Portlet Request Dispatcher in Java

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Portlet Request Dispatcher
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Your portlet can use a portlet request dispatcher to include the content from a servlet or JSP page. The portlet request dispatcher translates the portlet s render request and render response into servlet requests and responses. Then the portlet request dispatcher passes those servlet objects to the appropriate servlet or JSP resource. The resource processes the render request as if the request was an HttpServletRequest and adds its content to the portlet s render response. Each portlet has access to a portlet request dispatcher through the portlet s PortletContext object. The portlet request dispatcher is an object the portlet container creates that implements the PortletRequestDispatcher interface. Here are the two methods for retrieving a PortletRequestDispatcher object from the PortletContext object:
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public PortletRequestDispatcher getNamedDispatcher(String name) public PortletRequestDispatcher getRequestDispatcher(String path)
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Each of these methods retrieves a portlet request dispatcher for a servlet or JSP The difference is how the resource is found in the web application. The . getNamedDispatcher() method is used to get access to a servlet or JSP that is given a name in the web application deployment descriptor. It is also possible to name a servlet from your application server s administration tool; this is dependent on the application server used. The getRequestDispatcher() method is used to access a resource relative to the portlet s context root. The path argument must start with a / , and must be a valid path. For each of these methods, if the path or name is invalid, the methods will return null. The PortletRequestDispatcher object is very similar to the RequestDispatcher object from the servlet API. The major difference is that servlets may either include another servlet or forward a request, while portlets may only include another servlet s response. A portlet may not forward a request to a servlet, because that means that control would not return to the portlet. The portlet remains in control when it includes a servlet or a JSP When the servlet (or JSP) is finished writing output, con. trol passes back to the portlet that included the servlet. There is only one method on the PortletRequestDispatcher object:
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public void include(RenderRequest request, RenderResponse response) throws PortletException, java.io.IOException
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The include() method hides all of the details of loading and processing the servlet or JSP page, just like in the servlet API.
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Request Dispatcher
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A portlet may use a portlet request dispatcher to include the output of either a servlet or a JSP page. This example shows how to load a JSP page called homePage.jsp from the WEB-INF/jsp directory of your portlet application:
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PortletContext portletContext = getPortletContext(); PortletRequestDispatcher prd = PortletContext.getRequestDispatcher("/jsp/homePage.jsp"); prd.include(request,response);
The include() method on the PortletRequestDispatcher object throws a PortletException or an IOException. You may pass a query string on the path used for the getRequestDispatcher() method. For instance, our previous example could look like this:
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PortletContext portletContext = getPortletContext(); PortletRequestDispatcher prd = PortletContext.getRequestDispatcher("/jsp/homePage.jsp personalize=NONE"); prd.include(request,response);
The rule to remember is that any parameters passed in the query string to the
PortletRequestDispatcher override any existing parameters with the same name on
the request object. This can be useful for providing temporary overrides of parameters for one JSP page, while keeping the portlet s parameters intact for use on other JSP pages and servlets. If you would like to load a servlet through a request dispatcher, map the servlet to a path in your portlet application s web.xml deployment descriptor. Use the <servlet-mapping> element in the web.xml file, just as you would for a normal web application. Here is a snippet of code that includes a servlet with a request dispatcher:
PortletContext portletContext = getPortletContext(); PortletRequestDispatcher prd = portletContext.getRequestDispatcher("/patents"); prd.include(request,response);
Named Dispatcher
A named dispatcher is useful for loading servlets or JSP pages that have been given a name in the portlet application s web deployment descriptor. It also returns a PortletRequestDispatcher object. One important difference is that it is impossible to pass a query string to a servlet or JSP that is called through a named dispatcher. If we have a servlet named SingleSignOnServlet, we could include it when we render the portlet, using code like the following:
PortletContext portletContext = getPortletContext(); PortletRequestDispatcher prd = PortletContext.getNamedDispatcher("SingleSignOnServlet"); prd.include(request,response);
One important point is that portlets may not be included in the output of another portlet using dispatchers. Rendering one portlet s content inside another portlet should be accomplished by calling methods directly to get content, providing access to the portlet s templates, or another form of direct access.
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