2. Common Response Headers and their Meaning
Allow: What request methods (GET, POST, etc.) does the server support?
Content-Encoding: What method was used to encode the document?
You need to decode it to get the type specified by the
Content-Type header. Using gzip to compress the document can dramatically
reduce download times for HTML files, but it is only supported by Netscape
on Unix and IE 4 and 5 on Windows. On the other hand, gzipping HTML files can
dramatically reduce download times, and Java's GZIPOutputStream makes it easy.
So you should explicitly check if the browser supports this by
looking at the Accept-Encoding header (i.e. via
request.getHeader("Accept-Encoding")).
That way, you can return gzipped pages to browser that know how to unzip them,
but still return regular pages to other browsers.
Content-Length: How many bytes are being sent?
This information is only needed if the browser is using a persistent (keep-alive)
HTTP connection. If you want your servlet to take advantage of this when the
browser supports it, your servlet should write the document into a
ByteArrayOutputStream, look up its size when done, put that into the
Content-Length field, then send the
content via byteArrayStream.writeTo(response.getOutputStream()).
Content-Type: What is the MIME type of the following document? Default for servlets is
text/plain, but they usually explicitly specify text/html.
Setting this header is so common that there is a special method in
HttpServletResponse for it: setContentType.
Date: What is current time (in GMT)? Use the setDateHeader method to specify
this header. That saves you the trouble of formatting the date string properly.
Expires: At what time should content be considered out of date and thus no longer
cached?
Last-Modified: When was document last changed? Client can supply a date
via an If-Modified-Since request header. This is treated as a conditional
GET, with document only being returned if the Last-Modified date is later
than the specified date. Otherwise a 304 (Not Modified) status line
is returned. Again, use the setDateHeader method to specify
this header.
Location: Where should client go to get document? This is usually
set indirectly, along with a 302 status code, via the sendRedirect
method of HttpServletResponse.
Refresh: How soon should browser ask for an updated page (in seconds)?
Instead of just reloading current page, you can specify a specific page to load
via setHeader("Refresh", "5; URL=http://host/path"). Note that this is commonly set
via <META HTTP-EQUIV="Refresh" CONTENT="5; URL=http://host/path"> in
the HEAD section of the HTML page, rather than as an explicit header from the server.
This is because automatic reloading or forwarding is something often desired by HTML
authors who do not have CGI or servlet access. But for servlets, setting the header
directly is easier and clearer. Note that this header means "reload this page
or go to the specified URL in N seconds." It does not mean "reload this page or
go to the specified URL every N seconds." So you have to send a Refresh header
each time, and sending a 204 (No Content) status code stops the browser from
reloading further, regardless of whether you explicitly send the Refresh header
or use <META HTTP-EQUIV="Refresh" ...>. Note that this header
is not officially part of HTTP 1.1, but is an extension supported by both Netscape
and Internet Explorer.
Server: What server am I? Servlets don't usually set this;
the Web server itself does.
Set-Cookie: Specifies cookie associated with page. Servlets should not
use response.setHeader("Set-Cookie", ...), but instead use the special-purpose
addCookie method of HttpServletResponse. See separate section on
handling cookies.
WWW-Authenticate: What authorization type and realm should client supply
in their Authorization header? This header is required in responses that
have a 401 (Unauthorized) status line. E.g.
response.setHeader("WWW-Authenticate", "BASIC realm=\"executives\"").
Note that servlets do not usually handle this themselves, but instead let
password-protected Web pages be handled by the Web server's specialized
mechanisms (e.g. .htaccess).
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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