1.2. Support for Servlets
Like Java itself, servlets were designed for portability. Servlets are supported on all platforms that support Java, and servlets work with all the major web servers. Java servlets, as defined by the Java Software division of Sun Microsystems (formerly known as JavaSoft), are the first standard extension to Java. This means that servlets are officially blessed by Sun and are part of the Java language, but they are not part of the core Java API. Therefore, although they may work with any Java Virtual Machine (JVM), servlet classes need not be bundled with all JVMs. More information about the Java Extension Framework is available at http//java.sun.com/products/jdk/1.2/docs/guide/extensions.
To make it easy for you to develop servlets, Sun has made publicly available a set of classes that provide basic servlet support. The javax.servlet and javax.servlet.http packages constitute this Servlet API. Version 2.0 of these classes comes bundled with the Java Servlet Development Kit (JSDK) for use with the Java Development Kit version 1.1 and above; the JDSK is available for download from http://java.sun.com/products/servlet/. 
Many web server vendors have incorporated these classes into their servers to provide servlet support, and several have also provided additional functionality. Sun's Java Web Server, for instance, includes a proprietary interface to the server's security features.
It doesn't much matter where you get the servlet classes, as long as you have them on your system, since you need them to compile your servlets. In addition to the servlet classes, you need a servlet engine, so that you can test and deploy your servlets. Your choice of servlet engine depends in part on the web server(s) you are running. There are three flavors of servlet engines: standalone, add-on, and embeddable.
1.2.1. Standalone Servlet Engines
A standalone engine is a server that includes built-in support for servlets. Such an engine has the advantage that everything works right out of the box. One disadvantage, however, is that you have to wait for a new release of the web server to get the latest servlet support. Because servlets are still fairly new, this sort of server is still a bit of a rarity. As the various vendors upgrade their web servers, we expect that many of the servers will provide built-in support for servlets.
Standalone engines in web servers include the following:
Application servers are a fertile new area of development. An application server offers server-side support for developing enterprise-based applications. Here are two application servers that include servlet engines:
1.2.2. Add-on Servlet Engines
An add-on servlet engine functions as a plug-in to an existing server--it adds servlet support to a server that was not originally designed with servlets in mind. Add-on servlet engines have been written for many servers including Apache, Netscape's FastTrack Server and Enterprise Server, Microsoft's Internet Information Server and Personal Web Server, O'Reilly's WebSite, Lotus Domino's Go Webserver, StarNine's WebSTAR, and Apple's AppleShare IP. This type of engine acts as a stopgap solution until a future server release incorporates servlet support. A plug-in also can be used with a server that provides a poor or outdated servlet implementation.
Add-on servlet engines include these:
1.2.3. Embeddable Servlet Engines
An embeddable engine is generally a lightweight servlet deployment platform that can be embedded in another application. That application becomes the true server.
1.2.4. Additional Thoughts
Before proceeding, we feel obliged to point out that not all servlet engines are created equal. So, before you choose a servlet engine (and possibly a server) with which to deploy your servlets, take it out for a test drive. Kick its tires a little. Check the mailing lists. Always verify that your servlets behave as they do in the Java Web Server implementation. With servlets, you don't have to worry about the lowest-common-denominator implementation, so you should pick a servlet engine that has the functionality that you want.
For a complete, up-to-date list of available servlet engines, see the official list maintained by Sun at:
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