The Java EE 5 Tutorial (PDF) - Oracle Software Downloads

Accessing theWeb Context
Transferring Control to AnotherWeb Component
In some applications, you might want to have one web component do preliminary processing of
a request and have another component generate the response. For example, you might want to
partially process a request and then transfer to another component depending on the nature of
the request.
To transfer control to another web component, you invoke the forward method of a
RequestDispatcher. When a request is forwarded, the request URL is set to the path of the
forwarded page. The original URI and its constituent parts are saved as request attributes
javax.servlet.forward.[request-uri|context-path|servlet-path|path-info|query-string].
The
tut-install/javaeetutorial5/examples/web/bookstore2/src/java/com/sun/bookstore2/dispatcher/Di
servlet, used by a version of the Duke’s Bookstore application described in “The Example JSP
Pages” on page 224, saves the path information from the original URL, retrieves a
RequestDispatcher from the request, and then forwards to the JSP page,
tut-install/javaeetutorial5/examples/web/bookstore3/web/template/template.jsp.
public class Dispatcher extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response) {
RequestDispatcher dispatcher = request.
getRequestDispatcher("/template.jsp");
if (dispatcher != null)
dispatcher.forward(request, response);
}
public void doPost(HttpServletRequest request,
...
}
The forward method should be used to give another resource responsibility for replying to the
user. If you have already accessed a ServletOutputStream or PrintWriter object within the
servlet, you cannot use this method; doing so throws an IllegalStateException.
Accessing theWeb Context
124
The context in which web components execute is an object that implements the
ServletContext interface. You retrieve the web context using the getServletContext
method. The web context provides methods for accessing:
■ Initialization parameters
■ Resources associated with the web context
■ Object-valued attributes
■ Logging capabilities
The Java EE 5Tutorial • June 2010