Tomcat is an open source web server and servlet container developed by the Apache Software Foundation (ASF). Tomcat implements the Java Servlet and the JavaServer Pages (JSP) specifications from Sun Microsystems, and provides a “pure Java” HTTP web server environment for Java code to run in. Apache Tomcat includes tools for configuration and management, but can also be configured by editing XML configuration files.
Tomcat started off as a servlet reference implementation by James Duncan Davidson, a software architect at Sun Microsystems. He later helped make the project open source and played a key role in its donation by Sun Microsystems to the Apache Software Foundation. The Apache Ant software build automation tool was developed as a side-effect of the creation of Tomcat as an open source project.
Davidson had initially hoped that the project would become open sourced and, since many open source projects and O’Reilly books associated with them featuring an animal on the cover, he wanted to name the project after an animal. Davidson decided on Tomcat since he reasoned the animal represented something that could fend for itself.
Catalina is Tomcat’s servlet container. It implements Sun Microsystems’ specifications for servlet and JavaServer Pages (JSP). In Tomcat, a Realm element represents a “database” of usernames, passwords and roles (similar to Unix groups) assigned to those users. Different implementations of Realm allow Catalina to be integrated into environments where such authentication information is already being created and maintained, and then use that information to implement Container Managed Security as described in the Servlet Specification.
Coyote is Tomcat’s HTTP Connector component that supports HTTP 1.1 protocol for the web server or application container. Coyote listens for incoming connections on a specific TCP port on the server and forwards the request to the Tomcat Engine to process the request and send back a response to the requesting client. It can execute JSP’s and Servlets.
Jasper is Tomcat’s JSP Engine. Jasper parses JSP files to compile them into Java code as servlets (that can be handled by Catalina). At runtime, Jasper detects changes to JSP files and reorganizes them.
As of version 5, Tomcat uses Jasper 2, which is an implementation of the Sun Microsystems JSP 2.0 specification. From Jasper to Jasper 2, the important features added include:
- JSP Tag library pooling – Each tag markup in JSP file is handled by a tag handler class. Tag handler class objects can be pooled and reused in the whole JSP servlet.
- Background JSP compilation – While compiling modified JSP Java code, the older version is still available for server requests. The older JSP servlet is deleted once the new JSP servlet has finished being compiled.
- Recompile JSP when included page changes – Pages can be inserted and included into a JSP at runtime. The JSP will not only be recompiled with JSP file changes but also with included page changes.
- JDT Java compiler – Jasper 2 can use the Eclipse JDT (Java Development Tools) Java compiler instead of Ant and javac.
This component has been added to manage large applications. It is used for load balancing that can be achieved through many techniques. Clustering support currently requires the JDK version 1.5 or later.
A high-availability feature has been added to facilitate the scheduling of system upgrades (e.g. new releases, change requests) without affecting the live environment. This is done by dispatching live traffic requests to a temporary server on a different port while the main server is upgraded on the main port. It is very useful in handling user requests on high-traffic web applications.
It has also added user as well as system based web applications enhancement to add support for deployment across the variety of environments, while also trying to manage session as well as applications across the network.
Tomcat is building additional components. A number of additional components may be used with Apache Tomcat. These components may be built by users if they need them or they can be downloaded from one of the mirrors.
Tomcat 7.x implements the Servlet 3.0 and JSP 2.2 specifications. It requires Java version 1.6, although previous versions have run on Java 1.1 through 1.5 Versions 5 through 6 saw improvements in garbage collection, JSP parsing, performance and scalability. Native wrappers, known as “Tomcat Native”, are available for Microsoft Windows and Unix for platform integration.
Apache software is built as part of a community process that involves both user and developer mailing lists. The developer list is where discussion on building and testing the next release takes place, while the users can discuss their problems with the developers and other users.
Some of the free Apache Tomcat resources and communities include Tomcatexpert.com (a SpringSource sponsored community for developers and operators who are running Apache Tomcat in large-scale production environments) and MuleSoft’s Apache Tomcat Resource Center (which has instructional guides on installing, updating, configuring, monitoring, troubleshooting and securing various versions of Tomcat).