Java Network Programming, Second Edition

Configuring the Client Request MIME Header

In HTTP 1.0 and later, the client sends the server not only a request line, but also a MIME header. For example, here’s the MIME header that Netscape Navigator 4.6 for Windows uses:

Connection: Keep-Alive
User-Agent: Mozilla/4.6 [en] (WinNT; I)
Host: login.metalab.unc.edu:38309
Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, image/png, */*
Accept-Encoding: gzip
Accept-Language: en
Accept-Charset: iso-8859-1,*,utf-8

A simple web server can ignore this. A more sophisticated web server can use this information to serve different pages to different clients, to get and set cookies, to authenticate users through passwords, and more. All of this is done by placing different fields in the MIME headers that the client sends and the server responds with.

Note

It’s important to understand that this is not the MIME header that the server sends to the client, and that is read by the various getHeaderField( ) and getHeaderFieldKey( ) methods discussed previously. This is the MIME header that the client sends to the server.

Each concrete subclass of URLConnection sets a number of different name-value pairs in its MIME header by default. (Really, only HttpURLConnection does this, since HTTP is the only major protocol that uses MIME headers in this way.) For instance, here’s the MIME header that a connection from the SourceViewer2 program of Example 15.1 sends:

User-Agent: Java1.3beta Host: login.metalab.unc.edu:38358 Accept: text/html, image/gif, ...