## Class Loader How-To

### Table of Contents

- Overview
- Class Loader Definitions
- XML Parsers and Java
- Advanced configuration

### Overview

Like many server applications, Tomcat 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 2.4 — in particular, Sections 9.4 and 9.6.

In a Java 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. Note, that the model for web application class loaders differs slightly from this, as discussed below, but the main principles are the same.

When Tomcat 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
           /     \
      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 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 — Jakarta Annotations 3.0 classes.
  - catalina.jar — Implementation of the Catalina servlet container portion of Tomcat.
  - catalina-ant.jar — Optional. Tomcat Catalina Ant tasks for working with the Manager web application.
  - catalina-ha.jar — Optional. High availability package that provides session clustering functionality built on Tribes.
  - catalina-ssi.jar — Optional. Server-side Includes module.
  - catalina-storeconfig.jar — Optional. Generation of XML configuration files from current state.
  - catalina-tribes.jar — Optional. Group communication package used by the high availability package.
  - ecj-*.jar — Optional. Eclipse JDT Java compiler used to compile JSPs to Servlets.
  - el-api.jar — Optional. EL 6.0 API.
  - jakartaee-migration-*-shaded.jar — Optional. Provides conversion of web applications from Java EE 8 to Jakarta EE 9.
  - jasper.jar — Optional. Tomcat Jasper JSP Compiler and Runtime.
  - jasper-el.jar — Optional. Tomcat EL implementation.
  - jaspic-api.jar — Jakarta Authentication 3.1 API.
  - jsp-api.jar — Optional. Jakarta Pages 4.0 API.
  - servlet-api.jar — Jakarta Servlet 6.1 API.
  - tomcat-api.jar — Several interfaces defined by Tomcat.
  - tomcat-coyote.jar — Tomcat connectors and utility classes.
  - tomcat-dbcp.jar — Optional. Database connection pool implementation based on package-renamed copy of Apache Commons Pool 2 and Apache Commons DBCP 2.
  - 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 — Optional. An alternative database connection pool implementation, known as Tomcat JDBC pool. See documentation for more details.
  - tomcat-jni.jar — Provides the integration with the Tomcat Native library.
  - tomcat-util.jar — Common classes used by various components of Apache Tomcat.
  - tomcat-util-scan.jar — Provides the class scanning functionality used by Tomcat.
  - tomcat-websocket.jar — Optional. Jakarta WebSocket 2.2 implementation
  - websocket-api.jar — Optional. Jakarta WebSocket 2.2 API
  - websocket-client-api.jar — Optional. Jakarta WebSocket 2.2 Client 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. There are some exceptions such as the XML parser components which can be overridden using the upgradeable modules feature. Lastly, the web application class loader will always delegate first for Jakarta EE API classes for the specifications implemented by Tomcat (Servlet, JSP, EL, WebSocket). 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
- /WEB-INF/classes of your web application
- /WEB-INF/lib/*.jar of your web application
- System class loader classes (described above)
- Common class loader classes (described above)

If the web application class loader is configured with `<Loader delegate="true"/>` then the order becomes:

- Bootstrap classes of your JVM
- System class loader classes (described above)
- Common class loader classes (described above)
- /WEB-INF/classes of your web application
- /WEB-INF/lib/*.jar of your web application

### XML Parsers and Java

In older versions of Tomcat, you could simply replace the XML parser in the Tomcat libraries directory to change the parser used by all web applications. However, this technique will not be effective when you are running modern versions of Java, because the usual class loader delegation process will always choose the implementation inside the JDK in preference to this one.

Java supports a mechanism called upgradeable modules to allow replacement of APIs created outside of the JCP (i.e. DOM and SAX from W3C). It can also be used to update the XML parser implementation.

Note that overriding any JRE component carries risk. If the overriding component does not provide a 100% compatible API (e.g. the API provided by Xerces is not 100% compatible with the XML API provided by the JRE) then there is a risk that Tomcat and/or the deployed application will experience errors.

### Advanced configuration

A more complex class loader hierarchy may also be configured. See the diagram below. By default, the Server and Shared class loaders are not defined and the simplified hierarchy shown above is used. This more complex hierarchy may be use by defining values for the `server.loader` and/or `shared.loader` properties in `conf/catalina.properties`.

      Bootstrap
          |
        System
          |
        Common
         /  \
    Server  Shared
             /  \
       Webapp1  Webapp2 ...

The Server class loader is only visible to Tomcat internals and is completely invisible to web applications.

The Shared class loader is visible to all web applications and may be used to shared code across all web applications. However, any updates to this shared code will require a Tomcat restart.