RequestDispatcher in Servlet
Defines an object that receives
requests from the client and sends them to any resource (such as a servlet,
HTML file, or JSP file) on the server. The servlet container creates the RequestDispatcher object, which is used as a wrapper around
a server resource located at a particular path or given by a particular name.This interface is intended to wrap servlets, but a servlet container can create
RequestDispatcher objects to wrap any type of resource.
Version:
$Version$
Author:
Various
See Also:
ServletContext.getRequestDispatcher(java.lang.String),ServletContext.getNamedDispatcher(java.lang.String),ServletRequest.getRequestDispatcher(java.lang.String)|
Method Summary
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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. |
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Method Detail
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forward
void forward(ServletRequest request,
ServletResponse response)
throws ServletException,
java.io.IOException
Forwards a request from a servlet to another
resource (servlet, JSP file, or HTML file) on the server. This method allows
one servlet to do preliminary processing of a request and another resource to
generate the response.
For a
RequestDispatcher obtained via getRequestDispatcher(), the ServletRequest object has its path elements and
parameters adjusted to match the path of the target resource.forward should be called before the response has
been committed to the client (before response body output has been flushed). If
the response already has been committed, this method throws an IllegalStateException. Uncommitted output in the response
buffer is automatically cleared before the forward.
The
request and response parameters must be either the same objects as were passed
to the calling servlet's service method or be subclasses of the
ServletRequestWrapper or ServletResponseWrapper classes that wrap them.
Parameters:
Throws:
java.io.IOException - if the target resource throws this exceptionjava.lang.IllegalStateException - if the response was already committed
include
void include(ServletRequest request,
ServletResponse response)
throws ServletException,
java.io.IOException
Includes the content of a resource (servlet, JSP
page, HTML file) in the response. In essence, this method enables programmatic
server-side includes.
The
ServletResponse object has its path elements and
parameters remain unchanged from the caller's. The included servlet cannot
change the response status code or set headers; any attempt to make a change is
ignored.
The
request and response parameters must be either the same objects as were passed
to the calling servlet's service method or be subclasses of the
ServletRequestWrapper or ServletResponseWrapper classes that wrap them.
Parameters:
Throws:
java.io.IOException - if the included resource throws this exception
As you see in the above figure,
response of second servlet is sent to the client. Response of the first servlet
is not displayed to the user.
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As
you can see in the above figure, response of second servlet is included in
the response of the first servlet that is being sent to the client.
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How to
get the object of RequestDispatcher
The getRequestDispatcher()
method of ServletRequest interface returns the object of RequestDispatcher.
Syntax:
Syntax of
getRequestDispatcher method
1. public RequestDispatcher getRequestDispatcher(String resource);
Example of using
getRequestDispatcher method
1. RequestDispatcher rd=request.getRequestDispatcher("servlet2");
2. //servlet2 is the url-pattern of the second servlet
3.
4. rd.forward(request, response);//method may be include or forward
Example of
RequestDispatcher interface
In this example, we are
validating the password entered by the user. If password is servlet, it will
forward the request to the WelcomeServlet, otherwise will show an error
message: sorry username or password error!. In this program, we are cheking for
hardcoded information. But you can check it to the database also that we will
see in the development chapter. In this example, we have created following
files:
- index.html
file: for getting
input from the user.
- Login.java
file: a servlet
class for processing the response. If password is servet, it will forward
the request to the welcome servlet.
- WelcomeServlet.java
file: a servlet
class for displaying the welcome message.
- web.xml
file: a deployment
descriptor file that contains the information about the servlet.
index.html
1. <form action="servlet1" method="post">
2. Name:<input type="text" name="userName"/><br/>
3. Password:<input type="password" name="userPass"/><br/>
4. <input type="submit" value="login"/>
5. </form>
Login.java
1. import java.io.*;
2. import javax.servlet.*;
3. import javax.servlet.http.*;
4.
5.
6. public class Login extends HttpServlet {
7.
8. public void doPost(HttpServletRequest request, HttpServletResponse response)
9. throws ServletException, IOException {
10.
11. response.setContentType("text/html");
12. PrintWriter out = response.getWriter();
13.
14. String n=request.getParameter("userName");
15. String p=request.getParameter("userPass");
16.
17. if(p.equals("servlet"){
18. RequestDispatcher rd=request.getRequestDispatcher("servlet2");
19. rd.forward(request, response);
20. }
21. else{
22. out.print("Sorry UserName or Password Error!");
23. RequestDispatcher rd=request.getRequestDispatcher("/index.html");
24. rd.include(request, response);
25.
26. }
27. }
28.
29. }
WelcomeServlet.java
1. import java.io.*;
2. import javax.servlet.*;
3. import javax.servlet.http.*;
4.
5. public class WelcomeServlet extends HttpServlet {
6.
7. public void doPost(HttpServletRequest request, HttpServletResponse response)
8. throws ServletException, IOException {
9.
10. response.setContentType("text/html");
11. PrintWriter out = response.getWriter();
12.
13. String n=request.getParameter("userName");
14. out.print("Welcome "+n);
15. }
16.
17. }
web.xml
1. <web-app>
2. <servlet>
3. <servlet-name>Login</servlet-name>
4. <servlet-class>Login</servlet-class>
5. </servlet>
6. <servlet>
7. <servlet-name>WelcomeServlet</servlet-name>
8. <servlet-class>WelcomeServlet</servlet-class>
9. </servlet>
10.
11.
12. <servlet-mapping>
13. <servlet-name>Login</servlet-name>
14. <url-pattern>/servlet1</url-pattern>
15. </servlet-mapping>
16. <servlet-mapping>
17. <servlet-name>WelcomeServlet</servlet-name>
18. <url-pattern>/servlet2</url-pattern>
19. </servlet-mapping>
20.
21. <welcome-file-list>
22. <welcome-file>index.html</welcome-file>
23. </welcome-file-list>
24. </web-app>
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