The Commons HttpClient project is now end of life, and is no longer being developed. It has been replaced by the Apache HttpComponents project in its HttpClient and HttpCore modules, which offer better performance and more flexibility.

• Standards based, pure Java, implementation of HTTP versions 1.0 and 1.1
• Full implementation of all HTTP methods (GET, POST, PUT, DELETE, HEAD, OPTIONS, and TRACE) in an extensible OO framework.
• Supports encryption with HTTPS (HTTP over SSL) protocol.
• Granular non-standards configuration and tracking.
• Transparent connections through HTTP proxies.
• Tunneled HTTPS connections through HTTP proxies, via the CONNECT method.
• Transparent connections through SOCKS proxies (version 4 & 5) using native Java socket support.
• Authentication using Basic, Digest and the encrypting NTLM (NT Lan Manager) methods.
• Plug-in mechanism for custom authentication methods.
• Multi-Part form POST for uploading large files.
• Pluggable secure sockets implementations, making it easier to use third party solutions
• Connection management support for use in multi-threaded applications. Supports setting the maximum total connections as well as the maximum connections per host. Detects and closes stale connections.
• Automatic Cookie handling for reading Set-Cookie: headers from the server and sending them back out in a Cookie: header when appropriate.
• Plug-in mechanism for custom cookie policies.
• Request output streams to avoid buffering any content body by streaming directly to the socket to the server.
• Response input streams to efficiently read the response body by streaming directly from the socket to the server.
• Persistent connections using KeepAlive in HTTP/1.0 and persistance in HTTP/1.1
• Direct access to the response code and headers sent by the server.
• The ability to set connection timeouts.
• HttpMethods implement the Command Pattern to allow for parallel requests and efficient re-use of connections.
• Source code is freely available under the Apache Software License.

HttpClient implements the following specifications endorsed by the Internet Engineering Task Force (IETF) and the internet at large:

• RFC1945 Hypertext Transfer Protocol -- HTTP/1.0
• RFC2616 Hypertext Transfer Protocol -- HTTP/1.1
• RFC2617 HTTP Authentication: Basic and Digest Access Authentication
• RFC2109 HTTP State Management Mechanism (Cookies)
• RFC2396 Uniform Resource Identifiers (URI): Generic Syntax
• RFC1867 Form-based File Upload in HTML

The HTTP protocol is so ubiquitous on the internet that you can find other client side implementations written in Java. The jdk has the HttpUrlConnection which is limited and in many ways flawed. This is one reason why Jakarta, and others free and commercial vendors, have implemented independant HTTP clients. To help you choose the right solution, one of those commercial vendors, Oakland Software, has a fair product comparison.