webapps/docs/class-loader-howto.xml - tomcat - Git at Google

 <?xml version="1.0"?>
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 <!DOCTYPE document [
   <!ENTITY project SYSTEM "project.xml">
 ]>
 <document url="class-loader-howto.html">

     &project;

     <properties>
         <author email="craigmcc@apache.org">Craig R. McClanahan</author>
         <author email="yoavs@apache.org">Yoav Shapira</author>
         <title>Class Loader HOW-TO</title>
     </properties>

 <body>

 <section name="Table of Contents">
 <toc/>
 </section>

 <section name="Overview">

 <p>Like many server applications, Tomcat installs a variety of class loaders
 (that is, classes that implement <code>java.lang.ClassLoader</code>) 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.</p>

 <p>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.</p>

 <p>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:</p>

 <source>
       Bootstrap
           |
        System
           |
        Common
        /     \
   Webapp1   Webapp2 ...
 </source>

 <p>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.</p>

 </section>

 <section name="Class Loader Definitions">

 <p>As indicated in the diagram above, Tomcat creates the following class
 loaders as it is initialized:</p>
 <ul>
 <li><strong>Bootstrap</strong> - 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
     (<code>$JAVA_HOME/jre/lib/ext</code>).  <em>NOTE</em> - Some JVMs may
     implement this as more than one class loader, or it may not be visible
     (as a class loader) at all.</li>
 <li><strong>System</strong> - This class loader is normally initialized from
     the contents of the <code>CLASSPATH</code> environment variable.  All such
     classes are visible to both Tomcat internal classes, and to web
     applications.  However, the standard Tomcat startup scripts
     (<code>$CATALINA_HOME/bin/catalina.sh</code> or
     <code>%CATALINA_HOME%\bin\catalina.bat</code>) totally ignore the contents
     of the <code>CLASSPATH</code> environment variable itself, and instead
     build the System class loader from the following repositories:
     <ul>
     <li><em>$CATALINA_HOME/bin/bootstrap.jar</em> - Contains the main() method
         that is used to initialize the Tomcat server, and the class loader
         implementation classes it depends on.</li>
     <li><em>$CATALINA_HOME/bin/tomcat-juli.jar</em> - Package renamed Commons
         logging API, and java.util.logging LogManager.</li>
     </ul></li>
 <li><strong>Common</strong> - This class loader contains additional classes
     that are made visible to both Tomcat internal classes and to all web
     applications.  Normally, application classes should <strong>NOT</strong>
     be placed here.  All unpacked classes and resources in
     <code>$CATALINA_HOME/lib</code>, as well as classes and
     resources in JAR files are made visible through this
     class loader.  By default, that includes the following:
     <ul>
     <li><em>annotations-api.jar</em> - JEE annotations classes.</li>
     <li><em>catalina.jar</em> - Implementation of the Catalina servlet
         container portion of Tomcat.</li>
     <li><em>catalina-ant.jar</em> - Tomcat Catalina Ant tasks.</li>
     <li><em>catalina-ha.jar</em> - High availability package.</li>
     <li><em>catalina-tribes.jar</em> - Group communication package.</li>
     <li><em>el-api.jar</em> - EL 2.1 API.</li>
     <li><em>jasper.jar</em> - Jasper 2 Compiler and Runtime.</li>
     <li><em>jasper-el.jar</em> - Jasper 2 EL implementation.</li>
     <li><em>ecj-*.jar</em> - Eclipse JDT Java compiler.</li>
     <li><em>jsp-api.jar</em> - JSP 2.1 API.</li>
     <li><em>servlet-api.jar</em> - Servlet 3.0 API.</li>
     <li><em>tomcat-coyote.jar</em> - Tomcat connectors and utility classes.</li>
     <li><em>tomcat-dbcp.jar</em> - package renamed database connection
         pool based on Commons DBCP.</li>
     <li><em>tomcat-i18n-**.jar</em> - 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.</li>
     </ul></li>
 <li><strong>WebappX</strong> - A class loader is created for each web
     application that is deployed in a single Tomcat instance.  All unpacked
     classes and resources in the <code>/WEB-INF/classes</code> directory of
     your web application archive, plus classes and resources in JAR files
     under the <code>/WEB-INF/lib</code> directory of your web application
     archive, are made visible to the containing web application, but to
     no others.</li>
 </ul>

