JAVA EE: Servlets : RequestDispatcher Example

Thursday, 17 April 2014

Servlets : RequestDispatcher Example

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What is RequestDispatcher in Java Servlets?

The RequestDispatcher interface is an essential component of the javax.servlet package. It acts as a server-side traffic controller, allowing you to pass an incoming client request from one servlet to another web resource, such as another servlet, a JSP file, or an HTML file, within the same web application.

Unlike client-side redirection (sendRedirect), which asks the browser to initiate a new request, RequestDispatcher operates entirely on the server. This keeps the URL unchanged in the user's browser and preserves the original request and response objects throughout the process.

Key Methods of RequestDispatcher

The RequestDispatcher interface provides two primary methods to transfer or combine resources:

forward(ServletRequest request, ServletResponse response): Forwards a request from one servlet to another resource. The second resource takes over generating the complete response to the user.
include(ServletRequest request, ServletResponse response): Includes the content of another resource (such as a shared header, banner, or footer) directly inside the calling servlet's response.

Real-World Use Case: User Login Flow

A classic, practical example of using RequestDispatcher is an authentication workflow:

A user enters their username and password on a web login form.
LoginServlet receives the credentials and validates them against the database.
If authentication succeeds: LoginServlet forwards control to WelcomeServlet using dispatcher.forward(request, response) to render the user's dashboard.
If authentication fails: LoginServlet includes an error message and redisplays the login form using dispatcher.include(request, response).

Recommended Related Tutorials

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RequestDispatcherDemo Project Dir Structure

LoginServlet.java

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class LoginServlet extends HttpServlet
{

 private static final long serialVersionUID = 1L;

 public void init() throws ServletException
 {
  System.out
          .println("------------------------------------------------------");
  System.out.println(" Init method is called in "
          + this.getClass().getName());
  System.out
          .println("------------------------------------------------------");
 }

 public void doPost(HttpServletRequest request, HttpServletResponse response)
         throws ServletException, IOException
 {
  response.setContentType("text/html");
  PrintWriter out = response.getWriter();

  String userName = request.getParameter("username");
  String passWord = request.getParameter("password");

  if (userName.equalsIgnoreCase("ram") && passWord.equals("ram"))
  {
   RequestDispatcher requestDispatcher = request
           .getRequestDispatcher("/welcome");
   requestDispatcher.forward(request, response);
  }
  else
  {
   out.print("Sorry UserName or Password is Wrong!");
   RequestDispatcher requestDispatcher = request
           .getRequestDispatcher("/index.html");
   requestDispatcher.include(request, response);
  }
 }

 public void destroy()
 {
  System.out
          .println("------------------------------------------------------");
  System.out.println(" destroy method is called in "
          + this.getClass().getName());
  System.out
          .println("------------------------------------------------------");
 }
}

WelcomeServlet.java

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class WelcomeServlet extends HttpServlet
{

 private static final long serialVersionUID = 1L;

 public void init() throws ServletException
 {
  System.out
          .println("------------------------------------------------------");
  System.out.println(" Init method is called in "
          + this.getClass().getName());
  System.out
          .println("------------------------------------------------------");
 }

 public void doPost(HttpServletRequest request, HttpServletResponse response)
         throws ServletException, IOException
 {

  response.setContentType("text/html");
  PrintWriter out = response.getWriter();

  String userName = request.getParameter("username");
  out.print("Welcome " + userName);
 }

 public void destroy()
 {
  System.out
          .println("------------------------------------------------------");
  System.out.println(" destroy method is called in "
          + this.getClass().getName());
  System.out
          .println("------------------------------------------------------");
 }
}

web.xml

<?xml version="1.0" encoding="ISO-8859-1"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xmlns="http://java.sun.com/xml/ns/javaee"
 xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
 metadata-complete="true" version="3.0">
 <display-name>RequestDispatcherDemo</display-name>
 <description>
        This is a simple web application with a source code organization
        based on the recommendations of the Application Developer's Guide.
    </description>
 
 <servlet>
  <servlet-name>LoginServlet</servlet-name>
  <servlet-class>LoginServlet</servlet-class>
 </servlet>
 <servlet>
  <servlet-name>WelcomeServlet</servlet-name>
  <servlet-class>WelcomeServlet</servlet-class>
 </servlet>


 <servlet-mapping>
  <servlet-name>LoginServlet</servlet-name>
  <url-pattern>/login</url-pattern>
 </servlet-mapping>
 <servlet-mapping>
  <servlet-name>WelcomeServlet</servlet-name>
  <url-pattern>/welcome</url-pattern>
 </servlet-mapping>
 
</web-app>

index.html

<!DOCTYPE HTML>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>RequestDispatcherDemo</title>
</head>
<body>
 <form action="login" method="post">
  
  UserName :  <input type="text" name="username" /><br /> 
  
  Password :&nbsp; &nbsp; <input type="password" name="password" /><br /> 
   
  <input type="submit" value="login" />
  
 </form>
</body>
</html>

Environment Used

JDK version :1.6.0_30
Tomcat version : 7.0.50

To Download RequestDispatcherDemoApp Project Click the below link
https://sites.google.com/site/javaee4321/servlets/RequestDispatcherDemoApp.zip?attredirects=0&d=1

See also:

Servlets Tutorial

All Design Patterns Links