1.6.5

Redirecting Requests

create/update

Request

Server 

JSP/Servlet

Side

Objects

Client

redirect request

JSP/Servlet

access/update

Response

It is common that the data to be sent to the client varies significantly depending on properties 

of the client that are either directly encoded in the request object or can be discovered based 

on some user/client profile (e.g. stored in a login database). In this case it is very convenient 

to have the initial JSP page determine details about the request, perhaps create and/or update 

some server side objects, and then, if necessary, redirect the request to a different JSP page.

This programming model is supported by the underlying Servlet APIs. The properties of the 

HTTP protocol are such that the redirect cannot be done if the response stream has started 

being sent back to the client; this characteristic makes the description of some common 

situations quite inconvenient. To address this, the JSP specification by default indicates 

buffering on the output stream. The JSP page can redirect the request at any point before 

flushing the output buffer.

Buffering is also very convenient for error page handling, since that is done by redirecting 

the request.

Presentation JSP pages and Front JSP pages

In a slight variation of this model, the front component (a Servlet or a JSP) only creates and/

or updates the server side objects. In this organization, the 

front

 component does no 

presentation at all; instead all presentation is done by a 

presentation

 component. Although 

the front component could be written as a Servlet since it does no presentation, writing it as 

a JSP page enables the use of custom actions for the creation and update of the server side 

objects. The presentation component will almost in all cases be a JSP page, and it will most 

likely access the server side objects through custom actions

1

. 

1. Readers of the original JSP 0.92 draft will recognize the combination  

front component is servlet and presentation 

component is JSP

  as the model 2 mentioned in that draft.

Chapter 1

Overview

32