 <p>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 <em>WebappX</em> class loader is processed,
 this class loader will look in the local repositories <strong>first</strong>,
 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 Tomcat follow the usual delegation pattern.</p>

 <p>Therefore, from the perspective of a web application, class or resource
 loading looks in the following repositories, in this order:</p>
 <ul>
 <li>Bootstrap classes of your JVM</li>
 <li>System class loader classes (described above)</li>
 <li><em>/WEB-INF/classes</em> of your web application</li>
 <li><em>/WEB-INF/lib/*.jar</em> of your web application</li>
 <li><em>$CATALINA_HOME/lib</em></li>
 <li><em>$CATALINA_HOME/lib/*.jar</em></li>
 </ul>

 </section>


 <section name="XML Parsers and JSE 5">

 <p>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.</p>

 <p>In previous versions of Tomcat, you could simply replace the XML parser
 in the <code>$CATALINA_HOME/common/lib</code> 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.</p>

 <p>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:
 <a href="http://java.sun.com/j2se/1.5/docs/guide/standards/index.html">
 http://java.sun.com/j2se/1.5/docs/guide/standards/index.html</a>.</p>

 <p>Tomcat utilizes this mechanism by including the system property setting
 <code>-Djava.endorsed.dirs=$JAVA_ENDORSED_DIRS</code> in the
 command line that starts the container.</p>

 </section>


 <section name="Running under a security manager">

 <p>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 <a href="security-manager-howto.html">Security Manager HOW-TO</a>
 for further information.</p>

 </section>


 </body>

 </document>
	<?xml version="1.0"?>
	<!--
	Licensed to the Apache Software Foundation (ASF) under one or more
	contributor license agreements. See the NOTICE file distributed with
	this work for additional information regarding copyright ownership.
	The ASF licenses this file to You under the Apache License, Version 2.0
	(the "License"); you may not use this file except in compliance with
	the License. You may obtain a copy of the License at

	http://www.apache.org/licenses/LICENSE-2.0

	Unless required by applicable law or agreed to in writing, software
	distributed under the License is distributed on an "AS IS" BASIS,
	WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	See the License for the specific language governing permissions and
	limitations under the License.
	-->
	<!DOCTYPE document [
	<!ENTITY project SYSTEM "project.xml">
	]>
	<document url="class-loader-howto.html">

	&project;

	<properties>
	<author email="craigmcc@apache.org">Craig R. McClanahan</author>
	<author email="yoavs@apache.org">Yoav Shapira</author>
	<title>Class Loader HOW-TO</title>
	</properties>

	<body>

	<section name="Table of Contents">
	<toc/>
	</section>

	<section name="Overview">

	<p>Like many server applications, Tomcat installs a variety of class loaders
	(that is, classes that implement <code>java.lang.ClassLoader</code>) 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.</p>

