sendRedirect() vs. forward()

Forward( ) :   javax.Servlet.RequestDispatcher interface. 

•   RequestDispatcher.forward( ) works on the Server. 
•   The forward( ) works inside the WebContainer.
•   The forward( ) restricts you to redirect only to a resource in the same web-Application. 
•   After executing the forward( ), the control will return back to the same method from where the forward method was called. 
•   The forward( ) will redirect in the application server itself, it does'n come back to the client. 
•   The forward( ) is faster than Sendredirect( ) . 

To use the forward( ) of the requestDispatcher interface, the first thing to do is to obtain RequestDispatcher Object. The Servlet technology provides in three ways. 

1. By using the getRequestDispatcher( ) of the javax.Servlet.ServletContext interface , passing a String containing the path of the other resources, path is relative to the root of the ServletContext.

RequestDispatcher rd=request.getRequestDispatcher ("secondServlet");

Rd.forward(request, response); 

2.   getRequestDispatcher( ) of the javax.Servlet.Request interface  , the path is relative to current HtpRequest.

RequestDispatcher rd=getServletContext( ).getRequestDispatcher("servlet/secondServlet"); 

Rd.forward(request, response); 

3. By using the getNameDispatcher( ) of the javax.Servlet.ServletContext interface.

RequestDispatcher rd=getServletContext( ).getNameDispatcher("secondServlet"); 

Rd.forward(request, response); 

Sendredirect( ) : javax.Servlet.Http.HttpServletResponce interface 

• RequestDispatcher.SendRedirect( ) works on the browser. 
• The SendRedirect( ) allows you to redirect trip to the Client. 
• The SendRedirect( ) allows you to redirect to any URL. 
• After executing the SendRedirect( ) the control will not return back to same method. 
• The Client receives the Http response code 302 indicating that temporarly the client is being redirected to the specified location , if the specified location is relative , this method converts it into an absolute URL before redirecting. 
• The SendRedirect( ) will come to the Client and go back,.. ie URL appending will happen. 
• Response. SendRedirect( "absolute path"); 
• Absolutepath – other than application ,   relative path - same application.

When you invoke a forward request, the request is sent to another resource on the server, without the client being informed that a different resource is going to process the request. This process occurs completely with in the web container. When a sendRedirtect method is invoked, it causes the web container to return to the browser indicating that a new URL should be requested. Because the browser issues a completely new request any object that are stored as request attributes before the redirect occurs will be lost. This extra round trip a redirect is slower than forward. 

Code:

sendRedirect()

import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.util.*;
public class MyServlet extends HttpServlet {    
 public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, java.io.IOException {         
       //redirect the user depending on the value of the 'go' param
        String destination = getInitParameter("go");         String contextPath = request.getContextPath();         
         if(destination == null || destination.equals(""))             throw new ServletException(              "Missing or invalid 'go' parameter in " +                getClass().getName());         
        if(destination.equals("weather"))         //ensure URL rewriting
            response.sendRedirect(response.encodeRedirectURL(contextPath + "/weather") );         
         if(destination.equals("maps"))         //ensure URL rewriting
            response.sendRedirect(response.encodeRedirectURL(contextPath + "/maps") );     }
}

forward():

import java.io.IOException;
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 Main extends HttpServlet {
  protected void service(HttpServletRequest request, HttpServletResponse response)       throws ServletException, IOException {
    RequestDispatcher dispatcher = request.getRequestDispatcher("/p.jsp");
    dispatcher.forward(request, response);   }

}

(from http://www.sap-img.com/java/difference-between-response-sendredirect-and-request-forward.htm)