. As you often create only a handful of threads to

solve a problem, this is typically not much of a limit, yet in a more general

design it becomes a constraint.

A significant nonintuitive issue in threading is that, because of thread

scheduling, you can typically make your applications run faster by

inserting calls to sleep( ) inside run( )'s main loop. This definitely

makes it feel like an art, in particular when the longer delays seem to

speed up performance. Of course, the reason this happens is that shorter

delays can cause the end-of-sleep( ) scheduler interrupt to happen before

the running thread is ready to go to sleep, forcing the scheduler to stop it

and restart it later so it can finish what it was doing and then go to sleep.

It takes extra thought to realize how messy things can get.

One thing you might notice missing in this chapter is an animation

example, which is one of the most popular things to do with applets.

However, a complete solution (with sound) to this problem comes with

the Java JDK (available at java.sun.com) in the demo section. In addition,

we can expect better animation support to become part of future versions

of Java, while completely different non-Java, non-programming solutions

to animation for the Web are appearing that will probably be superior to

traditional approaches. For explanations about how Java animation

works, see Core Java 2 by Horstmann & Cornell, Prentice-Hall, 1997. For

more advanced discussions of threading, see Concurrent Programming

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in Java by Doug Lea, Addison-Wesley, 1997, or Java Threads by Oaks &

Wong, O'Reilly, 1997.

Exercises

Solutions to selected exercises can be found in the electronic document The Thinking in Java

Annotated Solution Guide, available for a small fee from .

Inherit a class from Thread and override the run( ) method.

Inside run( ), print a message, and then call sleep( ). Repeat this

three times, then return from run( ). Put a start-up message in

the constructor and override finalize( ) to print a shut-down

message. Make a separate thread class that calls System.gc( )

and System.runFinalization( ) inside run( ), printing a

message as it does so. Make several thread objects of both types

and run them to see what happens.

Modify Sharing2.java to add a synchronized block inside the

run( ) method of TwoCounter instead of synchronizing the

entire run( ) method.

Create two Thread subclasses, one with a run( ) that starts up,

captures the reference of the second Thread object and then calls

wait( ). The other class' run( ) should call notifyAll( ) for the

first thread after some number of seconds have passed, so the first

thread can print a message.

In Counter5.java inside Ticker2, remove the yield( ) and

explain the results. Replace the yield( ) with a sleep( ) and

explain the results.

In ThreadGroup1.java, replace the call to sys.suspend( ) with

a call to wait( ) for the thread group, causing it to wait for two

seconds. For this to work correctly you must acquire the lock for

sys inside a synchronized block.

Change Daemons.java so that main( ) has a sleep( ) instead of

a readLine( ). Experiment with different sleep times to see what

happens.

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901

In Chapter 8, locate the GreenhouseControls.java example,

which consists of three files. In Event.java, the class Event is

based on watching the time. Change Event so that it is a Thread,

and change the rest of the design so that it works with this new

Thread-based Event.

Modify Exercise 7 so that the java.util.Timer class found in JDK

1.3 is used to run the system.

Starting with SineWave.java from Chapter 13, create a program

(an applet/application using the Console class) that draws an

animated sine wave that appears to scrolls past the viewing

window like an oscilloscope, driving the animation with a

Thread. The speed of the animation should be controlled with a

java.swing.JSlider control.

10.

Modify Exercise 9 so that multiple sine wave panels are created

within the application. The number of sine wave panels should be

controlled by HTML tags or command-line parameters.

11.

Modify Exercise 9 so that the java.swing.Timer class is used to

drive the animation. Note the difference between this and

java.util.Timer.

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Thinking in Java

15: Distributed

Computing

Historically, programming across multiple machines has

been error-prone, difficult, and complex.

The programmer had to know many details about the network and

sometimes even the hardware. You usually needed to understand the

various "layers" of the networking protocol, and there were a lot of

different functions in each different networking library concerned with

connecting, packing, and unpacking blocks of information; shipping those

blocks back and forth; and handshaking. It was a daunting task.

However, the basic idea of distributed computing is not so difficult, and is

abstracted very nicely in the Java libraries. You want to:

! Get some information from that machine over there and move it to

this machine here, or vice versa. This is accomplished with basic

network programming.

! Connect to a database, which may live across a network. This is

accomplished with Java DataBase Connectivity (JDBC), which is

an abstraction away from the messy, platform-specific details of

SQL (the structured query language used for most database

transactions).

! Provide services via a Web server. This is accomplished with Java's

servlets and Java Server Pages (JSPs).

! Execute methods on Java objects that live on remote machines

transparently, as if those objects were resident on local machines.

This is accomplished with Java's Remote Method Invocation

(RMI).

903

! Use code written in other languages, running on other

architectures. This is accomplished using the Common Object

Request Broker Architecture (CORBA), which is directly

supported by Java.

! Isolate business logic from connectivity issues, especially

connections with databases including transaction management

and security. This is accomplished using Enterprise JavaBeans

(EJBs). EJBs are not actually a distributed architecture, but the

resulting applications are usually used in a networked client-

server system.

! Easily, dynamically, add and remove devices from a network

representing a local system. This is accomplished with Java's Jini.

Each topic will be given a light introduction in this chapter. Please note

that each subject is voluminous and by itself the subject of entire books,

so this chapter is only meant to familiarize you with the topics, not make

you an expert (however, you can go a long way with the information

presented here on network programming, servlets and JSPs).

Network programming

One of Java's great strengths is painless networking. The Java network

library designers have made it quite similar to reading and writing files,

except that the "file" exists on a remote machine and the remote machine

can decide exactly what it wants to do about the information you're

requesting or sending. As much as possible, the underlying details of

networking have been abstracted away and taken care of within the JVM

and local machine installation of Java. The programming model you use is

that of a file; in fact, you actually wrap the network connection (a

"socket") with stream objects, so you end up using the same method calls

as you do with all other streams. In addition, Java's built-in

multithreading is exceptionally handy when dealing with another

networking issue: handling multiple connections at once.

This section introduces Java's networking support using easy-to-

understand examples.

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Identifying a machine

Of course, in order to tell one machine from another and to make sure

that you are connected with a particular machine, there must be some way

of uniquely identifying machines on a network. Early networks were

satisfied to provide unique names for machines within the local network.

However, Java works within the Internet, which requires a way to

uniquely identify a machine from all the others in the world. This is

accomplished with the IP (Internet Protocol) address which can exist in

two forms:

The familiar DNS (Domain Name System) form. My domain name

is bruceeckel.com, and if I have a computer called Opus in my

domain, its domain name would be Opus.bruceeckel.com. This

is exactly the kind of name that you use when you send email to

people, and is often incorporated into a World Wide Web address.

Alternatively, you can use the "dotted quad" form, which is four

numbers separated by dots, such as 123.255.28.120.

In both cases, the IP address is represented internally as a 32-bit number1

(so each of the quad numbers cannot exceed 255), and you can get a

special Java object to represent this number from either of the forms

above by using the static InetAddress.getByName( ) method that's in

java.net. The result is an object of type InetAddress that you can use to

build a "socket," as you will see later.

As a simple example of using InetAddress.getByName( ), consider

what happens if you have a dial-up Internet service provider (ISP). Each

time you dial up, you are assigned a temporary IP address. But while

you're connected, your IP address has the same validity as any other IP

address on the Internet. If someone connects to your machine using your

IP address then they can connect to a Web server or FTP server that you

have running on your machine. Of course, they need to know your IP

1 This means a maximum of just over four billion numbers, which is rapidly running out.

The new standard for IP addresses will use a 128-bit number, which should produce

enough unique IP addresses for the foreseeable future.

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905

address, and since a new one is assigned each time you dial up, how can

you find out what it is?

The following program uses InetAddress.getByName( ) to produce

your IP address. To use it, you must know the name of your computer. On

Windows 95/98, go to "Settings," "Control Panel," "Network," and then

select the "Identification" tab. "Computer name" is the name to put on the

command line.

//: c15:WhoAmI.java

// Finds out your network address when

// you're connected to the Internet.

import java.net.*;

public class WhoAmI {

public static void main(String[] args)

throws Exception {

if(args.length != 1) {

System.err.println(

"Usage: WhoAmI MachineName");

System.exit(1);

}

InetAddress a =

InetAddress.getByName(args[0]);

System.out.println(a);

}

} ///:~

In this case, the machine is called "peppy." So, once I've connected to my

ISP I run the program:

java WhoAmI peppy

I get back a message like this (of course, the address is different each

time):

peppy/199.190.87.75

If I tell my friend this address and I have a Web server running on my

computer, he can connect to it by going to the URL http://199.190.87.75

(only as long as I continue to stay connected during that session). This can

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Thinking in Java

sometimes be a handy way to distribute information to someone else, or

to test out a Web site configuration before posting it to a "real" server.

Servers and clients

The whole point of a network is to allow two machines to connect and talk

to each other. Once the two machines have found each other they can

have a nice, two-way conversation. But how do they find each other? It's

like getting lost in an amusement park: one machine has to stay in one

place and listen while the other machine says, "Hey, where are you?"

The machine that "stays in one place" is called the server, and the one

that seeks is called the client. This distinction is important only while the

client is trying to connect to the server. Once they've connected, it

becomes a two-way communication process and it doesn't matter

anymore that one happened to take the role of server and the other

happened to take the role of the client.

