Links User Guide Reference Apache Tomcat Development | Class Loader HOW-TO| 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.
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| 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 — JavaEE 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.
- ecj-*.jar — Eclipse JDT Java compiler.
- el-api.jar — EL 2.2 API.
- jasper.jar — Tomcat Jasper JSP Compiler and Runtime.
- jasper-el.jar — Tomcat Jasper EL implementation.
- jsp-api.jar — JSP 2.2 API.
- servlet-api.jar — Servlet 3.0 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. See
documentation for more details.
- tomcat-util.jar — Common classes used by various components of
Apache Tomcat.
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
- /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 configuered with
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
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| XML Parsers and Java |
Starting with Java 1.4 a copy of JAXP APIs and an XML parser are packed
inside the JRE. This has impacts on applications that wish to use their own
XML parser.
In old 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 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://docs.oracle.com/javase/1.5.0/docs/guide/standards/index.html.
Tomcat utilizes this mechanism by including the system property setting
-Djava.endorsed.dirs=$JAVA_ENDORSED_DIRS in the
command line that starts the container. The default value of this option is
$CATALINA_HOME/endorsed. This endorsed directory is not
created by default.
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