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 (or later) 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 (a later section of this manual describes deployment on Tomcat). 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.

To facilitate creation of a Web Application Archive file in the required format, it is convenient to arrange the "executable" files of your web application (that is, the files that Tomcat actually uses when executing your app) in the same organization as required by the WAR format itself. To do this, you will end up with 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, stylesheet files, and images) 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. If your classes are organized into Java packages, you must reflect this in the directory hierarchy under /WEB-INF/classes/. For example, a Java class named com.mycompany.mypackage.MyServlet would need to be stored in a file named /WEB-INF/classes/com/mycompany/mypackage/MyServlet.class.
  
  
• /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 2.2/2.3-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 made visible to other classes within 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 (or installation of global library files in your server) will be necessary.

Much of this information was extracted from Chapter 9 of the Servlet API Specification, version 2.3, which you should consult for more details.

Like most servlet containers, Tomcat 5 also supports mechanisms to install library JAR files (or unpacked classes) once, and make them visible to all installed web applications (without having to be included inside the web application itself. The details of how Tomcat locates and shares such classes are described in the Class Loader HOW-TO documentation. For the purposes of our discussion, there are two locations that are commonly used within a Tomcat 5 installation for shared code:

• $CATALINA_HOME/common/lib - JAR files placed here are visible both to web applications and internal Tomcat code. This is a good place to put JDBC drivers that are required for both your application and internal Tomcat use (such as for a JDBCRealm).
  
  
• $CATALINA_BASE/shared/lib - JAR files placed here are visible to all web applications, but not to internal Tomcat code. This is the right place for shared libraries that are specific to your application.
  
  

Out of the box, a standard Tomcat 5 installation includes a variety of pre-installed shared library files, including:

• The Servlet 2.4 and JSP 2.0 APIs that are fundamental to writing servlets and JavaServer Pages.
  
  
• An XML Parser compliant with the JAXP (version 1.2) APIs, so your application can perform DOM-based or SAX-based processing of XML documents.
  
  

The description below uses the variable name $CATALINA_HOME to refer to the directory into which you have installed Tomcat 5, and is the base directory against which most relative paths are resolved. However, if you have configured Tomcat 5 for multiple instances by setting a CATALINA_BASE directory, you should use $CATALINA_BASE instead of $CATALINA_HOME for each of these references.

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 the 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 2.3. 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 file is provided. This file includes comments that describe the purpose of each included element.

NOTE - The Servlet Specification includes a Document Type Descriptor (DTD) for the web application deployment descriptor, and Tomcat 5 enforces the rules defined here when processing your application's /WEB-INF/web.xml file. In particular, you must enter your descriptor elements (such as <filter>, <servlet>, and <servlet-mapping> in the order defined by the DTD (see Section 13.3).

The description below uses the variable name $CATALINA_HOME to refer to the directory into which you have installed Tomcat 5, and is the base directory against which most relative paths are resolved. However, if you have configured Tomcat 5 for multiple instances by setting a CATALINA_BASE directory, you should use $CATALINA_BASE instead of $CATALINA_HOME for each of these references.

A /META-INF/context.xml file can be used to define Tomcat specific configuration options, such as loggers, data sources, session manager configuration and more. This XML file must contain one Context element, which will be considered as if it was the child of the Host element corresponding to the Host to which the The Tomcat configuration documentation contains information on the Context element.

In order to be executed, a web application must be deployed on a servlet container. This is true even during development. We will describe using Tomcat 5 to provide the execution environment. A web application can be deployed in Tomcat by one of the following approaches:

• Copy unpacked directory hierarchy into a subdirectory in directory $CATALINA_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. Be sure to restart Tomcat after installing or updating your application.
  
  
• Copy the web application archive file into directory $CATALINA_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. NOTE - If you use this approach, and wish to update your application later, you must both replace the web application archive file AND delete the expanded directory that Tomcat created, and then restart Tomcat, in order to reflect your changes.
  
  
• Use the Tomcat 5 "Manager" web application to deploy and undeploy web applications. Tomcat 5 includes a web application, deployed by default on context path /manager, that allows you to deploy and undeploy applications on a running Tomcat server without restarting it. See the administrator documentation (TODO: hyperlink) for more information on using the Manager web application.
  
  
• Use "Manager" Ant Tasks In Your Build Script. Tomcat 5 includes a set of custom task definitions for the Ant build tool that allow you to automate the execution of commands to the "Manager" web application. These tasks are used in the Tomcat deployer.
  
  
• Use the Tomcat Deployer. Tomcat 5 includes a packaged tool bundling the Ant tasks, and can be used to automatically precompile JSPs which are part of the web application before deployment to the server.
  
  

Deploying your app on other servlet containers will be specific to each container, but all containers compatible with the Servlet API Specification (version 2.2 or later) are required to accept a web application archive file. Note that other containers are NOT required to accept an unpacked directory structure (as Tomcat does), or to provide mechanisms for shared library files, but these features are commonly available.