Class Loader HOW-TO
Like many server applications, JBoss Web 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 J2SE 2 (that is, J2SE 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 JBoss Web 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.
As indicated in the diagram above, JBoss Web 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
CLASSPATHenvironment variable. All such classes are visible to both Tomcat internal classes, and to web applications. However, the standard JBoss Web startup scripts (
%CATALINA_HOME%\bin\catalina.bat) totally ignore the contents of the
CLASSPATHenvironment 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 JBoss Web server, and the class loader implementation classes it depends on.
- $CATALINA_HOME/bin/tomcat-juli.jar - JBoss Logging API, and java.util.logging LogManager.
- Common - This class loader contains additional classes
that are made visible to both JBoss Web internal classes and to all web
applications. Normally, application classes should NOT
be placed here. All unpacked classes and resources in
$CATALINA_HOME/lib, as well as classes and resources in JAR files are made visible through this class loader. By default, that includes the following:
- annotations-api.jar - JEE annotations classes.
- catalina.jar - Implementation of the Catalina servlet container portion of JBoss Web.
- catalina-ant.jar - JBoss Web Catalina Ant tasks.
- el-api.jar - EL 2.1 API.
- jasper.jar - Jasper 2 Compiler and Runtime.
- jasper-el.jar - Jasper 2 EL implementation.
- jasper-jdt.jar - Eclipse JDT 3.2 Java compiler.
- jsp-api.jar - JSP 2.1 API.
- servlet-api.jar - Servlet 2.5 API.
- tomcat-coyote.jar - JBoss Web connectors and utility classes.
- tomcat-dbcp.jar - package renamed database connection pool based on 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.
- WebappX - A class loader is created for each web
application that is deployed in a single JBoss Web instance. All unpacked
classes and resources in the
/WEB-INF/classesdirectory of your web application archive, plus classes and resources in JAR files under the
/WEB-INF/libdirectory 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. Last, any JAR containing servlet API classes will be ignored by the classloader. All other class loaders in JBoss Web 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
Among many other changes, the JSE 5 release packages the JAXP APIs, and a version of Xerces, inside the JRE. This has impacts on applications that wish to use their own XML parser.
In previous versions of Tomcat, you could simply replace the XML parser
$CATALINA_HOME/common/lib directory to change the parser
used by all web applications. However, this technique will not be effective
when you are running on JSE 5, because the usual class loader delegation
process will always choose the implementation inside the JDK in preference
to this one.
JDK 1.5 supports a mechanism called the "Endorsed Standards Override Mechanism" to allow replacement of APIs created outside of the JCP (i.e. DOM and SAX from W3C). It can also be used to update the XML parser implementation. For more information, see: http://java.sun.com/j2se/1.5/docs/guide/standards/index.html.
JBoss Web utilizes this mechanism by including the system property setting
-Djava.endorsed.dirs=$CATALINA_HOME/endorsed in the
command line that starts the container.
When running under a security manager the locations from which classes are permitted to be loaded will also depend on the contents of your policy file. See Security Manager HOW-TO for further information.