RequestDispatcher object can forward a client’s request to a resource or include the resource itself in the response back to the client. A resource can be another servlet, or an HTML file, or a JSP file, etc.

You can also think of a RequestDispatcher object as a wrapper for the resource located at a given path that is supplied as an argument to the getRequestDispatcher method.

For constructing a RequestDispatcher object, you can use either the

ServletRequest.getRequestDispatcher()  method 

ServletContext.getRequestDispatcher()  method  

They both do the same thing, but impose slightly different constraints on the argument path. For the former, it looks for the resource in the same webapp to which the invoking servlet belongs and the path-name specified can be relative to invoking servlet. For the latter, the path-name must begin with ‘/’ and is interpreted relative to the root of the webapp.

For example:  

Suppose you want Servlet_A to invoke Servlet_B. If they are both in the same directory, you could accomplish this by incorporating the following code fragment in either the service method or the doGet method of Servlet_A:

   RequestDispatcher dispatcher = getRequestDispatcher(“Servlet_B”);

   dispatcher.forward( request, response );

where request, of type HttpServletRequest, is the first parameter of the enclosing service method (or the doGet method) and response, of type HttpServletResponse, the second. You could accomplish the same by

 RequestDispatcher dispatcher=getServletContext().getRequestDispatcher( “/servlet/Servlet_B” );

  dispatcher.forward( request, response );

source : jguru

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