Page Navigation Techniques

2 Ways

1. Client Pull

• 1.1 Client Pull

  • Taking the client to the next page in the NEXT request

    • User takes some action

      • eg : clicking on a button or link & then client browser generates new URL to take user to the next page.

• 1.2 Redirect Scenario

  • User doesn't take any action. Client browser automatically generates new URL to take user to the next page. (next page can be from same web application, or different web application on same server or any web page on any server).

  • Here a temporary redirection happens, web container creates a response such that,

    • body: Empty (printwriter buffer emptied before redirection).

    • location header: Location of the next page.

    • status code: 302

    • Cookie header: name: ..., value...(If any)

  • Client browser on getting the 302 status code redirects the client to the location based on location header, on next request-response cycle. This newly created request by the client browser will have,

    • Cookie: name...,value...

    • method: GET (default http method) Note: URL on client browser URL bar will change as per location on the location header received in previous response.

    //API of HttpServletResponse
    public void sendRedirect(String redirectURL)

    //eg : For redirecting client from Servlet1('/s1') to Servlet2('/s2'), use
    //written ins /s1 servlet page
    response.sendRedirect("s2");

Note: If the response already has been committed(printwriter flushed or closed), this method throws(WC) an IllegalStateException.(since WC can't redirect the client after response is already committed).

Note: If session is not created before flushing or committing the response, then it throws IllegalStateException .

2. Server Pull (Request Dispatching)

• Taking the client to the next page in the same request. Also known as resource chaining or request dispatching technique.

• Client sends the request to the servlet/JSP. Same request can be chained to the next page for further servicing of the request.

1. Forward Scenario

• Steps

  • Create Request Dispatcher object for wrapping the next page resource, which can be static or dynamic.

      //API of ServletRequest
      javax.servlet.RequestDispatcher getRequestDispatcher(String path)

      //API of RequestDispatcher
      public void forward(ServletRequest rq,ServletResponse rs) throws ServletException, IOException, IllegalStateException 

  • This method allows one servlet to do initial processing of a request and another resource to generate the response. (i.e division of responsibility)

  • Uncommitted output in the response buffer is automatically cleared before the forward just like send redirect scenario.

  • If the response already has been committed (by flushing the print writer or closing), this method throws an IllegalStateException as client cannot be forwarded after commiting the response.

  • If a post request is received from the client to server, the forward() scenario will forward to the next resource and call the next resource's doPost() method. Like wise for other http methods.

  

  RD-forward

  • Limitation

    • Only last page in the chain can generate dynamic response.

1. Include scenario

• Same as above, get a request dispatcher object from the request object and then on that call the include() method.

    // API of RequestDispatcher
    public void include(ServletRequest rq,ServletResponse rs) throws ServletException, IOException

• Includes the content of a resource @run time (servlet, JSP page, HTML file) in the response. -- server-side includes.

• If the response already has been committed (by flushing the print writer), the container doesn't need to clear the print writer buffer, rather when flush is called, the container just sends the reponse. It doesn't close the buffer and further responses get added to this buffer. Hence, no IllegalStateException is encountered.

• If a post request is received from the client to server, the forward() scenario will forward to the next resource and call the next resource's doPost() method. Like wise for other http methods.

• Limitation

  • The included servlet/JSP cannot change the response status code or set headers; any attempt to make these changes are ignored.

include-scenario

Forward v/s include