So the job of the server is to listen for a connection, and that's performed

by the special server object that you create. The job of the client is to try to

make a connection to a server, and this is performed by the special client

object you create. Once the connection is made, you'll see that at both

server and client ends, the connection is magically turned into an I/O

stream object, and from then on you can treat the connection as if you

were reading from and writing to a file. Thus, after the connection is made

you will just use the familiar I/O commands from Chapter 11. This is one

of the nice features of Java networking.

Testing programs without a network

For many reasons, you might not have a client machine, a server machine,

and a network available to test your programs. You might be performing

exercises in a classroom situation, or you could be writing programs that

aren't yet stable enough to put onto the network. The creators of the

Internet Protocol were aware of this issue, and they created a special

address called localhost to be the "local loopback" IP address for testing

without a network. The generic way to produce this address in Java is:

InetAddress addr = InetAddress.getByName(null);

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907

If you hand getByName( ) a null, it defaults to using the localhost.

The InetAddress is what you use to refer to the particular machine, and

you must produce this before you can go any further. You can't

manipulate the contents of an InetAddress (but you can print them out,

as you'll see in the next example). The only way you can create an

InetAddress is through one of that class's overloaded static member

methods getByName( ) (which is what you'll usually use),

getAllByName( ), or getLocalHost( ).

You can also produce the local loopback address by handing it the string

localhost:

InetAddress.getByName("localhost");

(assuming "localhost" is configured in your machine's "hosts" table), or by

using its dotted quad form to name the reserved IP number for the

loopback:

InetAddress.getByName("127.0.0.1");

All three forms produce the same result.

Port: a unique place

within the machine

An IP address isn't enough to identify a unique server, since many servers

can exist on one machine. Each IP machine also contains ports, and when

you're setting up a client or a server you must choose a port where both

client and server agree to connect; if you're meeting someone, the IP

address is the neighborhood and the port is the bar.

The port is not a physical location in a machine, but a software

abstraction (mainly for bookkeeping purposes). The client program knows

how to connect to the machine via its IP address, but how does it connect

to a desired service (potentially one of many on that machine)? That's

where the port numbers come in as a second level of addressing. The idea

is that if you ask for a particular port, you're requesting the service that's

associated with the port number. The time of day is a simple example of a

service. Typically, each service is associated with a unique port number on

a given server machine. It's up to the client to know ahead of time which

port number the desired service is running on.

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Thinking in Java

The system services reserve the use of ports 1 through 1024, so you

shouldn't use those or any other port that you know to be in use. The first

choice for examples in this book will be port 8080 (in memory of the

venerable old 8-bit Intel 8080 chip in my first computer, a CP/M

machine).

Sockets

The socket is the software abstraction used to represent the "terminals" of

a connection between two machines. For a given connection, there's a

socket on each machine, and you can imagine a hypothetical "cable"

running between the two machines with each end of the "cable" plugged

into a socket. Of course, the physical hardware and cabling between

machines is completely unknown. The whole point of the abstraction is

that we don't have to know more than is necessary.

In Java, you create a socket to make the connection to the other machine,

then you get an InputStream and OutputStream (or, with the

appropriate converters, Reader and Writer) from the socket in order to

be able to treat the connection as an I/O stream object. There are two

stream-based socket classes: a ServerSocket that a server uses to

"listen" for incoming connections and a Socket that a client uses in order

to initiate a connection. Once a client makes a socket connection, the

ServerSocket returns (via the accept( ) method) a corresponding

Socket through which communications will take place on the server side.

From then on, you have a true Socket to Socket connection and you

treat both ends the same way because they are the same. At this point,

you use the methods getInputStream( ) and getOutputStream( ) to

produce the corresponding InputStream and OutputStream objects

from each Socket. These must be wrapped inside buffers and formatting

classes just like any other stream object described in Chapter 11.

The use of the term ServerSocket would seem to be another example of

a confusing naming scheme in the Java libraries. You might think

ServerSocket would be better named "ServerConnector" or something

without the word "Socket" in it. You might also think that ServerSocket

and Socket should both be inherited from some common base class.

Indeed, the two classes do have several methods in common, but not

enough to give them a common base class. Instead, ServerSocket's job

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909

is to wait until some other machine connects to it, then to return an actual

Socket. This is why ServerSocket seems to be a bit misnamed, since its

job isn't really to be a socket but instead to make a Socket object when

someone else connects to it.

However, the ServerSocket does create a physical "server" or listening

socket on the host machine. This socket listens for incoming connections

and then returns an "established" socket (with the local and remote

endpoints defined) via the accept( ) method. The confusing part is that

both of these sockets (listening and established) are associated with the

same server socket. The listening socket can accept only new connection

requests and not data packets. So while ServerSocket doesn't make

much sense programmatically, it does "physically."

When you create a ServerSocket, you give it only a port number. You

don't have to give it an IP address because it's already on the machine it

represents. When you create a Socket, however, you must give both the

IP address and the port number where you're trying to connect.

(However, the Socket that comes back from ServerSocket.accept( )

already contains all this information.)

A simple server and client

This example makes the simplest use of servers and clients using sockets.

All the server does is wait for a connection, then uses the Socket

produced by that connection to create an InputStream and

OutputStream. These are converted to a Reader and a Writer, then

wrapped in a BufferedReader and a PrintWriter. After that,

everything it reads from the BufferedReader it echoes to the

PrintWriter until it receives the line "END," at which time it closes the

connection.

The client makes the connection to the server, then creates an

OutputStream and performs the same wrapping as in the server. Lines

of text are sent through the resulting PrintWriter. The client also

creates an InputStream (again, with appropriate conversions and

wrapping) to hear what the server is saying (which, in this case, is just the

words echoed back).

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Thinking in Java

Both the server and client use the same port number and the client uses

the local loopback address to connect to the server on the same machine

so you don't have to test it over a network. (For some configurations, you

might need to be connected to a network for the programs to work, even if

you aren't communicating over that network.)

Here is the server:

//: c15:JabberServer.java

// Very simple server that just

// echoes whatever the client sends.

import java.io.*;

import java.net.*;

public class JabberServer {

// Choose a port outside of the range 1-1024:

public static final int PORT = 8080;

public static void main(String[] args)

throws IOException {

ServerSocket s = new ServerSocket(PORT);

System.out.println("Started: " + s);

try {

// Blocks until a connection occurs:

Socket socket = s.accept();

try {

System.out.println(

"Connection accepted: "+ socket);

BufferedReader in =

new BufferedReader(

new InputStreamReader(

socket.getInputStream()));

// Output is automatically flushed

// by PrintWriter:

PrintWriter out =

new PrintWriter(

new BufferedWriter(

new OutputStreamWriter(

socket.getOutputStream())),true);

while (true) {

String str = in.readLine();

if (str.equals("END")) break;

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911

System.out.println("Echoing: " + str);

out.println(str);

}

// Always close the two sockets...

} finally {

System.out.println("closing...");

socket.close();

}

} finally {

s.close();

}

} ///:~

You can see that the ServerSocket just needs a port number, not an IP

address (since it's running on this machine!). When you call accept( ),

the method blocks until some client tries to connect to it. That is, it's there

waiting for a connection, but other processes can run (see Chapter 14).

When a connection is made, accept( ) returns with a Socket object

representing that connection.

The responsibility for cleaning up the sockets is crafted carefully here. If

the ServerSocket constructor fails, the program just quits (notice we

must assume that the constructor for ServerSocket doesn't leave any

open network sockets lying around if it fails). For this case, main( )

throws IOException so a try block is not necessary. If the

ServerSocket constructor is successful then all other method calls must

be guarded in a try-finally block to ensure that, no matter how the block

is left, the ServerSocket is properly closed.

The same logic is used for the Socket returned by accept( ). If accept( )

fails, then we must assume that the Socket doesn't exist or hold any

resources, so it doesn't need to be cleaned up. If it's successful, however,

the following statements must be in a try-finally block so that if they fail

the Socket will still be cleaned up. Care is required here because sockets

use important nonmemory resources, so you must be diligent in order to

clean them up (since there is no destructor in Java to do it for you).

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Both the ServerSocket and the Socket produced by accept( ) are

printed to System.out. This means that their toString( ) methods are

automatically called. These produce:

ServerSocket[addr=0.0.0.0,PORT=0,localport=8080]

Socket[addr=127.0.0.1,PORT=1077,localport=8080]

Shortly, you'll see how these fit together with what the client is doing.

The next part of the program looks just like opening files for reading and

writing except that the InputStream and OutputStream are created

from the Socket object. Both the InputStream and OutputStream

objects are converted to Reader and Writer objects using the

"converter" classes InputStreamReader and OutputStreamWriter,

respectively. You could also have used the Java 1.0 InputStream and

OutputStream classes directly, but with output there's a distinct

advantage to using the Writer approach. This appears with

PrintWriter, which has an overloaded constructor that takes a second

argument, a boolean flag that indicates whether to automatically flush

the output at the end of each println( ) (but not print( )) statement.

Every time you write to out, its buffer must be flushed so the information

goes out over the network. Flushing is important for this particular

example because the client and server each wait for a line from the other

party before proceeding. If flushing doesn't occur, the information will not

be put onto the network until the buffer is full, which causes lots of

problems in this example.

