Tomcat Class Loader Definition

Class Loader Definitions

Tomcat 5 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 5 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 5 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.

$CATALINA_HOME/bin/commons-logging-api-x.y.z.jar - Jakarta commons logging API.

$CATALINA_HOME/bin/commons-daemon.jar - Jakarta commons daemon API.

jmx.jar - The JMX 1.2 implementation.

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 the $CATALINA_HOME/commons/endorsed, $CATALINA_HOME/commons/i18n and $CATALINA_HOME/common/lib directories, are made visible through this class loader. By default, that includes the following:
commons-el.jar - Jakarta commons el, implementing the expression language used by Jasper.

jasper-compiler.jar - The JSP 2.0 compiler.

jasper-compiler-jdt.jar - The Eclipse JDT Java compiler.

jasper-runtime.jar - The JSP 2.0 runtime.
jsp-api.jar - The JSP 2.0 API.

naming-common.jar - The JNDI implementation used by Tomcat 5 to represent in-memory naming contexts.

naming-factory.jar - The JNDI implementation used by Tomcat 5 to resolve references to enterprise resources (EJB, connection pools).

naming-factory-dbcp.jar - Jakarta commons DBCP, providing a JDBC connection pool to web applications. The classes have been moved out of their default org.apache.commons package.
naming-java.jar - Handler for the java: namespace.

naming-resources.jar - The specialized JNDI naming context implementation used to represent the static resources of a web application. This is not related to the support of the J2EE ENC, and cannot be removed.

servlet-api.jar - The Servlet 2.4 API.

tomcat-i18n-**.jar - Optional JARs containing resource bundles for other languages. As default bundles are also included in each individual JAR, they can be safely removed if no internationalization of messages is needed.

Catalina - This class loader is initialized to include all classes and resources required to implement Tomcat 5 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 5.

catalina-ant.jar - Some Ant tasks which can be used to manage Tomcat using the manager web application.

catalina-optional.jar - Some optional components of Catalina.

commons-modeler.jar - A model MBeans implementation used by Tomcat to expose its internal objects through JMX.

servlets-xxxxx.jar - The classes associated with each internal servlet that provides part of Tomcat's functionality. These are separated so that they can be completely removed if the corresponding service is not required, or they can be subject to specialized security manager permissions.

tomcat-coyote.jar - Coyote API.

tomcat-http.jar - Standalone Java HTTP/1.1 connector.

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

tomcat-util.jar - Utility classes required by some Tomcat connectors.

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, in which case you should put them in the Common class loader instead). All unpacked classes and resources in $CATALINA_BASE/shared/classes, as well as classes and resources in JAR files under $CATALINA_BASE/shared/lib, are made visible through this class loader. If multiple Tomcat instances are run from the same binary using the $CATALINA_BASE environment variable, then this classloader repositories are relative to $CATALINA_BASE rather than $CATALINA_HOME.
WebappX - A class loader is created for each web application that is deployed in a single Tomcat 5 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 9.7.2 Web Application Classloader). 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. There are exceptions. Classes which are part of the JRE base classes cannot be overriden. For some classes (such as the XML parser components in J2SE 1.4+), the J2SE 1.4 endorsed feature can be used (see the common classloader definition above). Last, any JAR containing servlet API classes will be ignored by the classloader. All other class loaders in Tomcat 5 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:
Bootstrap classes of your JVM
System class loader classses (described above)
/WEB-INF/classes of your web application
/WEB-INF/lib/*.jar of your web application
$CATALINA_HOME/common/classes
$CATALINA_HOME/common/endorsed/*.jar
$CATALINA_HOME/common/i18n/*.jar
$CATALINA_HOME/common/lib/*.jar
$CATALINA_BASE/shared/classes
$CATALINA_BASE/shared/lib/*.jar