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Web Design & Development ­ CS506
VU
Lesson 27
Creating a Simple
Web Application in Tomcat
In this handout, we'll discuss the standard tomcat directory structure, a pre-requisite for building any web
application. Different nuts and bolts of Servlets will also be discussed. In the later part of this handout,
we'll also learn how to make a simple web application using servlet.
Standard Directory Structure of a J2EE Web Application
A web application is defined as a hierarchy of directories and files in a standard layout. Such hierarchies
can be used in two forms
Unpack
o  Where each directory & file exists in the file system separately
o  Used mostly during development
Pack
Known as Web Archive (WAR) file
o
Mostly used to deploy web applications
o
The webapps folder is the top-level Tomcat directory that contains all the web applications deployed on the
server. Each application is deployed in a separate folder often referred as "context".
To make a new application e.g myapp in tomcat you need a specific folder hierarchy.
-Create a folder named myapp in C:\jakarta-tomcat-5.5.9\webapps folder. This name will also appear
in the URL for your application. For example
http://localhost:8080/myapp/index.html
-All JSP and html files will be kept in main application folder (C:\jakartatomcat-5.5.9\webapps\myapp)
-Create another folder inside myapp folder and change its name to WEB-INF. Remember WEB-INF is
case sensitive and it is not WEB_INF
-Configuration  files  such  as  web.xml
will
go
in
WEB-INF
folder
(C:\jakarta-tomcat-
5.5.9\webapps\myapp\WEB-INF)
-Create another folder inside WEB-INF folder and change its name to classes. Remember classes name
is also case sensitive.
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-Servlets and Java Beans will go in classes folder (C:\jakarta-tomcat5.5.9\webapps\myapp\WEB-
INF\classes)
That's the minimum directory structure required in order to get started. This is also shown in the figure
below:
-To test application hierarchy, make a simple html file e.g. index.html file. Write some basic HTML code
into it and save it in main application directory i.e.
C:\jakarta-tomcat-5.5.9\webapps\myapp\
-Restart the server and access it by using the URL
http://localhost:8080/myapp/index.html
-A more detailed view of the Tomcat standard directory structure is given below.
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-Here you can see some other folders like lib& tags under the WEB-INF.
-The lib folder is required if you want to use some achieve files (.jar). For example an API in jar format
that can help generating .pdf files.
-Similarly tags folder is helpful for building custom tags or for using .tagfiles.
Note: Restart Tomcat every time you create a new directory structure, a servlet or a java bean so that it
can recognize it. For JSP and html files you don't have to restart the server.
Writing Servlets
Servlet Types
-Servlet related classes are included in two main packages javax.servletand javax.servlet.http.
-Every servlet must implement the javax.servlet.Servlet interface, it contains the servlet's life cycle
methods etc. (Life cycle methods will be discussed in next handout)
-In order to write your own servlet, you can subclass from GernericServlet or HttpServlet
GenericServlet class
Available in javax.servlet package
Implements javax.servlet.Servlet
Extend your class from this class if you are interested in writing protocol independent servlets
HttpServlet class
Available in javax.servlet.http package
Extends from GenericServletclass
Adds functionality for writing HTTP specific servlets as compared to GernericServlet
Extend your class from HttpServlet, if you want to write HTTP based servlets
Servlet Class Hierarchy
The Servlet class hierarchy is given below. Like all java classes GenericServletalso inherits from Object
class. Apart from GenericServlet and HttpServlet classes, ServletRequest, HttpServletRequest,
ServeltResponse and HttpServletResponse are also helpful in writing a servlet.
As you can guess ServletRequest & ServletResponse are used in conjunction with GenericServlet. These
classes are used for processing protocol independent requests and generating protocol independent
responses respectively.
javax.servlet.http
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Types of HTTP requests
HTTP supports different types of request to be sent over to server. Each request has some specific purpose.
The most important ones are get & post. Given below a brief overview of each request type is given. You
can refer to RFC of HTTP for further details.
-GET: Requests a page from the server. This is the normal request used when browsing web pages.
-POST: This request is used to pass information to the server. Its most common use is with HTML forms.
-PUT: Used to put a new web page on a server.
-DELETE: Used to delete a web page from the server.
-OPTIONS: Intended for use with the web server, listing the supported options.
-TRACE: Used to trace servers
GET & POST, HTTP request types
Some details on GET and POST HTTP request types are given below.
. GET
-Attribute-Value pair is attached with requested URL after `?'.
-For  example  if  attribute  is  `name'  and  value  is  `ali'  then  the  request  will  be
http://www.gmail.com/register?name=ali
-For HTTP based servlet, override doGet() methods of HttpServlet class to handle these type of
requests.
