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/.
NEXT
This tutorial is now available as a book: Core Servlets and JavaServer Pages by Marty Hall, published by Sun Microsystems Press.
Read all about it at CoreServlets.com
Server-Side Web Applications using Java Servlets versions 2.1/2.2 and JavaServer Pages (JSP) version 1.0: A Tutorial
© 1999-2000 Marty Hall.
All source code freely available for unrestricted use.
Created for work in the Research and Technology Development Center of the Johns Hopkins University Applied Physics Lab, for courses in the Johns Hopkins Part-Time MS Program in Computer Science, and for various industry seminars and on-site Java short courses.
Please note that this is a first draft of the tutorial, so please send corrections, comments, and suggestions to me at hall@apl.jhu.edu.
Reprinted with permission from the author. Click here to visit the original version
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