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Web Design & Development ­ CS506
VU
Lesson 22
Serialization
What?
. You want to send an object to a stream.
Motivation
A lot of code involves boring conversion from a file to memory
As you might recall that AddressBook program reads data from file and then parses it
This is a common problem
Revisiting AddressBook
We read record from a text file named persons.txt. The person record was present in the file in the
following format.
persons.txt
The code that was used to construct Person objects after reading information from the file is given below.
Here only the part of code is shown, for complete listing, see AddressBook code in your earlier handout.
FileReader fr = new FileReader("persons.txt");BufferedReader br = new
BufferedReader(fr);
String line = br.readLine();
while ( line != null ) {
tokens = line.split(",");
name = tokens[0];
add = tokens[1];
ph = tokens[2];
PersonInfo p = new PersonInfo(name, add, ph); // you can add p into arraylist, if
needed line = br.readLine();
}
As you have seen a lot of parsing code is required for converting a line into PersonInfo objects.
Serialization mechanism eases developer's life by achieving all above in a very simple way.
Serialization in Java
Java provides an extensive support for serialization
Object knows how to read or write themselves to streams
Problem:
As you know, objects get created on heap and have some values therefore Objects have some state
in memory
You need to save and restore that state.
The good news is that java serialization takes care of it automatically
Serializable Interface
By implementing this interface a class declares that it is willing to be read/written by automatic
serialization machinery
Found in java.iopackage
Tagging interface ­ has no methods and serves only to identify the semantics of being serializable
Automatic Writing
System knows how to recursively write out the state of an object to stream
If an object has the reference of another object, the java serialization mechanism takes care of it
and writes it too.
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Automatic Reading
System knows how to read the data from Stream and re-create object in memory
The recreated object is of type "Object" therefore Down-casting is required to convert it into actual
type.
Serialization: How it works?
To write an object of PersonInfo, ObejctOutputStream and its method writeObject( ) will be used
PersonInfo p = new PersonInfo( );
ObejctOutputStream out;
// writing PersonInfo's object p
out.writeObject(p);
To read that object back, ObejctInputStream and its method readObject()will be used
ObejctInputStream in;
// reading PersonInfo's object. Remember type casting// is required
PersonInfo obj = (PersonInfo)in.readObject( );
Example Code 22.1: Reading / Writing PersonInfo objects
We want to send PersonInfo object to stream. You have already seen this class number of times before.
Here it will also implement serializable interface.
PersonInfo.java
import javax.swing.*;
import java.io.*
class PersonInfo implements Serializable{
String name;
String address;
String phoneNum;
//parameterized constructorpublic PresonInfo(String n, String a, String p) {
name = n;
address = a;
phoneNm = p;
}
//method for displaying person record on GUIpublic void printPersonInfo( ) {
JOptionPane.showMessageDialog(null ,
"name: " + name + "address:" +address +
"phone no:" + phoneNum);
}
} // end class
WriteEx.java
The following class will serialize PersonInfo object to a file
import java.io*;
public class WriteEx{
public static void main(String args[ ]){
PersonInfo pWrite =new PersonInfo("ali", "defence", "9201211");
try {
// attaching FileOutput stream with "ali.dat"
FileOutputStream fos =new FileOutputStream("ali.dat");
// attaching ObjectOutput stream over FileOutput// stream
ObjectOutputStream out =new ObjectOutputStream(fos);
//serialization
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// writing object to `ali.dat'
out.writeObject(pWrite);
// closing streams
out.close();
fos.close();
} catch (Exception ex){
System.out.println(ex)
}
} // end class
ReadEx.java
The following class will read serialized object of PersonInfo from file i.e "ali.data"
import java.io*; public class ReadEx{ public
static void main(String args[ ]){ try { //
attaching FileInput stream with "ali.dat"
FileInputStream fin = new FileInputStream("ali.dat");
// attaching FileInput stream over ObjectInput stream
ObjectInputStream in = new ObjectInputStream(fis);
//de-serialization
// reading object from `ali.dat'
PersonInfo pRead = (PersoInfo)in.ReadObject( );
// calling printPersonInfo method to confirm that// object contains same set of values
before// serializatoion
pRead.printPersonInfo();
// closing streams
in.close();
fis.close();
} catch (Exception ex){
System.out.println(ex)
}
} // end class
Compile & Execute
After compilation, first run the WriteEx.java file and visit the "ali.dat" file. Then run ReadEx.java from
different command or same command prompt.