When writing network programs you need to be careful about using

automatic flushing. Every time you flush the buffer a packet must be

created and sent. In this case, that's exactly what we want, since if the

packet containing the line isn't sent then the handshaking back and forth

between server and client will stop. Put another way, the end of a line is

the end of a message. But in many cases, messages aren't delimited by

lines so it's much more efficient to not use auto flushing and instead let

the built-in buffering decide when to build and send a packet. This way,

larger packets can be sent and the process will be faster.

Note that, like virtually all streams you open, these are buffered. There's

an exercise at the end of this chapter to show you what happens if you

don't buffer the streams (things get slow).

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The infinite while loop reads lines from the BufferedReader in and

writes information to System.out and to the PrintWriter out. Note

that in and out could be any streams, they just happen to be connected to

the network.

When the client sends the line consisting of "END," the program breaks

out of the loop and closes the Socket.

Here's the client:

//: c15:JabberClient.java

// Very simple client that just sends

// lines to the server and reads lines

// that the server sends.

import java.net.*;

import java.io.*;

public class JabberClient {

public static void main(String[] args)

throws IOException {

// Passing null to getByName() produces the

// special "Local Loopback" IP address, for

// testing on one machine w/o a network:

InetAddress addr =

InetAddress.getByName(null);

// Alternatively, you can use

// the address or name:

// InetAddress addr =

InetAddress.getByName("127.0.0.1");

// InetAddress addr =

InetAddress.getByName("localhost");

System.out.println("addr = " + addr);

Socket socket =

new Socket(addr, JabberServer.PORT);

// Guard everything in a try-finally to make

// sure that the socket is closed:

try {

System.out.println("socket = " + socket);

BufferedReader in =

new BufferedReader(

new InputStreamReader(

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socket.getInputStream()));

// Output is automatically flushed

// by PrintWriter:

PrintWriter out =

new PrintWriter(

new BufferedWriter(

new OutputStreamWriter(

socket.getOutputStream())),true);

for(int i = 0; i < 10; i ++) {

out.println("howdy " + i);

String str = in.readLine();

System.out.println(str);

}

out.println("END");

} finally {

System.out.println("closing...");

socket.close();

}

} ///:~

In main( ) you can see all three ways to produce the InetAddress of the

local loopback IP address: using null, localhost, or the explicit reserved

address 127.0.0.1. Of course, if you want to connect to a machine across

a network you substitute that machine's IP address. When the

InetAddress addr is printed (via the automatic call to its toString( )

method) the result is:

localhost/127.0.0.1

By handing getByName( ) a null, it defaulted to finding the localhost,

and that produced the special address 127.0.0.1.

Note that the Socket called socket is created with both the

InetAddress and the port number. To understand what it means when

you print one of these Socket objects, remember that an Internet

connection is determined uniquely by these four pieces of data:

clientHost, clientPortNumber, serverHost, and

serverPortNumber. When the server comes up, it takes up its assigned

port (8080) on the localhost (127.0.0.1). When the client comes up, it is

allocated to the next available port on its machine, 1077 in this case,

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which also happens to be on the same machine (127.0.0.1) as the server.

Now, in order for data to move between the client and server, each side

has to know where to send it. Therefore, during the process of connecting

to the "known" server, the client sends a "return address" so the server

knows where to send its data. This is what you see in the example output

for the server side:

Socket[addr=127.0.0.1,port=1077,localport=8080]

This means that the server just accepted a connection from 127.0.0.1 on

port 1077 while listening on its local port (8080). On the client side:

Socket[addr=localhost/127.0.0.1,PORT=8080,localport=1077]

which means that the client made a connection to 127.0.0.1 on port 8080

using the local port 1077.

You'll notice that every time you start up the client anew, the local port

number is incremented. It starts at 1025 (one past the reserved block of

ports) and keeps going up until you reboot the machine, at which point it

starts at 1025 again. (On UNIX machines, once the upper limit of the

socket range is reached, the numbers will wrap around to the lowest

available number again.)

Once the Socket object has been created, the process of turning it into a

BufferedReader and PrintWriter is the same as in the server (again,

in both cases you start with a Socket). Here, the client initiates the

conversation by sending the string "howdy" followed by a number. Note

that the buffer must again be flushed (which happens automatically via

the second argument to the PrintWriter constructor). If the buffer isn't

flushed, the whole conversation will hang because the initial "howdy" will

never get sent (the buffer isn't full enough to cause the send to happen

automatically). Each line that is sent back from the server is written to

System.out to verify that everything is working correctly. To terminate

the conversation, the agreed-upon "END" is sent. If the client simply

hangs up, then the server throws an exception.

You can see that the same care is taken here to ensure that the network

resources represented by the Socket are properly cleaned up, using a

try-finally block.

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Sockets produce a "dedicated" connection that persists until it is explicitly

disconnected. (The dedicated connection can still be disconnected

unexplicitly if one side, or an intermediary link, of the connection

crashes.) This means the two parties are locked in communication and the

connection is constantly open. This seems like a logical approach to

networking, but it puts an extra load on the network. Later in this chapter

you'll see a different approach to networking, in which the connections

are only temporary.

Serving multiple clients

The JabberServer works, but it can handle only one client at a time. In

a typical server, you'll want to be able to deal with many clients at once.

The answer is multithreading, and in languages that don't directly support

multithreading this means all sorts of complications. In Chapter 14 you

saw that multithreading in Java is about as simple as possible,

considering that multithreading is a rather complex topic. Because

threading in Java is reasonably straightforward, making a server that

handles multiple clients is relatively easy.

The basic scheme is to make a single ServerSocket in the server and call

accept( ) to wait for a new connection. When accept( ) returns, you take

the resulting Socket and use it to create a new thread whose job is to

serve that particular client. Then you call accept( ) again to wait for a

new client.

In the following server code, you can see that it looks similar to the

JabberServer.java example except that all of the operations to serve a

particular client have been moved inside a separate thread class:

//: c15:MultiJabberServer.java

// A server that uses multithreading

// to handle any number of clients.

import java.io.*;

import java.net.*;

class ServeOneJabber extends Thread {

private Socket socket;

private BufferedReader in;

private PrintWriter out;

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public ServeOneJabber(Socket s)

throws IOException {

socket = s;

in =

new BufferedReader(

new InputStreamReader(

socket.getInputStream()));

// Enable auto-flush:

out =

new PrintWriter(

new BufferedWriter(

new OutputStreamWriter(

socket.getOutputStream())), true);

// If any of the above calls throw an

// exception, the caller is responsible for

// closing the socket. Otherwise the thread

// will close it.

start(); // Calls run()

}

public void run() {

try {

while (true) {

String str = in.readLine();

if (str.equals("END")) break;

System.out.println("Echoing: " + str);

out.println(str);

}

System.out.println("closing...");

} catch(IOException e) {

System.err.println("IO Exception");

} finally {

try {

socket.close();

} catch(IOException e) {

System.err.println("Socket not closed");

}

public class MultiJabberServer {

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Thinking in Java

static final int PORT = 8080;

public static void main(String[] args)

throws IOException {

ServerSocket s = new ServerSocket(PORT);

System.out.println("Server Started");

try {

while(true) {

// Blocks until a connection occurs:

Socket socket = s.accept();

try {

new ServeOneJabber(socket);

} catch(IOException e) {

// If it fails, close the socket,

// otherwise the thread will close it:

socket.close();

}

} finally {

s.close();

}

} ///:~

The ServeOneJabber thread takes the Socket object that's produced

by accept( ) in main( ) every time a new client makes a connection.

Then, as before, it creates a BufferedReader and auto-flushed

PrintWriter object using the Socket. Finally, it calls the special

Thread method start( ), which performs thread initialization and then

calls run( ). This performs the same kind of action as in the previous

example: reading something from the socket and then echoing it back

until it reads the special "END" signal.

The responsibility for cleaning up the socket must again be carefully

designed. In this case, the socket is created outside of the

ServeOneJabber so the responsibility can be shared. If the

ServeOneJabber constructor fails, it will just throw the exception to the

caller, who will then clean up the thread. But if the constructor succeeds,

then the ServeOneJabber object takes over responsibility for cleaning

up the thread, in its run( ).

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Notice the simplicity of the MultiJabberServer. As before, a

ServerSocket is created and accept( ) is called to allow a new

connection. But this time, the return value of accept( ) (a Socket) is

passed to the constructor for ServeOneJabber, which creates a new

thread to handle that connection. When the connection is terminated, the

thread simply goes away.

If the creation of the ServerSocket fails, the exception is again thrown

through main( ). But if the creation succeeds, the outer try-finally

guarantees its cleanup. The inner try-catch guards only against the

failure of the ServeOneJabber constructor; if the constructor succeeds,

then the ServeOneJabber thread will close the associated socket.

To test that the server really does handle multiple clients, the following

program creates many clients (using threads) that connect to the same

server. The maximum number of threads allowed is determined by the

final int MAX_THREADS.

