Tutorials : Server-Side Web Applications Using Servlets and JSP :
Reading HTTP Request Headers :

An Overview of Request Headers

When an HTTP client (e.g. a browser) sends a request, it is required to supply a request line (usually GET or POST). If it wants to, it can also send a number of headers, all of which are optional except for Content-Length, which is required only for POST requests. Here are the most common headers:

• Accept The MIME types the browser prefers.
• Accept-Charset The character set the browser expects.
• Accept-Encoding The types of data encodings (such as gzip) the browser knows how to decode. Servlets can explicitly check for gzip support and return gzipped HTML pages to browsers that support them, setting the Content-Encoding response header to indicate that they are gzipped. In many cases, this can reduce page download times by a factor of five or ten.
• Accept-Language The language the browser is expecting, in case the server has versions in more than one language.
• Authorization Authorization info, usually in response to a WWW-Authenticate header from the server.
• Connection Use persistent connection? If a servlet gets a Keep-Alive value here, or gets a request line indicating HTTP 1.1 (where persistent connections are the default), it may be able to take advantage of persistent connections, saving significant time for Web pages that include several small pieces (images or applet classes). To do this, it needs to send a Content-Length header in the response, which is most easily accomplished by writing into a ByteArrayOutputStream, then looking up the size just before writing it out.
• Content-Length (for POST messages, how much data is attached)
• Cookie (one of the most important headers; see separate section in this tutorial on handling cookies)
• From (email address of requester; only used by Web spiders and other custom clients, not by browsers)
• Host (host and port as listed in the original URL)
• If-Modified-Since (only return documents newer than this, otherwise send a 304 "Not Modified" response)
• Pragma (the no-cache value indicates that the server should return a fresh document, even if it is a proxy with a local copy)
• Referer (the URL of the page containing the link the user followed to get to current page)
• User-Agent (type of browser, useful if servlet is returning browser-specific content)
• UA-Pixels, UA-Color, UA-OS, UA-CPU (nonstandard headers sent by some Internet Explorer versions, indicating screen size, color depth, operating system, and cpu type used by the browser's system)

For full details on HTTP headers, see the specifications at http://www.w3.org/Protocols/.

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