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100

Chapter 4 Handling the Client Request: HTTP Request Headers

Authorization

This header is used by clients to identify themselves when accessing 

password protected Web pages. See Section 4.5 (Restricting Access to 

Web Pages) for an example.

Cache Control

This header can be used by the client to specify a number of options for 

how pages should be cached by proxy servers. The request header is 

usually ignored by servlets, but the 

Cache Control

 response header 

can be valuable to indicate that a page is constantly changing and 

shouldn't be cached. See Chapter 7 (Generating the Server Response: 

HTTP Response Headers) for details.

Connection

This header tells whether or not the client can handle persistent HTTP 

connections. These let the client or other browser retrieve multiple files 

(e.g., an HTML file and several associated images) with a single socket 

connection, saving the overhead of negotiating several independent 

connections. With an HTTP 1.1 request, persistent connections are the 

default, and the client must specify a value of 

close

 for this header to 

use old style connections. In HTTP 1.0, a value of 

keep alive

 means 

that persistent connections should be used.

Each HTTP request results in a new invocation of a servlet, regardless 

of whether the request is a separate connection. That is, the server 

invokes the servlet only after the server has already read the HTTP 

request. This means that servlets need help from the server to handle 

persistent connections. Consequently, the servlet's job is just to make it 

possible for the server to use persistent connections, which is done by 

sending a 

Content Length

 response header. Section 7.4 (Using Per 

sistent HTTP Connections) has a detailed example.

Content Length

This header is only applicable to 

POST

 requests and gives the size of the 

POST

 data in bytes. Rather than calling 

request.getIntHeader("Con 

tent Length")

, you can simply use 

request.getContentLength()

. 

However, since servlets take care of reading the form data for you (see 

Chapter 3,  Handling the Client Request: Form Data ), you are 

unlikely to use this header explicitly.

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

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