//: c15:MultiJabberClient.java

// Client that tests the MultiJabberServer

// by starting up multiple clients.

import java.net.*;

import java.io.*;

class JabberClientThread extends Thread {

private Socket socket;

private BufferedReader in;

private PrintWriter out;

private static int counter = 0;

private int id = counter++;

private static int threadcount = 0;

public static int threadCount() {

return threadcount;

}

public JabberClientThread(InetAddress addr) {

System.out.println("Making client " + id);

threadcount++;

try {

socket =

new Socket(addr, MultiJabberServer.PORT);

} catch(IOException e) {

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Thinking in Java

System.err.println("Socket failed");

// If the creation of the socket fails,

// nothing needs to be cleaned up.

}

try {

in =

new BufferedReader(

new InputStreamReader(

socket.getInputStream()));

// Enable auto-flush:

out =

new PrintWriter(

new BufferedWriter(

new OutputStreamWriter(

socket.getOutputStream())), true);

start();

} catch(IOException e) {

// The socket should be closed on any

// failures other than the socket

// constructor:

try {

socket.close();

} catch(IOException e2) {

System.err.println("Socket not closed");

}

// Otherwise the socket will be closed by

// the run() method of the thread.

}

public void run() {

try {

for(int i = 0; i < 25; i++) {

out.println("Client " + id + ": " + i);

String str = in.readLine();

System.out.println(str);

}

out.println("END");

} catch(IOException e) {

System.err.println("IO Exception");

} finally {

// Always close it:

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try {

socket.close();

} catch(IOException e) {

System.err.println("Socket not closed");

}

threadcount--; // Ending this thread

}

public class MultiJabberClient {

static final int MAX_THREADS = 40;

public static void main(String[] args)

throws IOException, InterruptedException {

InetAddress addr =

InetAddress.getByName(null);

while(true) {

if(JabberClientThread.threadCount()

< MAX_THREADS)

new JabberClientThread(addr);

Thread.currentThread().sleep(100);

}

} ///:~

The JabberClientThread constructor takes an InetAddress and uses

it to open a Socket. You're probably starting to see the pattern: the

Socket is always used to create some kind of Reader and/or Writer (or

InputStream and/or OutputStream) object, which is the only way

that the Socket can be used. (You can, of course, write a class or two to

automate this process instead of doing all the typing if it becomes

painful.) Again, start( ) performs thread initialization and calls run( ).

Here, messages are sent to the server and information from the server is

echoed to the screen. However, the thread has a limited lifetime and

eventually completes. Note that the socket is cleaned up if the constructor

fails after the socket is created but before the constructor completes.

Otherwise the responsibility for calling close( ) for the socket is relegated

to the run( ) method.

The threadcount keeps track of how many JabberClientThread

objects currently exist. It is incremented as part of the constructor and

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Thinking in Java

decremented as run( ) exits (which means the thread is terminating). In

MultiJabberClient.main( ), you can see that the number of threads is

tested, and if there are too many, no more are created. Then the method

sleeps. This way, some threads will eventually terminate and more can be

created. You can experiment with MAX_THREADS to see where your

particular system begins to have trouble with too many connections.

Datagrams

The examples you've seen so far use the Transmission Control Protocol

(TCP, also known as stream-based sockets), which is designed for

ultimate reliability and guarantees that the data will get there. It allows

retransmission of lost data, it provides multiple paths through different

routers in case one goes down, and bytes are delivered in the order they

are sent. All this control and reliability comes at a cost: TCP has a high

overhead.

There's a second protocol, called User Datagram Protocol (UDP), which

doesn't guarantee that the packets will be delivered and doesn't guarantee

that they will arrive in the order they were sent. It's called an "unreliable

protocol" (TCP is a "reliable protocol"), which sounds bad, but because it's

much faster it can be useful. There are some applications, such as an

audio signal, in which it isn't so critical if a few packets are dropped here

or there but speed is vital. Or consider a time-of-day server, where it

really doesn't matter if one of the messages is lost. Also, some applications

might be able to fire off a UDP message to a server and can then assume,

if there is no response in a reasonable period of time, that the message

was lost.

Typically, you'll do most of your direct network programming with TCP,

and only occasionally will you use UDP. There's a more complete

treatment of UDP, including an example, in the first edition of this book

(available on the CD ROM bound into this book, or as a free download

from ).

Using URLs from within an applet

It's possible for an applet to cause the display of any URL through the

Web browser the applet is running within. You can do this with the

following line:

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923

getAppletContext().showDocument(u);

in which u is the URL object. Here's a simple example that redirects you

to another Web page. Although you're just redirected to an HTML page,

you could also redirect to the output of a CGI program.

//: c15:ShowHTML.java

// <applet code=ShowHTML width=100 height=50>

// </applet>

import javax.swing.*;

import java.awt.*;

import java.awt.event.*;

import java.net.*;

import java.io.*;

import com.bruceeckel.swing.*;

public class ShowHTML extends JApplet {

JButton send = new JButton("Go");

JLabel l = new JLabel();

public void init() {

Container cp = getContentPane();

cp.setLayout(new FlowLayout());

send.addActionListener(new Al());

cp.add(send);

cp.add(l);

}

class Al implements ActionListener {

public void actionPerformed(ActionEvent ae) {

try {

// This could be a CGI program instead of

// an HTML page.

URL u = new URL(getDocumentBase(),

"FetcherFrame.html");

// Display the output of the URL using

// the Web browser, as an ordinary page:

getAppletContext().showDocument(u);

} catch(Exception e) {

l.setText(e.toString());

}

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Thinking in Java

public static void main(String[] args) {

Console.run(new ShowHTML(), 100, 50);

}

} ///:~

The beauty of the URL class is how much it shields you from. You can

connect to Web servers without knowing much at all about what's going

on under the covers.

Reading a file from the server

A variation on the above program reads a file located on the server. In this

case, the file is specified by the client:

//: c15:Fetcher.java

// <applet code=Fetcher width=500 height=300>

// </applet>

import javax.swing.*;

import java.awt.*;

import java.awt.event.*;

import java.net.*;

import java.io.*;

import com.bruceeckel.swing.*;

public class Fetcher extends JApplet {

JButton fetchIt= new JButton("Fetch the Data");

JTextField f =

new JTextField("Fetcher.java", 20);

JTextArea t = new JTextArea(10,40);

public void init() {

Container cp = getContentPane();

cp.setLayout(new FlowLayout());

fetchIt.addActionListener(new FetchL());

cp.add(new JScrollPane(t));

cp.add(f); cp.add(fetchIt);

}

public class FetchL implements ActionListener {

public void actionPerformed(ActionEvent e) {

try {

URL url = new URL(getDocumentBase(),

f.getText());

t.setText(url + "\n");

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InputStream is = url.openStream();

BufferedReader in = new BufferedReader(

new InputStreamReader(is));

String line;

while ((line = in.readLine()) != null)

t.append(line + "\n");

} catch(Exception ex) {

t.append(ex.toString());

}

public static void main(String[] args) {

Console.run(new Fetcher(), 500, 300);

}

} ///:~

The creation of the URL object is similar to the previous example--

getDocumentBase( ) is the starting point as before, but this time the

name of the file is read from the JTextField. Once the URL object is

created, its String version is placed in the JTextArea so we can see what

it looks like. Then an InputStream is procured from the URL, which in

this case will simply produce a stream of the characters in the file. After

converting to a Reader and buffering, each line is read and appended to

the JTextArea. Note that the JTextArea has been placed inside a

JScrollPane so that scrolling is handled automatically.

More to networking

There's actually a lot more to networking than can be covered in this

introductory treatment. Java networking also provides fairly extensive

support for URLs, including protocol handlers for different types of

content that can be discovered at an Internet site. You can find other Java

networking features fully and carefully described in Java Network

Programming by Elliotte Rusty Harold (O'Reilly, 1997).

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Java Database

Connectivity (JDBC)

It has been estimated that half of all software development involves

client/server operations. A great promise of Java has been the ability to

build platform-independent client/server database applications. This has

come to fruition with Java DataBase Connectivity (JDBC).

One of the major problems with databases has been the feature wars

between the database companies. There is a "standard" database

language, Structured Query Language (SQL-92), but you must usually

know which database vendor you're working with despite the standard.

JDBC is designed to be platform-independent, so you don't need to worry

about the database you're using while you're programming. However, it's

still possible to make vendor-specific calls from JDBC so you aren't

restricted from doing what you must.

One place where programmers may need to use SQL type names is in the

SQL TABLE CREATE statement when they are creating a new database

table and defining the SQL type for each column. Unfortunately there are

significant variations between SQL types supported by different database

products. Different databases that support SQL types with the same

semantics and structure may give those types different names. Most

major databases support an SQL data type for large binary values: in

Oracle this type is called a LONG RAW, Sybase calls it IMAGE, Informix

calls it BYTE, and DB2 calls it LONG VARCHAR FOR BIT DATA.

Therefore, if database portability is a goal you should try to use only

generic SQL type identifiers.

Portability is an issue when writing for a book where readers may be

testing the examples with all kinds of unknown data stores. I have tried to

write these examples to be as portable as possible. You should also notice

that the database-specific code has been isolated in order to centralize any

changes that you may need to perform to get the examples operational in

your environment.

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JDBC, like many of the APIs in Java, is designed for simplicity. The

method calls you make correspond to the logical operations you'd think of

doing when gathering data from a database: connect to the database,

create a statement and execute the query, and look at the result set.

To allow this platform independence, JDBC provides a driver manager

that dynamically maintains all the driver objects that your database

queries will need. So if you have three different kinds of vendor databases

to connect to, you'll need three different driver objects. The driver objects

you can force the loading using Class.forName( ).

