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

104

Chapter 4 Handling the Client Request: HTTP Request Headers

check the 

Accept Encoding

 header. Admittedly, this is not always 

possible, but when it is not, you should ask yourself if the browser spe 

cific feature you are using really adds enough value to be worth the 

maintenance cost. 

Most Internet Explorer versions list a  Mozilla  (Netscape) version first 

in their 

User Agent

 line, with the real browser version listed paren 

thetically. This is done for compatibility with JavaScript, where the 

User Agent

 header is sometimes used to determine which JavaScript 

features are supported. Also note that this header can be easily spoofed, 

a fact that calls into question the reliability of sites that use this header 

to  show  market penetration of various browser versions. Hmm, mil 

lions of dollars in marketing money riding on statistics that could be 

skewed by a custom client written in less than an hour, and I should take 

those numbers as accurate ones?

Via

This header is set by gateways and proxies to show the intermediate 

sites the request passed through.

Warning

This rarely used catchall header lets clients warn about caching or con 

tent transformation errors.

4.4 Sending Compressed Web 

Pages

Several recent browsers know how to handle gzipped content, automatically

uncompressing documents that are marked with the 

Content Encoding

header and then treating the result as though it were the original document.

Sending such compressed content can be a real timesaver, since the time

required to compress the document on the server and then uncompress it on

the client is typically dwarfed by the savings in download time, especially

when dialup connections are used. 

Browsers that support content encoding include most versions of Netscape

for Unix, most versions of Internet Explorer for Windows, and Netscape 4.7

and later for Windows. Earlier Netscape versions on Windows and Internet

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

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