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Calling Servlets from Servlets

To have your servlet call another servlet, you can either:

• A servlet can make an HTTP request of another servlet. Opening a connection to a URL is discussed in the Working with URLs lesson in this tutorial.
   
• A servlet can call another servlet's public methods directly, if the two servlets run within the same server.

This section addresses the second option. To call another servlet's public methods directly, you must:

• Know the name of the servlet that you want to call.
   
• Gain access to that servlet's Servlet object
   
• Call the servlet's public method

To gain access to the Servlet object, use the ServletContext object's getServlet method. Get the ServletContext object from the ServletConfig object stored in the Servlet object. An example should make this clear. When the BookDetail servlet calls the BookDB servlet, the BookDetail servlet obtains the BookDB servlet's Servlet object like this:

public class BookDetailServlet extends HttpServlet {

    public void doGet (HttpServletRequest request,
                       HttpServletResponse response)
        throws ServletException, IOException
    {
        ...
            BookDBServlet database = (BookDBServlet)
                getServletConfig().getServletContext().getServlet("bookdb");
        ...
    }
}

Once you have the servlet object, you can call any of that servlet's public methods. For example, the BookDetail servlet calls the BookDB servlet's getBookDetails method:

public class BookDetailServlet extends HttpServlet {

    public void doGet (HttpServletRequest request,
                       HttpServletResponse response)
        throws ServletException, IOException
    {
        ...
            BookDBServlet database = (BookDBServlet)
                getServletConfig().getServletContext().getServlet("bookdb");
            BookDetails bd = database.getBookDetails(bookId);
        ...
    }
}

You must exercise caution when you call another servlet's methods. If the servlet that you want to call implements the SingleThreadModel interface, your call could violate the called servlet's single threaded nature. (The server has no way to intervene and make sure your call happens when the servlet is not interacting with another client.) In this case, your servlet should make an HTTP request of the other servlet instead of calling the other servlet's methods directly.

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