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Java Servlet Programming by Jason Hunter

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Servlet Reuse

Another use for interservlet communication is to allow one servlet to reuse the abilities (the public methods) of another servlet. The major challenge with servlet reuse is for the “user” servlet to obtain the proper instance of “usee” servlet when the usee servlet has not yet been loaded into the server.

The obvious solutions don’t always work. getServlet() isn’t guaranteed to load the named servlet on all servers; it may just return null. Directly creating a new instance of the usee servlet doesn’t work either, as the newly created servlet doesn’t have access to its own ServletConfig and ServletContext objects. Plus, the server would be using a different instance to handle client requests, leaving the new instance of the servlet without the right state information.

The solution is for the user servlet to ask the server to load the usee servlet, then call getServlet() to get a reference to it. Unfortunately, the Servlet API distinctly lacks any methods whereby a servlet can control the servlet life cycle, for itself or for other servlets. This is considered a security risk and is officially “left for future consideration.”

Fortunately, there’s a back door we can use today. A servlet can open an HTTP connection to the server in which it’s running, ask for the unloaded servlet, and effectively force the server to load the servlet to handle the request. Then a call to getServlet() gets the proper instance.[44]

An Improved getServlet()

The com.oreilly.servlet.ServletUtils ...

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