Object Serialization & Network
. You can read / write to a network using sockets
. All you need to do is attach your stream with socket rather than file
. The class version should be same on both sides (client & network) of the network
Example Code 22.2: Sending/Reading Objects to/from Network
We are going to use same PersonInfo class listed in example code 22.1. An object of PersonInfo class will
be sent by client on network using sockets and then be read by server from network.
Sending Objects over Network
The following class ClientWriteNetEx.java will send an object on network
import java.net.*;import java.io.*;import
javax.swing.*;
public class ClientWriteNetEx{ public static void
main(String args[]){ try { PersonInfo p = new
PersonInfo("ali", "defence", "9201211"); // create a
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communication socket
Socket s = new Socket("localhost", 2222);
// Get I/O streams
OutputStream is = s.getOutputStream();
// attaching ObjectOutput stream over Input stream
ObjectOutputStream oos= new ObjectOutputStream(is);
// writing object to network
oos.write(p);
// closing communication socket
s.close();
}catch(Exception ex){
System.out.println(ex);
}
} }// end class
Reading Objects over Network
The following class ServerReadNetEx.java will read an object of PersonInfo sent by client.
import java.net.*;import java.io.*;import
javax.swing.*;
public class ServerReadNetEx{
public static void main(String rgs[])
{
try
{
// create a server socket
ServerSocket ss = new ServerSocket(2222);
System.out.println("Server started...");
/* Loop back to the accept method of the serversocket and wait for a new connection request.
Soserver will continuously listen for requests
*/
while(true) {
// wait for incoming connection
Socket s = ss.accept();
System.out.println("connection request recieved");
// Get I/O streams
InputStream is = s.getInputStream();
// attaching ObjectOutput stream over Input stream
ObjectInputStream ois = new ObjectInputStream(is);
// read PersonInfo object from network
PersonInfo p = (PersonInfo)ois.read( );
p.printPersonInfo();
// closing communication socket
s.close();
} // end while
}catch(Exception ex){ System.out.println(ex); } } } // end class
Compile & Execute
After compiling both files, run ServerReadNetEx.java first, from the command prompt window. Open
another command prompt window and run ClientWriteNetEx.javafrom it.
The ClientWriteNetEx.java will send an Object of PersonInfo to ServerReadNetEx.java that displays that
object values in dialog box after reading it from network.
Preventing Serialization
Often there is no need to serialize sockets, streams & DB connections etc because they do no
represent the state of object, rather connections to external resources
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To do so, transient keyword is used to mark a field that should not be serialized
So we can mark them as,
o
transient Socket s;
o
transient OutputStream os;
o
transient Connecction con;
Transient fields are returned as nullon reading
Example Code 22 . 3: transient
Assume that we do not want to serialize phoneNum attribute of PersonInfo class, this can be done as shown
below
PersonInfo.java
import javax.swing.*;
import java.io.*
class PersonInfo implements Serializable{
String name;
String address;
transient String phoneNum;
public PresonInfo(String n, String a, String p) { name = n;address = a;phoneNm = p;
}
public void printPersonInfo( ) {
JOptionPane.showMessageDialog(null , "name: " + name + "address:" +address +
"phone no:" + phoneNum);
}
} // end class
References
Entire material for this handout is taken from the book JAVA A Lab Course by Umair Javed. This
material is available just for the use of VU students of the course Web Design and Development and not for
any other commercial purpose without the consent of author.
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