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376

Chapter 15 Integrating Servlets and JSP

pages, or even static HTML) can send output to the client, they should not

try to set HTTP response headers. Here is an example: 

response.setContentType("text/html");

PrintWriter out = response.getWriter();

out.println("...");

RequestDispatcher dispatcher =

getServletContext().getRequestDispatcher("/path/resource");

dispatcher.include(request, response);

out.println("...");

The 

include

 method has many of the same features as the 

forward

method. If the original method uses 

POST

, so does the forwarded request.

Whatever request data was associated with the original request is also associ 

ated with the auxiliary request, and you can add new parameters (in version

2.2 only) by appending them to the URL supplied to 

getRequestDis 

patcher

. Also supported in version 2.2 is the ability to get a 

RequestDis 

patcher

 by name (

getNamedDispatcher

) or by using a relative URL (use

the 

getRequestDispatcher

 method of the 

HttpServletRequest

); see Sec 

tion 15.1 (Forwarding Requests) for details. However, 

include

 does one

thing that 

forward

 does not: it automatically sets up attributes in the 

Http 

ServletRequest

 object that describe the original request path in case the

included servlet or JSP page needs that information. These attributes,

available to the included resource by calling 

getAttribute

 on the 

Http 

ServletRequest

, are listed below:

javax.servlet.include.request_uri

javax.servlet.include.context_path

javax.servlet.include.servlet_path

javax.servlet.include.path_info

javax.servlet.include.query_string

Note that this type of file inclusion is not the same as the nonstandard

servlet chaining supported as an extension by several early servlet engines.

With servlet chaining, each servlet in a series of requests can see (and mod 

ify) the output of the servlet before it. With the 

include

 method of

RequestDispatcher

, the included resource cannot see the output generated

by the original servlet. In fact, there is no standard construct in the servlet

specification that reproduces the behavior of servlet chaining.

Second edition of this book: www.coreservlets.com; Sequel: www.moreservlets.com.

Servlet and JSP training courses by book's author: courses.coreservlets.com.