An Overview Of Servlets
The Web-Side Helpers

Servlets are modules that extend request/response-oriented servers, such as Java-enabled Web servers. For example, a servlet might be responsible for taking data in an HTML order-entry form and applying the business logic used to update a company's order database.
Servlets are to servers what applets are to browsers. Unlike applets, however, servlets have no graphical user interface.

Servlets can be embedded in many different servers because the servlet API, which you use to write servlets, assumes nothing about the server's environment or protocol. Servlets have become most widely used within HTTP servers; many Web servers support Java Servlet technology.
A servlet can handle multiple requests concurrently, and can synchronise requests. This allows servlets to support systems such as on-line conferencing.

Servlets can forward requests to other servers and servlets. Thus servlets can be used to balance load among several servers that mirror the same content, and to partition a single logical service over several servers, according to task type or organisational boundaries.

Servlets are an effective replacement for CGI scripts. They provide a way to generate dynamic documents that is both easier to write and faster to run. Servlets also address the problem of doing server-side programming with platform-specific APIs: they are developed with the Java Servlet API, a standard Java extension.

Why are servlets better than CGI programs written in languages such as Perl and C?

Servlets are cross-platform: Because servlets are written in Java, they can be used on multiple platforms without worrying about compatibility issues or recompilation.

Servlets are fast: Standard CGI programs are slow because a new process must start up and run for every client request. For example, if 25 users are using a Perl CGI search engine, 25 separate versions of the program have to be loaded into memory and executed... that's a lot of overhead.

Java servlets are fast because they are persistent. The same search engine as a servlet would load only once and then service all 25 clients using multiple threads. It would then stay loaded in memory and wait for more requests instead of shutting down.

Servlets are elegant; because Java was created from the start to be object-oriented, programs written in that language tend be organised more effectively into easily manageable parts. This makes servlets easier to maintain and understand.

Servlets are secure: Servlets are run in the Java security sandbox so can be insulated from disrupting the operating system or breaching security. Additionally, many security holes in traditional CGI languages result from those languages being weakly typed. Java's strong typing helps to ensure fewer accidental security mistakes.

Many servlets resources are available around the Web. A good place to start is on Sun's Java site, specifically, the servlet page. Among other things, you'll find a popular mailing list there.