The role of Tomcat server:

To create servlets, you will need access to a servlet development environment. The one used by this chapter is Tomcat. Tomcat is an open-source product maintained by the Jakarta Project of the Apache Software Foundation. It contains the class libraries, documentation, and runtime support that you will need to create and test servlets.

The examples in this chapter assume a Windows environment. The default location for Tomcat 5.5.17 is

C:\Program Files\Apache Software Foundation\Tomcat 5.5\

This is the location assumed by the examples in this book. If you load Tomcat in a different location, you will need to make appropriate changes to the examples. You may need to set the environmental variable JAVA_HOME to the top-level directory in which the Java Development Kit is installed.

To start Tomcat, select Configure Tomcat in the Start | Programs menu, and then press Start in the Tomcat Properties dialog.

When you are done testing servlets, you can stop Tomcat by pressing Stop in the Tomcat Properties dialog.

The directory

C:\Program Files\Apache Software Foundation\Tomcat 5.5\common\lib\

contains

servlet-api.jar

. This JAR file contains the classes and interfaces that are needed to build servlets. To make this file accessible, update your

CLASSPATH

environment variable so that it includes

C:\Program Files\Apache Software Foundation\Tomcat 5.5\common\lib\servlet-api.jar

Alternatively, you can specify this file when you compile the servlets. For example, the following command compiles the first servlet example:

javac HelloServlet.java -classpath

"C:\Program Files\Apache Software Foundation\Tomcat 5.5\common\lib\servlet-api.jar"

Once you have compiled a servlet, you must enable Tomcat to find it. This means putting it into a directory under Tomcat’s webapps directory and entering its name into a web.xml file. To keep things simple, the examples in this chapter use the directory and web.xml file that Tomcat supplies for its own example servlets. Here is the procedure that you will follow.

First, copy the servlet’s class file into the following directory:

C:\Program Files\Apache Software Foundation\Tomcat 5.5\webapps\servlets-examples\WEB-INF\classes

Next, add the servlet’s name and mapping to the web.xml file in the following directory:

C:\Program Files\Apache Software Foundation\Tomcat 5.5\webapps\servlets-examples\WEB-INF

For instance, assuming the first example, called

HelloServlet

, you will add the following lines in the section that defines the servlets:

<servlet>

<servlet-name>HelloServlet</servlet-name>

<servlet-class>HelloServlet</servlet-class>

</servlet>

Next, you will add the following lines to the section that defines the servlet mappings.

<servlet-mapping>

<servlet-name>HelloServlet</servlet-name>

<url-pattern>/servlet/HelloServlet</url-pattern>

</servlet-mapping>

There are five types of JSP Tags & are as follows:

Comment tag:

A comment tag opens with <%-- and closes with --%>, and is followed by a comment that usually describes the functionality of statements that follow the comment tag.

Declaration statement tags:

A declaration statement tag opens with <%! and is followed by a Java declaration statement(s) that define variables, objects, and methods that are available to other components of the JSP Program.

Directive Tag:

A directive tag opens with <%@ and commands the JSP virtual engine to perform a specific task, such as importing a Java package required by objects and methods used in a declaration statement. The directive tag closes with %>. There are three commonly used directives. These are

import, include, and taglib

. The

import tag

is used to import Java Program into the JSP Programs. The

include tag

inserts a specified file into the JSP program replacing the include tag. The taglib tag specifies a file that contains a tag library.

<%@ page import=" import java.sql.* "; %>

<%@ include file="keogh\books.html" %>

<%@ taglib uri="myTags.tld" %>

Expression Tag:

An Expression tag opens with <%= and is used for an expression statement whose result replaces the expression tag when the JSP virtual engine resolves JSP tags. An expression tags closes with %>.

Scriptlet tags:

A scriptlet tags opens with <% and contains commonly used Java control statements and loops. A scriptlet tag closes with %>.

The Cookie class encapsulates a cookie. A cookie is stored on a client and contains state information. Cookies are valuable for tracking user activities. For example, assume that a user visits an online store. A cookie can save the user’s name, address, and other information. The user does not need to enter this data each time he or she visits the store.

Three methods are central to the life cycle of a servlet. These are

init( )

,

service( )

, and

destroy( )

.

They are implemented by every servlet and are invoked at specific times by the server. Let us consider a typical user scenario to understand when these methods are called. First, assume that a user enters a Uniform Resource Locator (URL) to a web browser. The browser then generates an HTTP request for this URL. This request is then sent to the appropriate server.

Second, this HTTP request is received by the web server. The server maps this request to a particular servlet. The servlet is dynamically retrieved and loaded into the address space of the server.

Third, the server invokes the

init( )

method of the servlet. This method is invoked only when the servlet is first loaded into memory. It is possible to pass initialization parameters to the servlet so it may configure itself.

Fourth, the server invokes the

service( )

method of the servlet. This method is called to process the HTTP request. You will see that it is possible for the servlet to read data that has been provided in the HTTP request. It may also formulate an HTTP response for the client. The servlet remains in the server’s address space and is available to process any other HTTP requests received from clients. The

service()

method is called for each HTTP request.