Java Network Programming, 4th Edition

Chapter 6. HTTP

The Hypertext Transfer Protocol (HTTP) is a standard that defines how a web client talks to a server and how data is transferred from the server back to the client. Although HTTP is usually thought of as a means of transferring HTML files and the pictures embedded in them, HTTP is data format agnostic. It can be used to transfer TIFF pictures, Microsoft Word documents, Windows .exe files, or anything else that can be represented in bytes. To write programs that use HTTP, you’ll need to understand HTTP at a deeper level than the average web page designer. This chapter goes behind the scenes to show you what actually happens when you type http://www.google.com into the browser’s address bar and press Return.

The Protocol

HTTP is the standard protocol for communication between web browsers and web servers. HTTP specifies how a client and server establish a connection, how the client requests data from the server, how the server responds to that request, and finally, how the connection is closed. HTTP connections use the TCP/IP protocol for data transfer. For each request from client to server, there is a sequence of four steps:

1. The client opens a TCP connection to the server on port 80, by default; other ports may be specified in the URL.
2. The client sends a message to the server requesting the resource at a specified path. The request includes a header, and optionally (depending on the nature of the request) a blank line followed by data for the request.
3. The server ...