Java Network Programming, Second Edition

Reading the Header

HTTP servers provide a substantial amount of information in the MIME headers that precede each response. For example, here’s a typical MIME header returned by an Apache web server running on Solaris:

HTTP 1.1 200 OK
Date: Mon, 18 Oct 1999 20:06:48 GMT
Server: Apache/1.3.4 (Unix) PHP/3.0.6 mod_perl/1.17
Last-Modified: Mon, 18 Oct 1999 12:58:21 GMT
ETag: "1e05f2-89bb-380b196d"
Accept-Ranges: bytes
Content-Length: 35259
Connection: close
Content-Type: text/html

There’s a lot of information there. In general, an HTTP MIME header may include the content type of the requested document, the length of the document in bytes, the character set in which the content is encoded, the date and time, the date the content expires, and the date the content was last modified. However, the information sent depends on the server; some servers send all this information for each request, others send some information, and a few don’t send anything. The methods of this section allow you to query a URLConnection to find out what MIME information the server has provided.

Aside from HTTP, very few protocols use MIME headers. When writing your own subclass of URLConnection, it is often necessary to override these methods so that they return sensible values. The most important piece of information you may be lacking is the MIME content type. URLConnection provides some utility methods that help you guess the data’s content type, based on its filename or (in the worst case) the first few bytes ...