As indicated in the diagram above, Tomcat 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 ($JAVA_HOME/jre/lib/ext). Note: some JVMs may implement this as more than one class loader, or it may not be visible (as a class loader) at all.
System — This class loader is normally initialized from the contents of the CLASSPATH environment variable. All such classes are visible to both Tomcat internal classes, and to web applications. However, the standard Tomcat startup scripts ($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 server, and the class loader implementation classes it depends on.
- $CATALINA_BASE/bin/tomcat-juli.jar or $CATALINA_HOME/bin/tomcat-juli.jar — Logging implementation classes. These include enhancement classes to java.util.logging API, known as Tomcat JULI, and a package-renamed copy of Apache Commons
Logging library used internally by Tomcat. See logging documentation for more details.
- If tomcat-juli.jar is present in $CATALINA_BASE/bin, it is used instead of the one in$CATALINA_HOME/bin. It is useful in certain logging configurations
- $CATALINA_HOME/bin/commons-daemon.jar — The classes from Apache Commons Daemon project. This JAR file is not present in the CLASSPATH built by catalina.bat|.sh scripts, but is referenced from the manifest file of bootstrap.jar.
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. The locations searched by this class loader are defined by the common.loader property in $CATALINA_BASE/conf/catalina.properties. The default setting will search the following locations in the order they are listed:
- unpacked classes and resources in $CATALINA_BASE/lib
- JAR files in $CATALINA_BASE/lib
- unpacked classes and resources in $CATALINA_HOME/lib
- JAR files in $CATALINA_HOME/lib
By default, this includes the following:
- annotations-api.jar — JavaEE annotations classes.
- jar — Implementation of the Catalina servlet container portion of Tomcat.
- catalina-ant.jar — Tomcat Catalina Ant tasks.
- catalina-ha.jar — High availability package.
- catalina-storeconfig.jar — Generation of XML configuration files from current state
- catalina-tribes.jar — Group communication package.
- ecj-*.jar — Eclipse JDT Java compiler.
- el-api.jar — EL 3.0 API.
- jar — Tomcat Jasper JSP Compiler and Runtime.
- jasper-el.jar — Tomcat Jasper EL implementation.
- jsp-api.jar — JSP 2.3 API.
- servlet-api.jar — Servlet 3.1 API.
- tomcat-api.jar — Several interfaces defined by Tomcat.
- tomcat-coyote.jar — Tomcat connectors and utility classes.
- tomcat-dbcp.jar — Database connection pool implementation based on package-renamed copy of Apache Commons Pool and Apache Commons DBCP.
- 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.
- tomcat-jdbc.jar — An alternative database connection pool implementation, known as Tomcat JDBC pool.
- tomcat-spdy.jar — SPDY implementation
- tomcat-util.jar — Common classes used by various components of Apache Tomcat.
- tomcat-websocket.jar — WebSocket 1.0 implementation
- websocket-api.jar — WebSocket 1.0 API
WebappX — A class loader is created for each web application that is deployed in a single Tomcat instance. All unpacked classes and resources in the /WEB-INF/classes directory of your web application, plus classes and resources in JAR files under the /WEB-INF/lib directory of your web application, are made visible to this web application, but not to other ones.
As mentioned above, the web application class loader diverges from the default Java delegation model (in accordance with the recommendations in the Servlet Specification, version 2.4, 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 overridden. For some classes (such as the XML parser components in J2SE 1.4+), the J2SE 1.4 endorsed feature can be used. Last, any JAR file that contains Servlet API classes will be explicitly ignored by the classloader — Do not include such JARs in your web application. All other class loaders in Tomcat 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 classes (described above)
- /WEB-INF/classes of your web application
- /WEB-INF/lib/*.jar of your web application
- Common class loader classes (described above)
