1. RequestDispatcher 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.
2. 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.
3. This interface is
intended to wrap servlets, but a servlet container can create
RequestDispatcher objects
to wrap any type of resource.
Method Summary
void forward(ServletRequest request, ServletResponse
response)
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 sub classes of the ServletRequestWrapper
or ServletResponseWrapper classes
that wrap them.
packagecom.jsl.servlet;
packagecom.jsl.servlet;
importjava.io.IOException;
importjavax.servlet.*;
importjavax.servlet.annotation.WebServlet;
importjavax.servlet.http.*;
@WebServlet("/login")
public class LoginServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.sendRedirect("index.html");
request.getRequestDispatcher("/welcome").forward(request,response);
}
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
}
}
In
this example the response has been committed, We are trying to forward to
another servlet, It throws the IllegalStateException
Login.html
<!DOCTYPEHTMLPUBLIC"-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
</html>
LoginServlet.java
packagecom.jsl.servlet;
importjava.io.IOException;
importjava.io.PrintWriter;
importjavax.servlet.*;
importjavax.servlet.annotation.WebServlet;
importjavax.servlet.http.*;
@WebServlet("/login")
publicclassLoginServletextendsHttpServlet {
privatestaticfinallongserialVersionUID = 1L;
protectedvoiddoGet(HttpServletRequest request, HttpServletResponse response) throwsServletException, IOException {
RequestDispatcherrd=null;
String email=request.getParameter("email");
String password=request.getParameter("password");
PrintWriter out=response.getWriter();
response.setContentType("text/html");
out.print("");
out.print("<html><head>Login Page </head>");
out.print("<body>");
if(email.equals(" ") &&password.equals(" ")){
out.print("Please verify user name and password");
rd=request.getRequestDispatcher("login.html");
rd.include(request, response);
}else{
if(email.equals("admin@jsltech.com") &&password.equals("admin")){
rd=request.getRequestDispatcher("welcome");
rd.forward(request, response);
}else{
out.print("Please verify user name and password");
rd=request.getRequestDispatcher("login.html");
rd.include(request, response);
}
}
out.print("</body>");
out.print("</html>");
}
protectedvoiddoPost(HttpServletRequest request, HttpServletResponse response) throwsServletException, IOException {
doGet(request, response);
}
}
Welcome.java
packagecom.jsl.servlet;
importjava.io.IOException;
importjava.io.PrintWriter;
importjavax.servlet.ServletException;
importjavax.servlet.annotation.WebServlet;
importjavax.servlet.http.HttpServlet;
importjavax.servlet.http.HttpServletRequest;
importjavax.servlet.http.HttpServletResponse;
@WebServlet("/welcome")
publicclass Welcome extendsHttpServlet {
privatestaticfinallongserialVersionUID = 1L;
protectedvoiddoPost(HttpServletRequest request, HttpServletResponse response) throwsServletException, IOException {
PrintWriter out=response.getWriter();
response.setContentType("text/html");
out.print("");
out.print("<html><head>Login Page </head>");
out.print("<body>");
out.print("
Welcome to JSL Tech
");
out.print("Login Page");
out.print("</body>");
out.print("</html>");
}
}
When click on
http://localhost:8080/Servlet_Example/login.html
If user name or password wrong, then along with the
error message the login page will be included
void include(ServletRequest request, ServletResponse response)
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 theServletRequestWrapper or ServletResponseWrapper classes that wrap them.
What is the difference between the
include() and forward() methods?
include()
|
forward()
|
The RequestDispatcherinclude() method
inserts the the contents of the specified resource directly in the flow of
the servlet response, as if it were part of the calling servlet.
|
The RequestDispatcherforward() method
is used to show a different resource in place of the servlet that was
originally called.
|
If you include a servlet or JSP
document, the included resource must not attempt to change the response
status code or HTTP headers, any such request will be ignored.
|
The forwarded resource may be
another servlet, JSP or static HTML document, but the response is issued
under the same URL that was originally requested. In other words, it is not
the same as a redirection.
|
The include() method
is often used to include common "boilerplate" text or template
markup that may be included by many servlets.
|
The forward() method
is often used where a servlet is taking a controller role; processing some
input and deciding the outcome by returning a particular response page.
|
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