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354

Chapter 15 Integrating Servlets and JSP

partially process the data, set up beans, then forward the results to one of a

number of different JSP pages, depending on the circumstances. In early JSP

specifications, this approach was known as the model 2 approach to JSP.

Rather than completely forwarding the request, the servlet can generate part

of the output itself, then include the output of one or more JSP pages to

obtain the final result.

15.1 Forwarding Requests

The key to letting servlets forward requests or include external content is to

use a 

RequestDispatcher

. You obtain a 

RequestDispatcher

 by calling the

getRequestDispatcher

 method of 

ServletContext

, supplying a URL rel 

ative to the server root. For example, to obtain a 

RequestDispatcher

 associ 

ated with 

http://yourhost/presentations/presentation1.jsp

, you

would do the following:

String url = "/presentations/presentation1.jsp";

RequestDispatcher dispatcher =

getServletContext().getRequestDispatcher(url);

Once you have a 

RequestDispatcher

, you use 

forward

 to completely

transfer control to the associated URL and use 

include

 to output the associ 

ated URL's content. In both cases, you supply the 

HttpServletRequest

 and

HttpServletResponse

 as arguments. Both methods throw 

Servlet 

Exception

 and 

IOException

. For example, Listing 15.1 shows a portion of a

servlet that forwards the request to one of three different JSP pages, depend 

ing on the value of the 

operation

 parameter. To avoid repeating the 

getRe 

questDispatcher

 call, I use a utility method called 

gotoPage

 that takes the

URL, the 

HttpServletRequest

 and the 

HttpServletResponse

; gets a

RequestDispatcher

; and then calls 

forward

 on it.

Using Static Resources

In most cases, you forward requests to a JSP page or another servlet. In some

cases, however, you might want to send the request to a static HTML page.

In an e commerce site, for example, requests that indicate that the user does

not have a valid account name might be forwarded to an account application

page that uses HTML forms to gather the requisite information. With 

GET

requests, forwarding requests to a static HTML page is perfectly legal and

requires no special syntax; just supply the address of the HTML page as the

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