. POST
-Attribute-Value pair attached within the request body. For your reference HTTP request diagram is
given below again:
-Override doPost() method of HttpServlet class to handle POST type requests.
Steps for making a Hello World Servlet
To get started we will make a customary "HelloWorldServlet". Let's see what are the steps involved in
writing a servlet that will produce "Hello World"
1. Create a directory structure for your application (i.e. helloapp). This is a one time process for any
application
2. Create a HelloWorldServlet source file by extending this class from HttpServlet and overriding
your desired method. For example doGet() or doPost().
3. Compile it (If get error of not having required packages, check your class path)
4. Place the class file of HelloWorldServlet in the classes folder of your web application (i.e. myapp).
Note:
If you are using packages then create a complete structure under classes folder
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5. Create a deployment descriptor (web.xml) and put it inside WEB-INF folder
6. Restart your server if already running
7. Access it using Web browser
Example Code: HelloWorldServlet.java
//File HelloWorldServlet.java
// importing required packages
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
// extending class from HttpServelt
public class HelloWorldServlet extends HttpServlet {
/* overriding doGet() method because writing a URL in the browserby default generate request of
GET type
As you can see, HttpServletRequest andHttpServletResponse are passed to this
method. Theseobjects will help in processing of HTTP request andgenerating
response for HTTP
This method can throw ServletException or IOException, so wemention these exception
types after method signature
*/
public void doGet(HttpServletRequest request,HttpServletResponse response)
throws ServletException, IOException
{
/* getting output stream i.e PrintWriter from responseobject by calling getWriter
method on it
As mentioned, for generating response, we will
useHttpServletResponse object*/
PrintWriter out = response.getWriter();
/* printing Hello World in the browser using PrintWriter
object. You can also write HTML like
out.println("<h1> Hello World </h1>") */
out.println("Hello World! ");
} // end doGet()
} // end HelloWorldServlet
Example Code: web.xml
eXtensible Markup Language (xml) contains custom defined tags which convey information about the
content. To learn more about XML visit http://ww.w3schools.com.
Inside web.xml, the <web-app> is the root tag representing the web application. All other tags come inside
of it.
<?xml version="1.0" encoding="ISO-8859-1"?>
<web-app>
<servlet> <servlet-name> HelloWorldServlet </servlet-name> <servlet-class>
HelloWorldServlet </servlet-class>
</servlet>
<servlet-mapping><servlet-name> HelloWorldServlet </servlet-
name><url-pattern> /myfirstservlet </url-
pattern></servlet-mapping> </web-app>
The <servlet> tag represents one's servlet name and its class. To specify the name of servlet, <servlet-
name> tag is used. Similarly to specify the class name of servlet (it is the same name you used for making
a servlet), <servlet-class>tag is used.
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Note: It is important to note here that you can specify any name for a servlet inside <servlet-name> tag.
This name is used for referring to servlet in later part of web.xml. You can think of it as your id assigned to
you by your university while you have actually different name (like <servlet-class>).
Next we will define the servlet mapping. By defining servlet mapping we are specifying URL to access a
servlet. <servlet-mapping> tag is used for this purpose.
Inside <servlet-mapping> tag, first you will write the name of the servlet for which you want to specify
the URL mapping using <servlet-name> tag and then you will define the URL pattern using <url-
pattern> tag. Notice the forward slash (/ ) is used before specifying the url. You can specify any name of
URL. The forward slash indicates the root of your application.
<url-pattern> /myfirstservlet </url-pattern>
Now you can access HelloWorldServelt (if it is placed in myapp application) by giving the following url in
the browser
http://localhost:8080/myapp/myfirstservlet
Note:
Save this web.xml file by placing double quotes("web.xml") around it as you did to save .java files.
Compiling and Invoking Servlets
-Compile HelloWorldServlet.java using javac command.
-Put HelloWorldServlet.class in C:\jakarta-tomcat5.5.9/webapps/myapp/WEB-INF/classes folder
-Put web.xml file in C:\jakarta-tomcat5.5.9/webapps/myapp/WEB-INF folder
-Invoke your servlet by writing following URL in web browser. Don't forget to restart your tomcat
server if already running
http://localhost:8080/myapp/myfirstservlet
Note:
By using IDEs like netBeans® 4.1, you don't have to write web.xml by yourself or even to worry
about creating directory structure and to copy files in appropriate locations. However manually
undergoing this process will strengthen your concepts and will help you to understand the
underlying mechanics.
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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