HTTP : Java Glossary

by Roedy Green ©1996-2008 Canadian Mind Products
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HTTP

Hypertext Transfer Protocol. A protocol used on the Internet by web browsers to transport text and graphics. It is focuses on grabbing a page at a time, rather setting up a session. Applets also use it to download jars, classes and resources. Browsers use to download files and images, not just HTML text.

Browser To Server	Sample Code
Server To Browser	Learning More
Language & Charset	Links

Message Headers From Browser To Server

Fields in the headers let browsers and servers communicate. For example:

HTTP Headers that Browsers Send Servers
Field	Typical Value	Meaning
User-Agent:	Opera ⇒ Opera/9.22 (Windows NT 6.0; U; en) Firefox ⇒ Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.8.1.6) Gecko/20070725 Firefox/2.0.0.6 Sea Monkey ⇒ Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.8.1.6) Gecko/20070802 SeaMonkey/1.1.4 Netscape ⇒ Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.8.1.5pre) Gecko/20070710 firefox/2.0.0.4 Navigator/9.0b2 IE ⇒ Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0; SLCC1; .NET CLR 2.0.50727; Media Center PC 5.0; .NET CLR 3.0.04506)	Which browser being used.
Host:	localhost:8081	destination url, server:port.
Accept:	application/xhtml+voice+xml;version=1.2, application/x-xhtml+voice+xml;version=1.2, application/x-shockwave-flash,text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,video/x-mng,image/png,image/jpeg,image/gif;q=0.2,text/css,/;q=0.1 or text/html, image/gif, image/jpeg, ; q=.2, /*; q=.2	MIME types the browser is willing to accept. The encoding of this field, is described in RFC 2616 section 14. and in the more friendly w3.org version . Roughly the q numbers define your preference. The higher the number the higher the preference. Default is 1. The q applys to the preceding MIME. You set this with URLConnection.setRequestProperty( "Accept", …); not "accept" as the Sun docs erroneously suggest.
Accept-Language:	en	Language the browser in willing to accept.
Accept-Charset:	windows-1252, utf-8, utf-16, iso-8859-1;q=0.6, *;q=0.1	Character set encodings the browser is willing to accept.
Accept-Encoding:	deflate, gzip, x-gzip, identity, *;q=0	compression schemes the browser is willing to accept.
Referer:	http://mindprod.com/jgloss/http.html	the web page that contained the link that triggered this requenst.
If-Modified-Since:	Mon, 06 Feb 2006 01:24:23 GMT	Only bother with the request if the file has changed since this date, otherwise the browser already has a copy in cache.
Connection:	Keep-Alive	requests server keep the socket open for further messages.
Content-Type:	application/x-www-form-urlencoded	MIME type of the payload to the server.
Content-Length:	114	length in encoded bytes of the payload to the server.

Beware using HttpURLConnection.setFollowRedirects( false); This reportedly causes trouble in recent JDKs.

Message Headers From Server To Browser

HTTP Headers that Servers Send Browsers
Field	Typical Value	Meaning
Server:	Apache/2.0.55 (NETWARE) mod_perl/1.99_12 Perl/v5.8.4	Which server software being used.
Accept-Ranges:	bytes	Inform the browser that the server supports downloading just parts of files, as small as a byte granularity.
Keep-Alive:	timeout=15, max=99	how long to keep this socket open for more messages.
Connection:	Keep-Alive	requests browser keep the socket open for further messages.
Content-Type:	image/png	MIME type of the payload from the server.
Content-disposition:	attachment;filename="smile.png"	Server suggests a filename to save this download under.
Content-Length:	842	length in encoded bytes of the payload from the server.

Language and Charset

You might wonder, where does the server encode the language and character set? Oddly not in the HTTP header, but embedded in the HTML documents, with tags like this:

<!-- embedding language and charset inside an HTML document -->
<meta http-equiv="Content-Language" content="en">
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">

Embedding this information makes it easier for web page authors to control, even if it makes finding the information slightly more difficult for the browser.

Learning More

RFC 1521 (obsolete) MIME part 1.

RFC 1522 (obsolete) MIME part 2, non ASCII.

RFC 1945 HTTP 1.0 specification.

RFC 2045 MIME Part One: Format of Internet Message Bodies, specifies the various headers used to describe the structure of MIME messages.

RFC 2046 MIME Part Two: Media Types, describes the general structure of the MIME media typing system and defines an initial set of media types.

RFC 2047 MIME Part Three: Message Header Extensions for non-ASCII text

RFC 2048 MIME Part Four: Registration Procedures

RFC 2049 MIME Part Five: Conformance Criteria and Examples, Provides some illustrative examples of MIME message formats

RFC 2183 MIME Part Five: Conformance Criteria and HTTP Content-Disposition

RFC 2068 (obsolete) HTTP/1.1 protocol, obsolete.

RFC 2616 updates the HTTP protocol

RFC 2617: for details on how to send username and password in http headers to restrict access

RFC 2183 MIME Part Five: Conformance Criteria and HTTP Content-Disposition

Sample Code

This code for doing GET and POST is from the com.mindprod.http package. You can download the whole package. Code to do a GET:

Code to do a POST:

Code to read the response either as bytes (readBytesBlocking) or converted to a String (readStringBlocking):

Code to build the &parm=value command string:

CGI
File I/O Amanuensis: to see how to write code that reads and writes via HTTP-CGI
forms
HTTP Client
MIME
network properties
remote file access
response codes
RFC

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