Servlet Engines

Servlets

Basically, a servlet is a .class file that can be "plugged" into a Java enabled web server. A servlet extends the javax.servlet package. More information can be found on Javasoft.
Running a servlet requires a servlet engine on the web server.
This articles explains what a servlet engine is, and compares some servlet engines.

Servlets Engines

A servlet engine does the following tasks:

• It loads the .class file in the Java virtual machine running on the server.
  Note that there's a single Java virtual machine on the server, and the servlet is loaded once when it is called.
• It runs the servlet and provides it with a runtime enviro

The .class file is not loaded again until the servlet changes. Usually, you have to restart the server when the .class file is modified, but now, some servlet engines can reload the .class when it's changed. This is a very nice feature.

The servlet stays in memory, and the execution is very fast. Compared to CGI, the web server doesn't need to start a shell or run perl to perform the execution.

Currently the servers written in Java with full built in servlet engine are Java Server and Netforge. Servlets are core components for them.
But, there are commercial and freeware web servers that have built-in support for the Java Servlet API.
Here is a list of servlet engines, provided by Sun on jserv.javasoft.com:

Third Party Server Implementations:

• NetForge
• O'Reilly WebSite Professional V2 Server
• JSDK-Apache Project
• Weblogic Tengah
• Jigsaw
• AcmeServe
• Mort Bay's Jetty
• Lotus Domino Go Webserver
• IBM Internet Connection Server

Third Party Server Add-Ons

• JRun
• Servlet Express
• ServletExec for Windows & MacOS
• Servlet CGI Development Kit
• Apache Week's patch to our JSDK to use servlets with Apache 1.2

Server Support Via JSDK

• Netscape FastTrack 2.0, Enterprise 2.0, Enterprise 3.0
• IIS 2.0, 3.0
• Apache 1.1.3

Key features

Some features are really important in a given implementation of a servlet engine. According to us, they are:

• Dynamic reloading of modified servlets.
  Under Windows NT, it's very annoying to lauch the control panel in order to restart the server each time a servlet is modified. Under Unix, it's possible to write a cron that monitors the servlets and restart the server when one of them is modified, but that's not convenient.
• Packaging of servlets in .zip/.jar files, and dynamic reloading of modified .zip/.jar files.
• Servlet initialization parameters.
• Servlet chaining and filtering.
  A servlet can send a response with a precise content-type, and this may invoke another servlet using this feature. It's a kind of pipe.
• Virtual server support, with separate directories for each virtual server.
  Multiple servers can run on the same machine, using IP aliasing.
• tag support.
  Servlets can be invoked using this tag.

Feature Comparison

Feature	Java Web Server	ServletExec 1.0	JRun 2.0	Lotus Domino Go WebServer	Servlet Express	JSDK Apache project
Servlet Chains				-		-
Servlet Filters				-		-
Reloading Servlets				-		-
.zip/.jar Servlet archives				-		-
.zip/.jar Auto Reload				-	-	-
Virtual Servers	-				-
Tag				-	-

This page was last updated 06 March 1998