|
Forwarding and Including Response from other Servlets
by Faisal Khan.
 Overview
If you are familiar with ASP 3.0 then you must be knowing about Server.Transfer
and Server.Execute methods. Server.Transfer forwards the
control to another ASP page while Server.Execute executes the given page
and after that gives the control back to the caller page. Servlet API also provides
us this functionality with the RequestDispatcher interface. This interface
has just two methods, forward() and include which do pretty
much what their names suggest.
In this article we will learn how to pass control from one Servlet to another using
RequestDispatcher.forward() method and how to include response from another
Servlet within the caller Servlet using RequestDispatcher.include() method.
RequestDispatcher Interface
This interface is present in the javax.servlet package and contains
only following two methods :
forward(ServletRequest request, ServletResponse response)
Forwards a request to another resource on the same server. That resource can
be a Servlet, JSP page or a simple HTML page.include(ServletRequest request, ServletResponse response)
Works like a server-side include ( SSI ) and includes the response from the given
resource ( Servlet, JSP page, HTML page ) within the caller response.
How to get a reference to RequestDispatcher Interface ?
In order to use forward() or include() methods we discussed
above we will have to get a reference to RequestDispatcher interface. There
are two ways you can do this :
ServletContext.getRequestDispatcher(String resource)ServletRequest.getRequestDispatcher(String resource)
If your Servlet is extending HttpServletRequest then you can simply call
getRequestDispatcher(String resource) to get reference to the RequestDispatcher
object for the given resource. This is what we will do in the demo
application later.
// req is HttpServletRequest object
RequestDispatcher rd;
rd = req.getRequestDispatcher("pathToServlet");
rd.forward(req, res);
Or you can use ServletContext's getRequestDispatcher(String resource)
to do the same.
RequestDispatcher rd;
rd = getServletContext().getRequestDispatcher("pathToServlet");
rd.forward(req, res);
Having learned the theory about including and forwarding response using
RequestDispatcher interface, it is time now that we build some demo
application and demonstrate the use of these methods. We will also be learning one
more thing which is setting and getting attributes from HttpServletRequest
object.
RequestDispatcher Demo Application
Our demo application will consist of two Servlets. First Servlet will call forward()
and include() methods to display response from second Servlet. To demonstrate the
use of HttpServletRequest attributes, we will set an attribute in the
first Servlet and display that attribute from within the second Servlet.
TestDispatcherServlet1
Create a new TestDispatcherServlet1.java file in CATALINA_HOME/webapps/star/WEB-INF/classes/com/stardeveloper/servlets/
folder. You can substitute CATALINA_HOME/webapps/star/ with the path of /WEB-INF/ folder of
any other application server if you want. Copy and paste the following code in it :
package com.stardeveloper.servlets;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class TestDispatcherServlet1 extends HttpServlet {
private static final String forwardTo
= "/servlet/com.stardeveloper.servlets." +
"TestDispatcherServlet2";
private static final String includeIn
= "/servlet/com.stardeveloper.servlets." +
"TestDispatcherServlet2";
public void doGet(HttpServletRequest req, HttpServletResponse res)
throws ServletException, IOException {
res.setContentType("text/html");
PrintWriter out = res.getWriter();
out.print("<html><head><style>");
out.print("p,form{font-family:tahoma;font-size:10pt;}");
out.print("input{width:20;height:20;}");
out.print("</style></head><body>");
// Displaying Form
out.print("<form action=\"");
out.print( req.getRequestURI() );
out.print("\" method=\"post\">");
out.print("<input type=\"hidden\" name=\"mode\" ");
out.print("value=\"forward\">");
out.print("<input type=\"submit\" value=\" \"");
out.print("> ");
out.print(" Forward to another Servlet ..");
out.print("</form>");
out.print("<form action=\"");
out.print( req.getRequestURI() );
out.print("\" method=\"post\">");
out.print("<input type=\"hidden\" name=\"mode\" ");
out.print("value=\"include\">");
out.print("<input type=\"submit\" ");
out.print("value=\" \"> ");
out.print(" Include another Servlet ..");
out.print("</form>");
out.print("</body></html>");
out.close();
}
public void doPost(HttpServletRequest req, HttpServletResponse res)
throws ServletException, IOException {
String mode = req.getParameter("mode");
// Forwarding to Servlet2
if(mode != null && mode.equals("forward")) {
req.setAttribute("mode", "Forwarding Response..");
req.getRequestDispatcher(forwardTo).forward(req, res);
}
// Including response from Servlet2
if(mode != null && mode.equals("include")) {
req.setAttribute("mode", "Including Response..");
req.getRequestDispatcher(includeIn).include(req, res);
}
}
}
Explanation
Our Servlet above extends HttpServlet class and overrides doGet()
and doPost() methods. We also declare two variables forwardTo
and includeIn which point to the path of the second Servlet. For this
example they both point to same Servlet.
| 
.NET
ASP (14) 