To open a database, you must create a "database URL" that specifies:

That you're using JDBC with "jdbc."

The "subprotocol": the name of the driver or the name of a

database connectivity mechanism. Since the design of JDBC was

inspired by ODBC, the first subprotocol available is the "jdbc-odbc

bridge," specified by "odbc."

The database identifier. This varies with the database driver used,

but it generally provides a logical name that is mapped by the

database administration software to a physical directory where the

database tables are located. For your database identifier to have

any meaning, you must register the name using your database

administration software. (The process of registration varies from

platform to platform.)

All this information is combined into one string, the "database URL." For

example, to connect through the ODBC subprotocol to a database

identified as "people," the database URL could be:

String dbUrl = "jdbc:odbc:people";

If you're connecting across a network, the database URL will contain the

connection information identifying the remote machine and can become a

bit intimidating. Here is an example of a CloudScape database being

called from a remote client utilizing RMI:

jdbc:rmi://192.168.170.27:1099/jdbc:cloudscape:db

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Thinking in Java

This database URL is really two jdbc calls in one. The first part

"jdbc:rmi://192.168.170.27:1099/" uses RMI to make the

connection to the remote database engine listening on port 1099 at IP

Address 192.168.170.27. The second part of the URL,

"jdbc:cloudscape:db" conveys the more typical settings using the

subprotocol and database name but this will only happen after the first

section has made the connection via RMI to the remote machine.

When you're ready to connect to the database, call the static method

DriverManager.getConnection( ) and pass it the database URL, the

user name, and a password to get into the database. You get back a

Connection object that you can then use to query and manipulate the

database.

The following example opens a database of contact information and looks

for a person's last name as given on the command line. It selects only the

names of people that have email addresses, then prints out all the ones

that match the given last name:

//: c15:jdbc:Lookup.java

// Looks up email addresses in a

// local database using JDBC.

import java.sql.*;

public class Lookup {

public static void main(String[] args)

throws SQLException, ClassNotFoundException {

String dbUrl = "jdbc:odbc:people";

String user = "";

String password = "";

// Load the driver (registers itself)

Class.forName(

"sun.jdbc.odbc.JdbcOdbcDriver");

Connection c = DriverManager.getConnection(

dbUrl, user, password);

Statement s = c.createStatement();

// SQL code:

ResultSet r =

s.executeQuery(

"SELECT FIRST, LAST, EMAIL " +

"FROM people.csv people " +

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"WHERE " +

"(LAST='" + args[0] + "') " +

" AND (EMAIL Is Not Null) " +

"ORDER BY FIRST");

while(r.next()) {

// Capitalization doesn't matter:

System.out.println(

r.getString("Last") + ", "

+ r.getString("fIRST")

+ ": " + r.getString("EMAIL") );

}

s.close(); // Also closes ResultSet

}

} ///:~

You can see the creation of the database URL as previously described. In

this example, there is no password protection on the database so the user

name and password are empty strings.

Once the connection is made with DriverManager.getConnection( ),

you can use the resulting Connection object to create a Statement

object using the createStatement( ) method. With the resulting

Statement, you can call executeQuery( ), passing in a string

containing an SQL-92 standard SQL statement. (You'll see shortly how

you can generate this statement automatically, so you don't have to know

much about SQL.)

The executeQuery( ) method returns a ResultSet object, which is an

iterator: the next( ) method moves the iterator to the next record in the

statement, or returns false if the end of the result set has been reached.

You'll always get a ResultSet object back from executeQuery( ) even if

a query results in an empty set (that is, an exception is not thrown). Note

that you must call next( ) once before trying to read any record data. If

the result set is empty, this first call to next( ) will return false. For each

record in the result set, you can select the fields using (among other

approaches) the field name as a string. Also note that the capitalization of

the field name is ignored--it doesn't matter with an SQL database. You

determine the type you'll get back by calling getInt( ), getString( ),

getFloat( ), etc. At this point, you've got your database data in Java

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Thinking in Java

native format and can do whatever you want with it using ordinary Java

code.

Getting the example to work

With JDBC, understanding the code is relatively simple. The confusing

part is making it work on your particular system. The reason this is

confusing is that it requires you to figure out how to get your JDBC driver

to load properly, and how to set up a database using your database

administration software.

Of course, this process can vary radically from machine to machine, but

the process I used to make it work under 32-bit Windows might give you

clues to help you attack your own situation.

Step 1: Find the JDBC Driver

The program above contains the statement:

Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");

This implies a directory structure, which is deceiving. With this particular

installation of JDK 1.1, there was no file called JdbcOdbcDriver.class,

so if you looked at this example and went searching for it you'd be

frustrated. Other published examples use a pseudo name, such as

"myDriver.ClassName," which is less than helpful. In fact, the load

statement above for the jdbc-odbc driver (the only one that actually comes

with the JDK) appears in only a few places in the online documentation

(in particular, a page labeled "JDBC-ODBC Bridge Driver"). If the load

statement above doesn't work, then the name might have been changed as

part of a Java version change, so you should hunt through the

documentation again.

If the load statement is wrong, you'll get an exception at this point. To test

whether your driver load statement is working correctly, comment out the

code after the statement and up to the catch clause; if the program

throws no exceptions it means that the driver is loading properly.

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Step 2: Configure the database

Again, this is specific to 32-bit Windows; you might need to do some

research to figure it out for your own platform.

First, open the control panel. You might find two icons that say "ODBC."

You must use the one that says "32bit ODBC," since the other one is for

backward compatibility with 16-bit ODBC software and will produce no

results for JDBC. When you open the "32bit ODBC" icon, you'll see a

tabbed dialog with a number of tabs, including "User DSN," "System

DSN," "File DSN," etc., in which "DSN" means "Data Source Name." It

turns out that for the JDBC-ODBC bridge, the only place where it's

important to set up your database is "System DSN," but you'll also want to

test your configuration and create queries, and for that you'll also need to

set up your database in "File DSN." This will allow the Microsoft Query

tool (that comes with Microsoft Office) to find the database. Note that

other query tools are also available from other vendors.

The most interesting database is one that you're already using. Standard

ODBC supports a number of different file formats including such

venerable workhorses as DBase. However, it also includes the simple

"comma-separated ASCII" format, which virtually every data tool has the

ability to write. In my case, I just took my "people" database that I've been

maintaining for years using various contact-management tools and

exported it as a comma-separated ASCII file (these typically have an

extension of .csv). In the "System DSN" section I chose "Add," chose the

text driver to handle my comma-separated ASCII file, and then un-

checked "use current directory" to allow me to specify the directory where

I exported the data file.

You'll notice when you do this that you don't actually specify a file, only a

directory. That's because a database is typically represented as a collection

of files under a single directory (although it could be represented in other

forms as well). Each file usually contains a single table, and the SQL

statements can produce results that are culled from multiple tables in the

database (this is called a join). A database that contains only a single table

(like my "people" database) is usually called a flat-file database. Most

problems that go beyond the simple storage and retrieval of data generally

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Thinking in Java

require multiple tables that must be related by joins to produce the

desired results, and these are called relational databases.

Step 3: Test the configuration

To test the configuration you'll need a way to discover whether the

database is visible from a program that queries it. Of course, you can

simply run the JDBC program example above, up to and including the

statement:

Connection c = DriverManager.getConnection(

dbUrl, user, password);

If an exception is thrown, your configuration was incorrect.

However, it's useful to get a query-generation tool involved at this point. I

used Microsoft Query that came with Microsoft Office, but you might

prefer something else. The query tool must know where the database is,

and Microsoft Query required that I go to the ODBC Administrator's "File

DSN" tab and add a new entry there, again specifying the text driver and

the directory where my database lives. You can name the entry anything

you want, but it's helpful to use the same name you used in "System

DSN."

Once you've done this, you will see that your database is available when

you create a new query using your query tool.

Step 4: Generate your SQL query

The query that I created using Microsoft Query not only showed me that

my database was there and in good order, but it also automatically created

the SQL code that I needed to insert into my Java program. I wanted a

query that would search for records that had the last name that was typed

on the command line when starting the Java program. So as a starting

point, I searched for a specific last name, "Eckel." I also wanted to display

only those names that had email addresses associated with them. The

steps I took to create this query were:

Start a new query and use the Query Wizard. Select the "people"

database. (This is the equivalent of opening the database

connection using the appropriate database URL.)

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Select the "people" table within the database. From within the

table, choose the columns FIRST, LAST, and EMAIL.

Under "Filter Data," choose LAST and select "equals" with an

argument of "Eckel." Click the "And" radio button.

Choose EMAIL and select "Is not Null."

Under "Sort By," choose FIRST.

The result of this query will show you whether you're getting what you

want.

Now you can press the SQL button and without any research on your part,

up will pop the correct SQL code, ready for you to cut and paste. For this

query, it looked like this:

SELECT people.FIRST, people.LAST, people.EMAIL

FROM people.csv people

WHERE (people.LAST='Eckel') AND

(people.EMAIL Is Not Null)

ORDER BY people.FIRST

Especially with more complicated queries it's easy to get things wrong, but

by using a query tool you can interactively test your queries and

automatically generate the correct code. It's hard to argue the case for

doing this by hand.

