Prentice Hall and Sun Microsystems. Personal use only; do not redistribute.
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.






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