When the HTTP client (e.g. a browser) sends a request, it is required to supply a request line (usually GET or POST).
A D V E R T I S E M E N T
If it wants to, it can also send the number of headers, all of which are optional except for Content-Length, which is required only for POST requests
HTTP Request Header Methods Available in Servlets
-getHeader (header name is not case sensitive)
-getAuthType and getRemoteUser
An Overview of Request Headers
When the 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 the 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 that the browser prefers.
Accept-Charset The character set that the browser expects.
Accept-Encoding The types of data encodings (such as gzip) that
the browser knows how to decode. Servlet 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 the one language.
Authorization Authorization info, usually in response to the
WWW-Authenticate header from the server.
Connection Use persistent connection? If a servlet get 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 the Content-Length header in the response,
which is most easily accomplished by writing into a
ByteArrayOutputStream, then looking up the size just before writing
Content-Length (for POST messages, how much the
data is attached)
Cookie (one of most important headers; see separate
section in this tutorial on handling cookies)
From (email address of the requester; only used by Web spiders
and other custom clients, not by browsers)
Host (host and the port as listed in the original URL)
If-Modified-Since (only return documents newer than this,
otherwise send the 304 "Not Modified" response)
Pragma (the no-cache value indicates that the
server should return the fresh document, even if it is a proxy with a local
Referer (the URL of the page containing the link the user
followed to get to the current page)
User-Agent (type of the browser, useful if servlet is returning
UA-Pixels, UA-Color, UA-OS,
UA-CPU (nonstandard header sent by some Internet Explorer versions,
indicating screen size, color depth, operating system, and cpu type used by
the browser's system)
Reading Request Headers from Servlets
Reading headers is the very straightforward; just call the getHeader
method of the HttpServletRequest, which returns a String
if the header was supplied on this request, null otherwise.
However, there are the couple of headers that are so commonly used that they have
special access methods. The getCookies method returns contents
of the Cookie header, parsed and stored in an array of Cookie
objects. See the separate section of this tutorial on cookies. The
getAuthType and getRemoteUser methods break
Authorization header into its component pieces. The getDateHeader
and getIntHeader methods read the specified header and then convert
them to Date and int values, respectively.
Rather than looking up one of the particular header, you can use the
getHeaderNames to get an Enumeration of all header names
received on this particular request.
Finally, in addition to looking up request headers, you can get
information on the main request line itself. The getMethod method
returns the main request method (normally GET or POST,
but things like HEAD, PUT, and DELETE are
possible). The getRequestURI method returns URI (the part of
the URL that came after the host and port, but before the form data). The
getRequestProtocol returns third part of the request line, which is
generally "HTTP/1.0" or "HTTP/1.1".
The following example called showHeaders.java prints all the headers in the browser