Step 5: Modify and paste in your query

You'll notice that the code above looks different from what's used in the

program. That's because the query tool uses full qualification for all of the

names, even when there's only one table involved. (When more than one

table is involved, the qualification prevents collisions between columns

from different tables that have the same names.) Since this query involves

only one table, you can optionally remove the "people" qualifier from

most of the names, like this:

SELECT FIRST, LAST, EMAIL

FROM people.csv people

WHERE (LAST='Eckel') AND

(EMAIL Is Not Null)

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Thinking in Java

ORDER BY FIRST

In addition, you don't want this program to be hard coded to look for only

one name. Instead, it should hunt for the name given as the command-

line argument. Making these changes and turning the SQL statement into

a dynamically-created String produces:

"SELECT FIRST, LAST, EMAIL " +

"FROM people.csv people " +

"WHERE " +

"(LAST='" + args[0] + "') " +

" AND (EMAIL Is Not Null) " +

"ORDER BY FIRST");

SQL has another way to insert names into a query called stored

procedures, which is used for speed. But for much of your database

experimentation and for your first cut, building your own query strings in

Java is fine.

You can see from this example that by using the tools currently available--

in particular the query-building tool--database programming with SQL

and JDBC can be quite straightforward.

A GUI version of the lookup

program

It's more useful to leave the lookup program running all the time and

simply switch to it and type in a name whenever you want to look

someone up. The following program creates the lookup program as an

application/applet, and it also adds name completion so the data will

show up without forcing you to type the entire last name:

//: c15:jdbc:VLookup.java

// GUI version of Lookup.java.

// <applet code=VLookup

// width=500 height=200></applet>

import javax.swing.*;

import java.awt.*;

import java.awt.event.*;

import javax.swing.event.*;

import java.sql.*;

Chapter 15: Distributed Computing

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import com.bruceeckel.swing.*;

public class VLookup extends JApplet {

String dbUrl = "jdbc:odbc:people";

String user = "";

String password = "";

Statement s;

JTextField searchFor = new JTextField(20);

JLabel completion =

new JLabel("

");

JTextArea results = new JTextArea(40, 20);

public void init() {

searchFor.getDocument().addDocumentListener(

new SearchL());

JPanel p = new JPanel();

p.add(new Label("Last name to search for:"));

p.add(searchFor);

p.add(completion);

Container cp = getContentPane();

cp.add(p, BorderLayout.NORTH);

cp.add(results, BorderLayout.CENTER);

try {

// Load the driver (registers itself)

Class.forName(

"sun.jdbc.odbc.JdbcOdbcDriver");

Connection c = DriverManager.getConnection(

dbUrl, user, password);

s = c.createStatement();

} catch(Exception e) {

results.setText(e.toString());

}

class SearchL implements DocumentListener {

public void changedUpdate(DocumentEvent e){}

public void insertUpdate(DocumentEvent e){

textValueChanged();

}

public void removeUpdate(DocumentEvent e){

textValueChanged();

}

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Thinking in Java

public void textValueChanged() {

ResultSet r;

if(searchFor.getText().length() == 0) {

completion.setText("");

results.setText("");

return;

}

try {

// Name completion:

r = s.executeQuery(

"SELECT LAST FROM people.csv people " +

"WHERE (LAST Like '" +

searchFor.getText() +

"%') ORDER BY LAST");

if(r.next())

completion.setText(

r.getString("last"));

r = s.executeQuery(

"SELECT FIRST, LAST, EMAIL " +

"FROM people.csv people " +

"WHERE (LAST='" +

completion.getText() +

"') AND (EMAIL Is Not Null) " +

"ORDER BY FIRST");

} catch(Exception e) {

results.setText(

searchFor.getText() + "\n");

results.append(e.toString());

return;

}

results.setText("");

try {

while(r.next()) {

results.append(

r.getString("Last") + ", "

+ r.getString("fIRST") +

": " + r.getString("EMAIL") + "\n");

}

} catch(Exception e) {

results.setText(e.toString());

}

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937

}

public static void main(String[] args) {

Console.run(new VLookup(), 500, 200);

}

} ///:~

Much of the database logic is the same, but you can see that a

DocumentListener is added to listen to the JTextField (see the

javax.swing.JTextField entry in the Java HTML documentation from

java.sun.com for details), so that whenever you type a new character it

first tries to do a name completion by looking up the last name in the

database and using the first one that shows up. (It places it in the

completion JLabel, and uses that as the lookup text.) This way, as soon

as you've typed enough characters for the program to uniquely find the

name you're looking for, you can stop.

Why the JDBC API

seems so complex

When you browse the online documentation for JDBC it can seem

daunting. In particular, in the DatabaseMetaData interface--which is

just huge, contrary to most of the interfaces you see in Java--there are

methods such as dataDefinitionCausesTransactionCommit( ),

getMaxColumnNameLength( ), getMaxStatementLength( ),

storesMixedCaseQuotedIdentifiers( ),

supportsANSI92IntermediateSQL( ),

supportsLimitedOuterJoins( ), and so on. What's this all about?

As mentioned earlier, databases have seemed from their inception to be in

a constant state of turmoil, primarily because the demand for database

applications, and thus database tools, is so great. Only recently has there

been any convergence on the common language of SQL (and there are

plenty of other database languages in common use). But even with an SQL

"standard" there are so many variations on that theme that JDBC must

provide the large DatabaseMetaData interface so that your code can

discover the capabilities of the particular "standard" SQL database that

it's currently connected to. In short, you can write simple, transportable

SQL, but if you want to optimize speed your coding will multiply

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Thinking in Java

tremendously as you investigate the capabilities of a particular vendor's

database.

This, of course, is not Java's fault. The discrepancies between database

products are just something that JDBC tries to help compensate for. But

bear in mind that your life will be easier if you can either write generic

queries and not worry quite as much about performance, or, if you must

tune for performance, know the platform you're writing for so you don't

need to write all that investigation code.

A more sophisticated example

A more interesting example2 involves a multitable database that resides

on a server. Here, the database is meant to provide a repository for

community activities and to allow people to sign up for these events, so it

is called the Community Interests Database (CID). This example will only

provide an overview of the database and its implementation, and is not

intended to be an in-depth tutorial on database development. There are

numerous books, seminars, and software packages that will help you in

the design and development of a database.

In addition, this example presumes the prior installation of an SQL

database on a server (although it could also be run on a local machine),

and the interrogation and discovery of an appropriate JDBC driver for

that database. Several free SQL databases are available, and some are

even automatically installed with various flavors of Linux. You are

responsible for making the choice of database and locating the JDBC

driver; the example here is based on an SQL database system called

"Cloudscape."

To keep changes in the connection information simple, the database

driver, database URL, user name, and password are placed in a separate

class:

//: c15:jdbc:CIDConnect.java

// Database connection information for

// the community interests database (CID).

2 Created by Dave Bartlett.

Chapter 15: Distributed Computing

939

public class CIDConnect {

// All the information specific to CloudScape:

public static String dbDriver =

"COM.cloudscape.core.JDBCDriver";

public static String dbURL =

"jdbc:cloudscape:d:/docs/_work/JSapienDB";

public static String user = "";

public static String password = "";

} ///:~

In this example, there is no password protection on the database so the

user name and password are empty strings.

The database consists of a set of tables that have a structure as shown

here:

EVTMEMS

EVENTS

MEM_ID

EVT_ID

TITLE (IE)

MEM_ORD

TYPE

LOC_ID

PRICE

DATETIME

MEMBERS

MEM_ID

LOCATIONS

MEM_UNAME (AK)

LOC_ID

MEM_LNAME (IE)

NAME (IE)

MEM_FNAME (IE)

CONTACT

ADDRESS

CITY

STATE

ZIP

PHONE

DIRECTIONS

"Members" contains community member information, "Events" and

"Locations" contain information about the activities and where they take

place, and "Evtmems" connects events and members that would like to

attend that event. You can see that a data member in one table produces a

key in another table.

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Thinking in Java

The following class contains the SQL strings that will create these

database tables (refer to an SQL guide for an explanation of the SQL

code):

//: c15:jdbc:CIDSQL.java

// SQL strings to create the tables for the CID.

public class CIDSQL {

public static String[] sql = {

// Create the MEMBERS table:

"drop table MEMBERS",

"create table MEMBERS " +

"(MEM_ID INTEGER primary key, " +

"MEM_UNAME VARCHAR(12) not null unique, "+

"MEM_LNAME VARCHAR(40), " +

"MEM_FNAME VARCHAR(20), " +

"ADDRESS VARCHAR(40), " +

"CITY VARCHAR(20), " +

"STATE CHAR(4), " +

"ZIP CHAR(5), " +

"PHONE CHAR(12), " +

"EMAIL VARCHAR(30))",

"create unique index " +

"LNAME_IDX on MEMBERS(MEM_LNAME)",

// Create the EVENTS table

"drop table EVENTS",

"create table EVENTS " +

"(EVT_ID INTEGER primary key, " +

"EVT_TITLE VARCHAR(30) not null, " +

"EVT_TYPE VARCHAR(20), " +

"LOC_ID INTEGER, " +

"PRICE DECIMAL, " +

"DATETIME TIMESTAMP)",

"create unique index " +

"TITLE_IDX on EVENTS(EVT_TITLE)",

// Create the EVTMEMS table

"drop table EVTMEMS",

"create table EVTMEMS " +

"(MEM_ID INTEGER not null, " +

"EVT_ID INTEGER not null, " +

"MEM_ORD INTEGER)",

Chapter 15: Distributed Computing

941

"create unique index " +

"EVTMEM_IDX on EVTMEMS(MEM_ID, EVT_ID)",

// Create the LOCATIONS table

"drop table LOCATIONS",

"create table LOCATIONS " +

"(LOC_ID INTEGER primary key, " +

"LOC_NAME VARCHAR(30) not null, " +

"CONTACT VARCHAR(50), " +

"ADDRESS VARCHAR(40), " +

"CITY VARCHAR(20), " +

"STATE VARCHAR(4), " +

"ZIP VARCHAR(5), " +

"PHONE CHAR(12), " +

"DIRECTIONS VARCHAR(4096))",

"create unique index " +

"NAME_IDX on LOCATIONS(LOC_NAME)",

};

} ///:~

The following program uses the CIDConnect and CIDSQL information

to load the JDBC driver, make a connection to the database, and then

create the table structure diagrammed above. To connect with the

database, you call the static method

DriverManager.getConnection( ), passing it the database URL, the

user name, and a password to get into the database. You get back a

Connection object that you can use to query and manipulate the

database. Once the connection is made you can simply push the SQL to

the database, in this case by marching through the CIDSQL array.

