JAVA: Creating a Simple Web Application in Tomcat

<< Java Servlets

JAVA: Servlets Lifecycle >>

Web Design & Development CS506

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

Mostly used to deploy web applications

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

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.

198

Web Design & Development CS506

-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.

199

Web Design & Development CS506

-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

200

Web Design & Development CS506

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

201

Web Design & Development CS506

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.

202

Web Design & Development CS506

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.

203

Table of Contents: