Class Loader INFO

Quick Start

The following rules cover about 95% of the decisions that application developers and deployers must make about where to place class and resource files to make them available to web applications:

For classes and resources specific to a particular web application, place unpacked classes and resources under /WEB-INF/classe of your web application archive, or place JAR files containing those classes and resources under /WEB-INF/lib of your web application archive.

For classes and resources that must be shared across all web applications, place unpacked classes and resources under $CATALINA_HOME/classes, or place JAR files containing those classes and resources under $CATALINA_HOME/lib.

Overview

Like many server applications, Tomcat 4 installs a variety of class loaders (that is, classes that implement java.lang.ClassLoader) to allow different portions of the container, and the web applications running on the container, to have access to different repositories of available classes and resources. This mechanism is used to provide the functionality defined in the Servlet Specification, version 2.3 -- in particular, Sections 9.4 and 9.6.

In a Java 2 (that is, JDK 1.2 or later) environment, class loaders are arranged in a parent-child tree. Normally, when a class loader is asked to load a particular class or resource, it delegates the request to a parent class loader first, and then looks in its own repositories only if the parent class loader(s) cannot find the requested class or resource. The model for web application class loaders differs slightly from this, as discussed below, but the main principles are the same.

When Tomcat 4 is started, it creates a set of class loaders that are organized into the following parent-child relationships, where the parent class loader is above the child class loader:
      Bootstrap
          |
       System
          |
       Common
      /      \
 Catalina   Shared
             /   \
        Webapp1  Webapp2 ... 
The characteristics of each of these class loaders, including the source of classes and resources that they make visible, are discussed in detail in the following section.

Class Loader Definitions

As indicated in the diagram above, Tomcat 4 creates the following class loaders as it is initialized:

Bootstrap - This class loader contains the basic runtime classes provided by the Java Virtual Machine, plus any classes from JAR files present in the System Extensions directory ($JAVA_HOME/jre/lib/ext). NOTE - Some JVMs may implement this as more than one class loader, or it may not be visible (as a class loader) at all.

System - This class loader is normally initialized from the contents of the CLASSPATH environment variable. All such classes are visible to both Tomcat internal classes, and to web applications. However, the standard Tomcat 4 startup scripts ($CATALINA_HOME/bin/catalina.sh or %CATALINA_HOME%\bin\catalina.bat) totally ignore the contents of the CLASSPATH environment variable itself, and instead build the System class loader from the following repositories:

$CATALINA_HOME/bin/bootstrap.jar - Contains the main() method that is used to initialize the Tomcat 4 server, and the class loader implementation classes it depends on.

$JAVA_HOME/lib/tools.jar - Contains the "javac" compiler used to convert JSP pages into servlet classes.

Common - This class loader contains additional classes that are made visible to both Tomcat internal classes and to all web applications. Normally, application classes should NOT be placed here. All unpacked classes and resources in $CATALINA_HOME/common/classes, as well as classes and resources in JAR files under $CATALINA_HOME/common/lib, are made visible through this class loader. By default, that includes the following:

activation.jar - The Java activation framework.

jdbc2_0-stdext.jar - JDBC 2.0 standard extension (included as a standard part of JDBC 3.0).

jndi.jar - The Java Naming and Directory Interface API classes (loaded ONLY on a JDK 1.2 system, because they are included automatically on JDK 1.3 and later).

jta-spec.jar - The Java Transaction API interfaces.

mail.jar - Java Mail.

naming-common.jar - The JNDI implementation used by Tomcat 4 to represent the default JNDI naming context provided to web applications.

naming-resources.jar - JNDI Directory Context implementations which are used to abstract access to the static resources of a web application.

servlet.jar - The Servlet and JSP API classes.

tyrex-0.9.7.jar - JTA/JTS/OTS compliant transaction manager and DataSource connection pool implementation.

xerces.jar - The Xerces 1.x XML parser (also includes the JAXP 1.1 interfaces).

Catalina - This class loader is initialized to include all classes and resources required to implement Tomcat 4 itself. These classes and resources are TOTALLY invisible to web applications. All unpacked classes and resources in $CATALINA_HOME/server/classes, as well as classes and resources in JAR files under $CATALINA_HOME/server/lib, are made visible through this class loader. By default, that includes the following:

catalina.jar - Implementation of the Catalina servlet container portion of Tomcat 4.

jakarta-regexp-X.Y.jar - The binary distribution of the Jakarta Regexp regular expression processing library, used in the implementation of request filters.

servlets-xxxx.jar - The standard suite of servlets which provide basic services, like static page serving, WebDAV support, CGI, SSI, and more.

tomcat-util.jar - Utility components used by modules from the jakarta-tomcat-connectors subproject.

tomcat-ajp.jar - The Java portion of the AJP 1.x connector (also know as the JK connector), which provides support for connecting native webservers, and load balancing between multiple Tomcat instances.

warp.jar - Classes for the Java portion of the mod_webapp web server connector, which allows Tomcat to run behind web servers such as Apache and iPlanet iAS and iWS.

Shared - This class loader is the place to put classes and resources that you wish to share across ALL web applications (unless Tomcat internal classes also need access, which is an unusual case). All unpacked classes and resources in $CATALINA_HOME/classes, as well as classes and resources in JAR files under $CATALINA_HOME/lib, are made visible through this class loader. By default, that includes the following:

jasper-compiler.jar - The Jasper JSP page compiler.

jasper-runtime.jar - The runtime support classes required to execute JSP pages that have already been translated into Java servlets and then compiled.

naming-factory.jar - JNDI object factories for resources supported by the default JNDI naming context provided to web applications.

WebappX - A class loader is created for each web application that is deployed in a single Tomcat 4 instance. All unpacked classes and resources in the /WEB-INF/classes directory of your web application archive, plus classes and resources in JAR files under the /WEB-INF/lib directory of your web application archive, are made visible to the containing web application, but to no others.

As mentioned above, the web application class loader diverges from the default Java 2 delegation model (in accordance with the recommendations in the Servlet Specification, version 2.3, section 9.6). When a request to load a class from the web application's WebappX class loader is processed, this class loader will look in the local repositories first, instead of delegating before looking. All other class loaders in Tomcat 4 follow the usual delegation pattern.

Therefore, from the perspective of a web application, class or resource loading looks in the following repositories, in this order:

/WEB-INF/classes of your web application

/WEB-INF/lib/*.jar of your web application

Bootstrap classes of your JVM

System class loader classses (described above)

$CATALINA_HOME/common/classes

$CATALINA_HOME/common/lib/*.jar

$CATALINA_HOME/classes

$CATALINA_HOME/lib/*.jar