However, the first time this program is run, the "drop table" command

will fail, causing an exception, which is caught, reported, and then

ignored. The reason for the "drop table" command is to allow easy

experimentation: you can modify the SQL that defines the tables and then

rerun the program, causing the old tables to be replaced by the new.

In this example, it makes sense to let the exceptions be thrown out to the

console:

//: c15:jdbc:CIDCreateTables.java

// Creates database tables for the

// community interests database.

import java.sql.*;

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Thinking in Java

public class CIDCreateTables {

public static void main(String[] args)

throws SQLException, ClassNotFoundException,

IllegalAccessException {

// Load the driver (registers itself)

Class.forName(CIDConnect.dbDriver);

Connection c = DriverManager.getConnection(

CIDConnect.dbURL, CIDConnect.user,

CIDConnect.password);

Statement s = c.createStatement();

for(int i = 0; i < CIDSQL.sql.length; i++) {

System.out.println(CIDSQL.sql[i]);

try {

s.executeUpdate(CIDSQL.sql[i]);

} catch(SQLException sqlEx) {

System.err.println(

"Probably a 'drop table' failed");

}

s.close();

c.close();

}

} ///:~

Note that all changes in the database can be controlled by changing

Strings in the CIDSQL table, without modifying CIDCreateTables.

executeUpdate( ) will usually return the number of rows that were

affected by the SQL statement. executeUpdate( ) is more commonly

used to execute INSERT, UPDATE, or DELETE statements that modify one

or more rows. For statements such as CREATE TABLE, DROP TABLE, and

CREATE INDEX, executeUpdate( ) always returns zero.

To test the database, it is loaded with some sample data. This requires a

series of INSERTs followed by a SELECT to produce result set. To make

additions and changes to the test data easy, the test data is set up as a

two-dimensional array of Objects, and the executeInsert( ) method

can then use the information in one row of the table to create the

appropriate SQL command.

Chapter 15: Distributed Computing

943

//: c15:jdbc:LoadDB.java

// Loads and tests the database.

import java.sql.*;

class TestSet {

Object[][] data = {

{ "MEMBERS", new Integer(1),

"dbartlett", "Bartlett", "David",

"123 Mockingbird Lane",

"Gettysburg", "PA", "19312",

"123.456.7890", "bart@you.net" },

{ "MEMBERS", new Integer(2),

"beckel", "Eckel", "Bruce",

"123 Over Rainbow Lane",

"Crested Butte", "CO", "81224",

"123.456.7890", "beckel@you.net" },

{ "MEMBERS", new Integer(3),

"rcastaneda", "Castaneda", "Robert",

"123 Downunder Lane",

"Sydney", "NSW", "12345",

"123.456.7890", "rcastaneda@you.net" },

{ "LOCATIONS", new Integer(1),

"Center for Arts",

"Betty Wright", "123 Elk Ave.",

"Crested Butte", "CO", "81224",

"123.456.7890",

"Go this way then that." },

{ "LOCATIONS", new Integer(2),

"Witts End Conference Center",

"John Wittig", "123 Music Drive",

"Zoneville", "PA", "19123",

"123.456.7890",

"Go that way then this." },

{ "EVENTS", new Integer(1),

"Project Management Myths",

"Software Development",

new Integer(1), new Float(2.50),

"2000-07-17 19:30:00" },

{ "EVENTS", new Integer(2),

"Life of the Crested Dog",

"Archeology",

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Thinking in Java

new Integer(2), new Float(0.00),

"2000-07-19 19:00:00" },

// Match some people with events

{ "EVTMEMS",

new Integer(1), // Dave is going to

new Integer(1), // the Software event.

new Integer(0) },

{ "EVTMEMS",

new Integer(2), // Bruce is going to

new Integer(2), // the Archeology event.

new Integer(0) },

{ "EVTMEMS",

new Integer(3), // Robert is going to

new Integer(1), // the Software event.

new Integer(1) },

{ "EVTMEMS",

new Integer(3), // ... and

new Integer(2), // the Archeology event.

new Integer(1) },

};

// Use the default data set:

public TestSet() {}

// Use a different data set:

public TestSet(Object[][] dat) { data = dat; }

}

public class LoadDB {

Statement statement;

Connection connection;

TestSet tset;

public LoadDB(TestSet t) throws SQLException {

tset = t;

try {

// Load the driver (registers itself)

Class.forName(CIDConnect.dbDriver);

} catch(java.lang.ClassNotFoundException e) {

e.printStackTrace(System.err);

}

connection = DriverManager.getConnection(

CIDConnect.dbURL, CIDConnect.user,

CIDConnect.password);

Chapter 15: Distributed Computing

945

statement = connection.createStatement();

}

public void cleanup() throws SQLException {

statement.close();

connection.close();

}

public void executeInsert(Object[] data) {

String sql = "insert into "

+ data[0] + " values(";

for(int i = 1; i < data.length; i++) {

if(data[i] instanceof String)

sql += "'" + data[i] + "'";

else

sql += data[i];

if(i < data.length - 1)

sql += ", ";

}

sql += ')';

System.out.println(sql);

try {

statement.executeUpdate(sql);

} catch(SQLException sqlEx) {

System.err.println("Insert failed.");

while (sqlEx != null) {

System.err.println(sqlEx.toString());

sqlEx = sqlEx.getNextException();

}

public void load() {

for(int i = 0; i< tset.data.length; i++)

executeInsert(tset.data[i]);

}

// Throw exceptions out to console:

public static void main(String[] args)

throws SQLException {

LoadDB db = new LoadDB(new TestSet());

db.load();

try {

// Get a ResultSet from the loaded database:

ResultSet rs = db.statement.executeQuery(

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Thinking in Java

"select " +

"e.EVT_TITLE, m.MEM_LNAME, m.MEM_FNAME "+

"from EVENTS e, MEMBERS m, EVTMEMS em " +

"where em.EVT_ID = 2 " +

"and e.EVT_ID = em.EVT_ID " +

"and m.MEM_ID = em.MEM_ID");

while (rs.next())

System.out.println(

rs.getString(1) + " " +

rs.getString(2) + ", " +

rs.getString(3));

} finally {

db.cleanup();

}

} ///:~

The TestSet class contains a default set of data that is produced if you

use the default constructor; however, you can also create a TestSet object

using an alternate data set with the second constructor. The set of data is

held in a two-dimensional array of Object because it can be any type,

including String or numerical types. The executeInsert( ) method uses

RTTI to distinguish between String data (which must be quoted) and

non-String data as it builds the SQL command from the data. After

printing this command to the console, executeUpdate( ) is used to send

it to the database.

The constructor for LoadDB makes the connection, and load( ) steps

through the data and calls executeInsert( ) for each record. cleanup( )

closes the statement and the connection; to guarantee that this is called, it

is placed inside a finally clause.

Once the database is loaded, an executeQuery( ) statement produces a

sample result set. Since the query combines several tables, it is an

example of a join.

There is more JDBC information available in the electronic documents

that come as part of the Java distribution from Sun. In addition, you can

find more in the book JDBC Database Access with Java (Hamilton,

Cattel, and Fisher, Addison-Wesley, 1997). Other JDBC books appear

regularly.

Chapter 15: Distributed Computing

947

Servlets

Client access from the Internet or corporate intranets is a sure way to

allow many users to access data and resources easily3. This type of access

is based on clients using the World Wide Web standards of Hypertext

Markup Language (HTML) and Hypertext Transfer Protocol (HTTP). The

Servlet API set abstracts a common solution framework for responding to

HTTP requests.

Traditionally, the way to handle a problem such as allowing an Internet

client to update a database is to create an HTML page with text fields and

a "submit" button. The user types the appropriate information into the

text fields and presses the "submit" button. The data is submitted along

with a URL that tells the server what to do with the data by specifying the

location of a Common Gateway Interface (CGI) program that the server

runs, providing the program with the data as it is invoked. The CGI

program is typically written in Perl, Python, C, C++, or any language that

can read from standard input and write to standard output. That's all that

is provided by the Web server: the CGI program is invoked, and standard

streams (or, optionally for input, an environment variable) are used for

input and output. The CGI program is responsible for everything else.

