RequestDispacher and Redirection in Java Web App

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This post only talks about RequestDispacher in Java web app. If you want to know more about using servlet in Java, I have another related post: Ajax and Java Servlet.

Dispach

This article from www.xyzws.com is very useful.

RequestDispacher is used to formard a client's request to another 'place', which including another servlet, another JSP, etc.

In a servlet, there are three ways to get RequestDispacher:

RequestDispatcher dispatch = request.getRequestDispatcher(path);

Using this, path can be:

  • either a path related to your current request, such as subhome.html

  • or a servlet name in your web.xml file

  • or a abosute path related to your current contaxt, such as /subpage/subjome.jsp

RequestDispatcher dispatch = getServletContext.getRequestDispatcher(path)

Using this, path must start with /, such as /subpage/subjome.jsp

RequestDispatcher dispatch = getServletContext.getNamedDispatcher("string")

This method must take the servlet name as its parameter. The servlet name is definded in web.xml as sub-element of the element. The request is dispatched to that corresponding servlet.

Example in web.xml:

<servlet>
    <!-- Use testProctor for getting RequestDispatcher -->
    <servlet-name>testProctor</servlet-name>
    <servlet-class>com.test.proctortest</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>testProctor</servlet-name>
    <url-pattern>/test_proctor</url-pattern>
</servlet-mapping>

Before/After get your RequestDispacher, you can use request.addAttribute(something) or other functions to add stuff to your request.

Go to another place

After you finish process HttpRequest or HttpResponse, there are three ways to 'leave' for another place:

RequestDispatcher.forward(request, response) | RequestDispatcher.include(request, response)

There is a greate article from way2java.com to explain the difference between RequestDispatcher.forward() and RequestDispatcher.include().

These are main differences bewtween these two methods:

  • include() will include another file in your current file, the send back to client. This is like <include /footer.jsp/> but on server side. forward() will forward the client to another page/servlet.

  • include() will return back to original servlet, but forward() will go directly to client.

  • So, if you need response from two servlet, use include(). If you only need reponse from last servlet, use forward(). Note include() does have limitation for second servlet accessing information from first servlet.

  • include() will give client the response from the same servlet which he requested. forward() will give client the response from a different servlet.

  • Generally, you want to use include() when static information needs to be included, such as Banner Content or Copyright Information. forward() should be used to include dynamic information, or when you want to use servlet as a Controller.

HttpResponse.sendRedirect(String)

The differnce between forward()/include() and HttpResponse.sendRedirect() is very obvious:

  • SendRedirect is just for re-direction. If you have any data, it has to be transferred by session or being included in URL. This request is visible in browser as a new request. i.e. it is a client side redirect.

  • forward()/include() is server side operation, which is not visible to browser. You can use request.getAttribute() to get data.

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