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130

Chapter 6 Generating the Server Response: HTTP Status Codes

tracking is discussed in Chapter 9, and it is a good idea to use 

enco 

deURL

 routinely so that you can add session tracking at a later time with 

minimal changes to the code.

Core Approach

If you redirect users to a page within your site, plan ahead for session 

tracking by using 

response.sendRedirect(response.encodeURL(url))

, 

rather than just 

response.sendRedirect(url)

.

This status code is sometimes used interchangeably with 301. For exam 

ple, if you erroneously ask for 

http://host/~user

 (missing the trailing 

slash), some servers will reply with a 301 code while others will use 302. 

Technically, browsers are only supposed to automatically follow the 

redirection if the original request was 

GET

. For details, see the discus 

sion of the 307 status code. 

303 (See Other)

The 303 (

SC_SEE_OTHER

) status is similar to 301 and 302, except that 

if the original request was 

POST

, the new document (given in the 

Location

 header) should be retrieved with 

GET

. This code is new in 

HTTP 1.1. 

304 (Not Modified)

When a client has a cached document, it can perform a conditional 

request by supplying an 

If Modified Since

 header to indicate that it 

only wants the document if it has been changed since the specified date. 

A value of 304 (

SC_NOT_MODIFIED

) means that the cached version is 

up to date and the client should use it. Otherwise, the server should 

return the requested document with the normal (200) status code. Serv 

lets normally should not set this status code directly. Instead, they 

should implement the 

getLastModified

 method and let the default 

service

 method handle conditional requests based upon this modifica 

tion date. An example of this approach is given in Section 2.8 (An Exam 

ple Using Servlet Initialization and Page Modification Dates).

Second edition of this book: www.coreservlets.com; Sequel: www.moreservlets.com.

Servlet and JSP training courses by book's author: courses.coreservlets.com.