Apache Tomcat Development
Class Loader HOW-TO
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 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 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:
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.bat) totally ignore the contents
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_HOME/bin/tomcat-juli.jar - Package renamed Commons
logging API, and java.util.logging LogManager.
- 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/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 Tomcat.
- catalina-ant.jar - Tomcat Catalina Ant tasks.
- catalina-ha.jar - High availability package.
- catalina-tribes.jar - Group communication package.
- el-api.jar - EL 2.1 API.
- jasper.jar - Jasper 2 Compiler and Runtime.
- jasper-el.jar - Jasper 2 EL implementation.
- ecj-*.jar - Eclipse JDT Java compiler.
- jsp-api.jar - JSP 2.1 API.
- servlet-api.jar - Servlet 3.0 API.
- tomcat-coyote.jar - Tomcat 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 Tomcat instance. All unpacked
classes and resources in the
/WEB-INF/classes directory of
your web application archive, plus classes and resources in JAR files
/WEB-INF/lib directory of your web application
archive, are made visible to the containing web application, but to
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
Last, any JAR containing servlet API classes will be ignored by the
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
|XML Parsers and JSE 5
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
$CATALINA_HOME/common/lib directory to change the parser
used by all web applications. However, this technique will not be effective
when you are running on JSE 5, because the usual class loader delegation
process will always choose the implementation inside the JDK in preference
to this one.
JDK 1.5 supports a mechanism called the "Endorsed Standards Override
Mechanism" to allow replacement of APIs created outside of the JCP (i.e.
DOM and SAX from W3C). It can also be used to update the XML parser
implementation. For more information, see:
Tomcat utilizes this mechanism by including the system property setting
-Djava.endorsed.dirs=$JAVA_ENDORSED_DIRS in the
command line that starts the container.