If you write JavaServer Pages or use Servlets to provide the functionality of your Web site, you're probably already aware of Tomcat. Tomcat is the Apache Foundation's reference implementation of the JavaServer Pages and Servlet technologies. Tomcat 3 covers the Servlet 2.2 and JSP 1.1 revisions, while Tomcat 4 covers Servlet 2.3 and JSP 1.2. Tomcat itself is part of the Jakarta Project, which is a suite of Java development tools developed through the Apache foundation.
Installing Tomcat itself is relatively easy — download the corresponding installer from the Tomcat pages at Apache, expand the files or run the installer, and then use the corresponding script to start up the Tomcat service. Tomcat has its own built-in HTTP service that handles and services requests from clients. We'll look at the specific steps later in this article.
To integrate the Tomcat server processes with the Apache HTTP server we need the mod_jk module, which implements the interface between Tomcat and Apache, combined with some small steps to configure Apache and Tomcat to our needs.
Tomcat is written in Java, while Apache and its modules are written in C, so how do the two work together?
Well, when Tomcat executes as well as creating an HTTP listener service, it also creates a number of "worker" processes responsible for executing JSPs/servlets. The mod_jk module is written in C, so it's incapable of interpreting Java classes directly. Instead, mod_jk communicates with the various worker processes created by Tomcat through a network connection. There is, of course, slightly more to it than that, but the core execution is handled through Apache to Tomcat and back again.
There are some advantages to this method, even though it looks convoluted. The primary advantage is flexibility. For example, if you wanted to run Apache (and mod_jk) on one physical server but the Tomcat service and the actual JSPs and servlets on another machine, you can. Some companies use this method to provide an additional level of security, with the Tomcat server behind another firewall only accessible from the Apache server.
Another advantage is stability — if a significant failure within Tomcat caused it to fail completely, it wouldn't render your entire Apache service unusable, just your servlets and JSP pages.
We'll be using Tomcat 4.1.18, although the connector pack we'll be using will actually work with any version of Tomcat you want to use.
Installing Tomcat
Tomcat is very easy to install, as the only prerequisite is Java. If you don't already have the Java SDK (you need the SDK rather than the runtime) you can get a copy of Java for most platforms here.
After you've installed Java, make sure you set your JAVA_HOME environment variable to point to your Java installation, as it's needed not only by Tomcat but also by other tools we use in this article. You'll probably also want to set your PATH variable to point to the bin directory within the Java directory so you have easy access to all the Java tools.
Now download Tomcat. There's no installer under Unix/OS X, so once downloaded, extract the package and then copy the directory created to where you want to store your Tomcat installation. Within Windows use the installer.
Once installed, you can startup and shutdown Tomcat using the provided scripts. These are in the bin directory and called startup (Windows) or startup.sh (Unix). By default Tomcat runs on port 8080. To change this, edit the file conf/server.xml in the Tomcat directory and replace 8080 with the port number you want to use.
You can test your installation by opening up a web browser and entering a suitable URL for your machine.
Shutdown scripts are provided in the bin directory if you want to shutdown your Tomcat installation.
Installing mod_jk
The easiest way to get hold of a suitable mod_jk library is to go to the project's build directory. From there you'll find a number of directories for different platforms. Within each directory are two files, mod_jk for use with Apache 1.3.x and the other for use with Apache 2.x.
Download the file and copy it into your libexec or modules directory within your Apache server installation.
Building mod_jk
If you really want (or need) to, you can build your own mod_jk from the source. You'll need to download the Jakarta Tomcat Connectors pack. You'll also need Ant, the Apache Java build engine.
Original date of publication, 05/03/2003
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