Developing Applications with Tomcat

Deployment Organization

Background

Before describing how to organize your source code directories, it is useful to examine the runtime organization of a web application. Prior to the Servlet API Specification, version 2.2, there was little consistency between server platforms. However, servers that conform to the 2.2 specification are required to accept a Web Application Archive in a standard format, which is discussed further below.

A web application is defined as a hierarchy of directories and files in a standard layout. Such a hierarchy can be accessed in its "unpacked" form, where each directory and file exists in the filesystem separately, or in a "packed" form known as a Web ARchive, or WAR file. The former format is more useful during development, while the latter is used when you distribute your application to be installed.

The top-level directory of your web application hierarchy is also the document root of your application. Here, you will place the HTML files and JSP pages that comprise your application's user interface. When the system administrator deploys your application into a particular server, he or she assigns a context path to your application. Thus, if the system administrator assigns your application to the context path /catalog, then a request URI referring to /catalog/index.html will retrieve the index.html file from your document root.

Standard Directory Layout

To facilitate creation of a Web Application Archive file in the required format, it is convenient to arrange the files that Tomcat actually uses when executing your application in the same organization as required by the WAR format itself. To do this, you will have the following contents in your application's "document root" directory:

• *.html, *.jsp, etc. - The HTML and JSP pages, along with other files that must be visible to the client browser (such as JavaScript and stylesheet files) for your application. In larger applications you may choose to divide these files into a subdirectory hierarchy, but for smaller apps, it is generally much simpler to maintain only a single directory for these files.

• WEB-INF/web.xml - The Web Application Deployment Descriptor for your application. This is an XML file describing the servlets and other components that make up your application, along with any initialization parameters and container-managed security constraints that you want the server to enforce for you. This file is discussed in more detail in the following subsection.

• WEB-INF/classes/ - This directory contains any Java class files (and associated resources) required for your application, including both servlet and non-servlet classes, that are not combined into JAR files.

• WEB-INF/lib/ - This directory contains JAR files that contain Java class files (and associated resources) required for your application, such as third party class libraries or JDBC drivers.

When you install an application into Tomcat (or any other 2.2-compatible server), the classes in the WEB-INF/classes/ directory, as well as all classes in JAR files found in the WEB-INF/lib/ directory, are added to the class path for your particular web application. Thus, if you include all of the required library classes in one of these places (be sure to check licenses for redistribution rights for any third party libraries you utilize), you will simplify the installation of your web application -- no adjustment to the system class path will be necessary.

Web Application Deployment Descriptor

As mentioned above, the WEB-INF/web.xml file contains the Web Application Deployment Descriptor for your application. As the filename extension implies, this file is an XML document, and defines everything about your application that a server needs to know (except the context path, which is assigned by t he system administrator when the application is deployed).

The complete syntax and semantics for the deployment descriptor is defined in Chapter 13 of the Servlet API Specification, version 2.2. Over time, it is expected that development tools will be provided that create and edit the deployment descriptor for you. In the meantime, to provide a starting point, a basic web.xml (text file) is provided in the doc/appdev. This file includes comments that describe the purpose of each included element.

Integration With Tomcat

In order to be executed, a web application must be integrated with, or installed in, a servlet container. This is true even during development. We will describe using Tomcat to provide the execution environment. A web application can be deployed in Tomcat by one of three different approaches:

• Copy unpacked directory hierarchy into a subdirectory in directory %TOMCAT_HOME%/webapps/. Tomcat will assign a context path to your application based on the subdirectory name you choose. We will use this technique in the build.xml file that we construct, because it is the quickest and easiest approach during development. That is, if you choose your sub-directory in webapps to be students for your application it will be similar to your context path.

• Copy the web application archive file into directory %TOMCAT_HOME%/webapps/. When Tomcat is started, it will automatically expand the web application archive file into its unpacked form, and execute the application that way. This approach would typically be used to install an additional application, provided by a third party vendor or by your internal development staff, into an existing Tomcat installation.

• Add a <Context> entry in the Tomcat server.xml configuration file. This approach is described briefly below, and allows you to position the document root of your web application at some point other than the %TOMCAT_HOME%/webapps/ directory.

Integrating your app with other servlet containers will be specific to each container, but all containers compatible with the Servlet API Specification (version 2.2) are required to accept a web application archive file.