First it looks at the data and decides whether the format is correct. If not,

the CGI program must produce HTML to describe the problem; this page

is handed to the Web server (via standard output from the CGI program),

which sends it back to the user. The user must usually back up a page and

try again. If the data is correct, the CGI program processes the data in an

appropriate way, perhaps adding it to a database. It must then produce an

appropriate HTML page for the Web server to return to the user.

It would be ideal to go to a completely Java-based solution to this

problem--an applet on the client side to validate and send the data, and a

servlet on the server side to receive and process the data. Unfortunately,

although applets are a proven technology with plenty of support, they

have been problematic to use on the Web because you cannot rely on a

3 Dave Bartlett was instrumental in the development of this material, and also the JSP

section.

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Thinking in Java

particular version of Java being available on a client's Web browser; in

fact, you can't rely on a Web browser supporting Java at all! In an

intranet, you can require that certain support be available, which allows a

lot more flexibility in what you can do, but on the Web the safest approach

is to handle all the processing on the server side and deliver plain HTML

to the client. That way, no client will be denied the use of your site

because they do not have the proper software installed.

Because servlets provide an excellent solution for server-side

programming support, they are one of the most popular reasons for

moving to Java. Not only do they provide a framework that replaces CGI

programming (and eliminates a number of thorny CGI problems), but all

your code has the platform portability gained from using Java, and you

have access to all the Java APIs (except, of course, the ones that produce

GUIs, like Swing).

The basic servlet

The architecture of the servlet API is that of a classic service provider with

a service( ) method through which all client requests will be sent by the

servlet container software, and life cycle methods init( ) and destroy( ),

which are called only when the servlet is loaded and unloaded (this

happens rarely).

public interface Servlet {

public void init(ServletConfig config)

throws ServletException;

public ServletConfig getServletConfig();

public void service(ServletRequest req,

ServletResponse res)

throws ServletException, IOException;

public String getServletInfo();

public void destroy();

}

getServletConfig( )'s sole purpose is to return a ServletConfig object

that contains initialization and startup parameters for this servlet.

getServletInfo( ) returns a string containing information about the

servlet, such as author, version, and copyright.

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The GenericServlet class is a shell implementation of this interface and

is typically not used. The HttpServlet class is an extension of

GenericServlet and is designed specifically to handle the HTTP

protocol-- HttpServlet is the one that you'll use most of the time.

The most convenient attribute of the servlet API is the auxiliary objects

that come along with the HttpServlet class to support it. If you look at the

service( ) method in the Servlet interface, you'll see it has two

parameters: ServletRequest and ServletResponse. With the

HttpServlet class these two object are extended for HTTP:

HttpServletRequest and HttpServletResponse. Here's a simple

example that shows the use of HttpServletResponse:

//: c15:servlets:ServletsRule.java

import javax.servlet.*;

import javax.servlet.http.*;

import java.io.*;

public class ServletsRule extends HttpServlet {

int i = 0; // Servlet "persistence"

public void service(HttpServletRequest req,

HttpServletResponse res) throws IOException {

res.setContentType("text/html");

PrintWriter out = res.getWriter();

out.print("<HEAD><TITLE>");

out.print("A server-side strategy");

out.print("</TITLE></HEAD><BODY>");

out.print("<h1>Servlets Rule! " + i++);

out.print("</h1></BODY>");

out.close();

}

} ///:~

ServletsRule is about as simple as a servlet can get. The servlet is

initialized only once by calling its init( ) method, on loading the servlet

after the servlet container is first booted up. When a client makes a

request to a URL that happens to represent a servlet, the servlet container

intercepts this request and makes a call to the service( ) method, after

setting up the HttpServletRequest and HttpServletResponse

objects.

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Thinking in Java

The main responsibility of the service( ) method is to interact with the

HTTP request that the client has sent, and to build an HTTP response

based on the attributes contained within the request. ServletsRule only

manipulates the response object without looking at what the client may

have sent.

After setting the content type of the response (which must always be done

before the Writer or OutputStream is procured), the getWriter( )

method of the response object produces a PrintWriter object, which is

used for writing character-based response data (alternatively,

getOutputStream( ) produces an OutputStream, used for binary

response, which is only utilized in more specialized solutions).

The rest of the program simply sends HTML back to the client (it's

assumed you understand HTML, so that part is not explained) as a

sequence of Strings. However, notice the inclusion of the "hit counter"

represented by the variable i. This is automatically converted to a String

in the print( ) statement.

When you run the program, you'll notice that the value of i is retained

between requests to the servlet. This is an essential property of servlets:

since only one servlet of a particular class is loaded into the container, and

it is never unloaded (unless the servlet container is terminated, which is

something that only normally happens if you reboot the server computer),

any fields of that servlet class effectively become persistent objects! This

means that you can effortlessly maintain values between servlet requests,

whereas with CGI you had to write values to disk in order to preserve

them, which required a fair amount of fooling around to get it right, and

resulted in a non-cross-platform solution.

Of course, sometimes the Web server, and thus the servlet container,

must be rebooted as part of maintenance or during a power failure. To

avoid losing any persistent information, the servlet's init( ) and

destroy( ) methods are automatically called whenever the servlet is

loaded or unloaded, giving you the opportunity to save data during

shutdown, and restore it after rebooting. The servlet container calls the

destroy( ) method as it is terminating itself, so you always get an

opportunity to save valuable data as long as the server machine is

configured in an intelligent way.

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There's one other issue when using HttpServlet. This class provides

doGet( ) and doPost( ) methods that differentiate between a CGI "GET"

submission from the client, and a CGI "POST." GET and POST vary only

in the details of the way that they submit the data, which is something

that I personally would prefer to ignore. However, most published

information that I've seen seems to favor the creation of separate

doGet( ) and doPost( ) methods instead of a single generic service( )

method, which handles both cases. This favoritism seems quite common,

but I've never seen it explained in a fashion that leads me to believe that

it's anything more than inertia from CGI programmers who are used to

paying attention to whether a GET or POST is being used. So in the spirit

of "doing the simplest thing that could possibly work,"4 I will just use the

service( ) method in these examples, and let it care about GETs vs.

POSTs. However, keep in mind that I might have missed something and

so there may in fact be a good reason to use doGet( ) and doPost( )

instead.

Whenever a form is submitted to a servlet, the HttpServletRequest

comes preloaded with all the form data, stored as key-value pairs. If you

know the names of the fields, you can just use them directly with the

getParameter( ) method to look up the values. You can also get an

Enumeration (the old form of the Iterator) to the field names, as is

shown in the following example. This example also demonstrates how a

single servlet can be used to produce the page that contains the form, and

to respond to the page (a better solution will be seen later, with JSPs). If

the Enumeration is empty, there are no fields; this means no form was

submitted. In this case, the form is produced, and the submit button will

re-call the same servlet. If fields do exist, however, they are displayed.

//: c15:servlets:EchoForm.java

// Dumps the name-value pairs of any HTML form

import javax.servlet.*;

import javax.servlet.http.*;

import java.io.*;

import java.util.*;

4 A primary tenet of Extreme Programming (XP). See www.xprogramming.com.

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Thinking in Java

public class EchoForm extends HttpServlet {

public void service(HttpServletRequest req,

HttpServletResponse res) throws IOException {

res.setContentType("text/html");

PrintWriter out = res.getWriter();

Enumeration flds = req.getParameterNames();

if(!flds.hasMoreElements()) {

// No form submitted -- create one:

out.print("<html>");

out.print("<form method=\"POST\"" +

" action=\"EchoForm\">");

for(int i = 0; i < 10; i++)

out.print("<b>Field" + i + "</b> " +

"<input type=\"text\""+

" size=\"20\" name=\"Field" + i +

"\" value=\"Value" + i + "\"><br>");

out.print("<INPUT TYPE=submit name=submit"+

" Value=\"Submit\"></form></html>");

} else {

out.print("<h1>Your form contained:</h1>");

while(flds.hasMoreElements()) {

String field= (String)flds.nextElement();

String value= req.getParameter(field);

out.print(field + " = " + value+ "<br>");

}

out.close();

}

} ///:~

One drawback you'll notice here is that Java does not seem to be designed

with string processing in mind--the formatting of the return page is

painful because of line breaks, escaping quote marks, and the "+" signs

necessary to build String objects. With a larger HTML page it becomes

unreasonable to code it directly into Java. One solution is to keep the page

as a separate text file, then open it and hand it to the Web server. If you

have to perform any kind of substitution to the contents of the page, it's

not much better since Java has treated string processing so poorly. In

these cases you're probably better off using a more appropriate solution

(Python would be my choice; there's a version that embeds itself in Java

called JPython) to generate the response page.

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Servlets and multithreading

The servlet container has a pool of threads that it will dispatch to handle

client requests. It is quite likely that two clients arriving at the same time

could be processing through your service( ) at the same time. Therefore

the service( ) method must written in a thread-safe manner. Any access

to common resources (files, databases) will need to be guarded by using

the synchronized keyword.

The following simple example puts a synchronized clause around the

thread's sleep( ) method. This will block all other threads until the