SERVLET TECHNOLOGY

Introduction to Request Dispatcher

RequestDispatcher is an interface, implementation of which defines an object which can dispatch request to any resources(such as HTML, Image, JSP, Servlet) on the server.


Methods of RequestDispatcher

RequestDispatcher interface provides two important methods

MethodsDescription
void forward(ServletRequest request, ServletResponse response)forwards a request from a servlet to another resource (servlet, JSP file, or HTML file) on the server
void include(ServletRequest request, ServletResponse response) includes the content of a resource (servlet, JSP page, HTML file) in the response

How to get an Object of RequestDispatcher

getRequestDispatcher() method of ServletRequest returns the object of RequestDispatcher.

RequestDispatcher rs = request.getRequestDispatcher("hello.html");
rs.forward(request,response);

RequestDispatcher detail and instantiation using forward

OR

RequestDispatcher rs = request.getRequestDispatcher("hello.html");
rs.include(request,response);

RequestDispatcher detail and instantiation using include


Example demonstrating usage of RequestDispatcher

In this example, we will show you how RequestDispatcher is used to forward or include response of a resource in a Servlet. Here we are using index.html to get username and password from the user, Validate Servlet will validate the password entered by the user, if the user has entered "studytonight" as password, then he will be forwarded to Welcome Servlet else the user will stay on the index.html page and an error message will be displayed.

Files to be created :

  • index.html will have form fields to get user information.
  • Validate.java will validate the data entered by the user.
  • Welcome.java will be the welcome page.
  • web.xml , the deployment descriptor.

index.html

<form method="post" action="Validate">
Name:<input type="text" name="user" /><br/>
Password:<input type="password" name="pass" ><br/>
<input type="submit" value="submit">
</form>

Validate.java

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class Validate extends HttpServlet {

   protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
      response.setContentType("text/html;charset=UTF-8");
     PrintWriter out = response.getWriter();
     try {
         String name = request.getParameter("user");
        String password = request.getParameter("pass");

        if(password.equals("studytonight"))
        {
           RequestDispatcher rd = request.getRequestDispatcher("Welcome");
           rd.forward(request, response);
        }
        else
         {
         out.println("<font color='red'><b>You have entered incorrect password</b></font>");
                RequestDispatcher rd = request.getRequestDispatcher("index.html");
                rd.include(request, response);
            }
        }finally {            
            out.close();
        }
        
    }
}

Welcome.java

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class Welcome extends HttpServlet {

   protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        try {
            
            out.println("<h2>Welcome user</h2>");
        } finally {            
            out.close();
        }
    }
}

web.xml

<web-app>
    <servlet>
        <servlet-name>Validate</servlet-name>
        <servlet-class>Validate</servlet-class>
    </servlet>
    <servlet>
        <servlet-name>Welcome</servlet-name>
        <servlet-class>Welcome</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>Validate</servlet-name>
        <url-pattern>/Validate</url-pattern>
    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>Welcome</servlet-name>
        <url-pattern>/Welcome</url-pattern>
    </servlet-mapping>
    <welcome-file-list>
        <welcome-file>index.html</welcome-file>
        </welcome-file-list>
</web-app>

This will be the first screen. You can enter your Username and Password here.

request dispatcher example


When you click on Submit, Password will be validated, if it is not 'studytonight' , error message will be displayed.

request dispatcher example


Enter any Username, but enter 'studytonight' as password.

request dispatcher example


Password will be successfully validated and you will be directed to the Welcome Servlet.

request dispatcher example