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Java Servlet Technology Overview

Servlets are the Java platform technology of choice for extending and enhancing Web servers. Servlets provide a component-based, platform-independent method for building Web-based applications, without the performance limitations of CGI programs. And unlike proprietary server extension mechanisms (such as the Netscape Server API or Apache modules), servlets are server- and platform-independent. This leaves you free to select a "best of breed" strategy for your servers, platforms, and tools.

Servlets have access to the entire family of Java APIs, including the JDBC API to access enterprise databases. Servlets can also access a library of HTTP-specific calls and receive all the benefits of the mature Java language, including portability, performance, reusability, and crash protection.

Today servlets are a popular choice for building interactive Web applications. Third-party servlet containers are available for Apache Web Server, Microsoft IIS, and others. Servlet containers are usually a component of Web and application servers, such as BEA WebLogic Application Server, IBM WebSphere, Sun Java System Web Server, Sun Java System Application Server, and others.

You might want to check out the latest information on JavaServer Pages (JSP) technology. JSP technology is an extension of the servlet technology created to support authoring of HTML and XML pages. It makes it easier to combine fixed or static template data with dynamic content. Even if you're comfortable writing servlets, there are several compelling reasons to investigate JSP technology as a complement to your existing work.

 

How a Servlet is Loaded and Instantiated
The servlet engine instantiates and loads a servlet. The instantiation and loading can
occur when the engine starts, when it needs the servlet in order to respond to a
request, or any time in between.
The servlet engine loads a servlet using the Java class loading facility. The servlet
engine can load the servlet from the local file system, a remote file system, or a
network source.

 

How a Servlet is Destroyed
The servlet engine is not required to keep a servlet loaded for any period of time or
for the life of the server. Servlet engines are free to use servlets or retire them at any
time. Therefore, you should not rely on class or instance members to store state
information.
When the servlet engine determines that a servlet should be destroyed (for example,
if the engine is shut down or needs to conserve resources), the engine must allow the
servlet to release any resources it is using and save persistent state. To do this, the
engine calls the servlet’s destroy method.
The servlet engine must allow any calls to the service method either to complete
or to end with a time out (as the engine defines a time out) before the engine can
destroy the servlet. Once the engine destroys a servlet, the engine cannot route any
more requests to the servlet. The engine must release the servlet and make it eligible
for garbage collection.