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What is an HTTP Server?


HTTP is a large protocol. A full-featured HTTP server must respond to requests for files, convert URLs into filenames on the local system, respond to POST and GET requests, handle requests for files that don't exist, interpret MIME types, launch CGI programs, and much, much more. However, many HTTP servers don't need all of these features. For example, many sites simply display an "under construction" message. Clearly, Apache is overkill for a site like this. Such a site is a candidate for a custom server that does only one thing. Java's network class library makes writing simple servers like this almost trivial.

Custom servers aren't useful only for small sites. High-traffic sites like Yahoo! are also candidates for custom servers because a server that does only one thing can often be much faster than a general purpose server such as Apache or Netscape. It is easy to optimize a special purpose server for a particular task; the result is often much more efficient than a general purpose server that needs to respond to many different kinds of requests. For instance, icons and images that are used repeatedly across many pages or on high-traffic pages might be better handled by a server that read all the image files into memory on startup, and then served them straight out of RAM rather than having to read them off disk for each request. Furthermore, this server could avoid wasting time on logging if you didn't want to track the image request separately from the requests for the pages they were included in.

Finally, Java isn't a bad language for feature-full web servers meant to compete with the likes of Apache or AOLServer. Although CPU-intensive Java programs are demonstrably slower than CPU-intensive C and C++ programs, even when run under a JIT, most HTTP servers are limited by bandwidth, not by CPU speed. Consequently, Java's other advantages, such as its half-compiled/half-interpreted nature, dynamic class loading, garbage collection, and memory protection, really get a chance to shine. In particular, sites that make heavy use of dynamic content through CGI scripts, PHP pages, or other mechanisms can often run much faster when reimplemented on top of a pure or mostly pure Java web server. Indeed, there are several production web servers written in Java such as the W3C's testbed server Jigsaw ( Many other web servers written in C now include substantial Java components to support the Java Servlet API and Java Server Pages. On many sites, these are replacing the traditional CGIs, ASPs, and server-side includes, mostly because the Java equivalents are faster and less resource-intensive.

Investigation of HTTP servers begins with a server that always sends out the same file, no matter who or what the request. This is shown in Example, SingleFileHTTPServer. The filename, local port, and content encoding are read from the command line. If the port is omitted, port 80 is assumed. If the encoding is omitted, ASCII is assumed.

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