Specifying HTTP Response Headers
A response from a Web server normally consists of a status line,
one or more response headers, a blank line, and the document.
Setting the HTTP response headers often goes hand in hand with setting
the status codes in the status line. For example, several of
the "document moved" status codes have an
accompanying Location header, and a 401 (Unauthorized)
code must include an accompanying WWW-Authenticate header.
However, specifying headers can play a useful role even when no unusual
status code is set. Response headers can be used to specify cookies,
to supply the modification date (for caching), to instruct the browser
to reload the page after a designated interval, to say how long the file is
so that persistent HTTP connections can be used, and many other tasks.
The most general way to specify headers is by the setHeader
method of HttpServletResponse, which takes two strings: the header name
and the header value. Like setting the status codes, this
must be done before any document content is sent.
There are also two specialized methods to set headers that contain
dates (setDateHeader) and integers (setIntHeader). The first saves you
the trouble of translating a Java date in milliseconds since the epoch
(as returned by System.currentTimeMillis or the getTime method applied
to a Date object) into a GMT time string. The second spares you
the minor inconvenience of converting an int to a String.
Rather than setting a header outright, you can add a new header, in case a header
with that name already exists. Use addHeader,
addDateHeader, and addIntHeader for this. If it really
matters to you whether a specific header has already been set, use
containsHeader to check.
Finally, HttpServletResponse also supplies a number of convenience methods
for specifying common headers.
- The
setContentType method sets the
Content-Type header, and is used by the majority of servlets.
- The
setContentLength method sets the Content-Length header,
useful if the browser supports persistent (keep-alive) HTTP connections.
- The
addCookie method sets a cookie (there is no corresponding setCookie,
since it is normal to have multiple Set-Cookie lines).
- And, as discussed
in the previous
section, the
sendRedirect method sets the Location header
as well as setting the status code to 302.
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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