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Tomcat three covers the Servlet 2.2 and JSP 1.1 revisions, while Tomcat 4 covers Servlet 2.3 and JSP 1.2. Tomcat is the Apache Foundation's reference implementation of the JavaServer Pages and Servlet technologies. Tomcat itself is part of the Jakarta Project, which is a suite of Java development tools developed through the Apache foundation. It is a good idea to 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.
In order 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? When Tomcat executes as well as creating an HTTP listener service, it also creates a number of worker processes that are responsible for executing JSPs/servlets. The mod_jk module is written in C, so it is incapable of interpreting Java classes directly. Instead, mod_jk communicates with the various worker processes created by Tomcat through a network connection.
The main benefit to this is flexibility. For example, if you wanted to run Apache 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. Stability is another advantage. If a significant failure within Tomcat caused it to fail completely, it would not render your entire Apache service unusable, just your servlets and JSP pages.
Tomcat is very easy to install, as the only prerequisite is Java. If you do not already have the Java SDK you can get a copy of Java for most platforms here. After you have 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.
There is 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. 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.
It is possible to test your installation by opening up a web browser and entering a suitable URL for your machine. In addition, shutdown scripts are provided in the bin directory if you want to shutdown your Tomcat installation.
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