	<p>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.</p>

	<p>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:</p>

	<source>
	Bootstrap
	\|
	System
	\|
	Common
	/ \
	Webapp1 Webapp2 ...
	</source>

	<p>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.</p>

	</section>

	<section name="Class Loader Definitions">

	<p>As indicated in the diagram above, Tomcat creates the following class
	loaders as it is initialized:</p>
	<ul>
	<li><strong>Bootstrap</strong> - 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
	(<code>$JAVA_HOME/jre/lib/ext</code>). <em>NOTE</em> - Some JVMs may
	implement this as more than one class loader, or it may not be visible
	(as a class loader) at all.</li>
	<li><strong>System</strong> - This class loader is normally initialized from
	the contents of the <code>CLASSPATH</code> environment variable. All such
	classes are visible to both Tomcat internal classes, and to web
	applications. However, the standard Tomcat startup scripts
	(<code>$CATALINA_HOME/bin/catalina.sh</code> or
	<code>%CATALINA_HOME%\bin\catalina.bat</code>) totally ignore the contents
	of the <code>CLASSPATH</code> environment variable itself, and instead
	build the System class loader from the following repositories:
	<ul>
	<li><em>$CATALINA_HOME/bin/bootstrap.jar</em> - Contains the main() method
	that is used to initialize the Tomcat server, and the class loader
	implementation classes it depends on.</li>
	<li><em>$CATALINA_HOME/bin/tomcat-juli.jar</em> - Package renamed Commons
	logging API, and java.util.logging LogManager.</li>
	</ul></li>
	<li><strong>Common</strong> - This class loader contains additional classes
	that are made visible to both Tomcat internal classes and to all web
	applications. Normally, application classes should <strong>NOT</strong>
	be placed here. All unpacked classes and resources in
	<code>$CATALINA_HOME/lib</code>, as well as classes and
	resources in JAR files are made visible through this
	class loader. By default, that includes the following:
	<ul>
	<li><em>annotations-api.jar</em> - JEE annotations classes.</li>
	<li><em>catalina.jar</em> - Implementation of the Catalina servlet
	container portion of Tomcat.</li>
	<li><em>catalina-ant.jar</em> - Tomcat Catalina Ant tasks.</li>
	<li><em>catalina-ha.jar</em> - High availability package.</li>
	<li><em>catalina-tribes.jar</em> - Group communication package.</li>
	<li><em>el-api.jar</em> - EL 2.1 API.</li>
	<li><em>jasper.jar</em> - Jasper 2 Compiler and Runtime.</li>
	<li><em>jasper-el.jar</em> - Jasper 2 EL implementation.</li>
	<li><em>ecj-*.jar</em> - Eclipse JDT Java compiler.</li>
	<li><em>jsp-api.jar</em> - JSP 2.1 API.</li>
	<li><em>servlet-api.jar</em> - Servlet 3.0 API.</li>
	<li><em>tomcat-coyote.jar</em> - Tomcat connectors and utility classes.</li>
	<li><em>tomcat-dbcp.jar</em> - package renamed database connection
	pool based on Commons DBCP.</li>
	<li><em>tomcat-i18n-**.jar</em> - 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.</li>
	</ul></li>
	<li><strong>WebappX</strong> - A class loader is created for each web
	application that is deployed in a single Tomcat instance. All unpacked
	classes and resources in the <code>/WEB-INF/classes</code> directory of
	your web application archive, plus classes and resources in JAR files
	under the <code>/WEB-INF/lib</code> directory of your web application
	archive, are made visible to the containing web application, but to
	no others.</li>
	</ul>

	<p>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 <em>WebappX</em> class loader is processed,
	this class loader will look in the local repositories <strong>first</strong>,
	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 Tomcat follow the usual delegation pattern.</p>

	<p>Therefore, from the perspective of a web application, class or resource
	loading looks in the following repositories, in this order:</p>
	<ul>
	<li>Bootstrap classes of your JVM</li>
	<li>System class loader classes (described above)</li>
	<li><em>/WEB-INF/classes</em> of your web application</li>
	<li><em>/WEB-INF/lib/*.jar</em> of your web application</li>
	<li><em>$CATALINA_HOME/lib</em></li>
	<li><em>$CATALINA_HOME/lib/*.jar</em></li>
	</ul>

	</section>


	<section name="XML Parsers and JSE 5">

	<p>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.</p>

	<p>In previous versions of Tomcat, you could simply replace the XML parser
	in the <code>$CATALINA_HOME/common/lib</code> 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.</p>

	<p>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:
	<a href="http://java.sun.com/j2se/1.5/docs/guide/standards/index.html">
	http://java.sun.com/j2se/1.5/docs/guide/standards/index.html</a>.</p>

	<p>Tomcat utilizes this mechanism by including the system property setting
	<code>-Djava.endorsed.dirs=$JAVA_ENDORSED_DIRS</code> in the
	command line that starts the container.</p>

	</section>


	<section name="Running under a security manager">

	<p>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 <a href="security-manager-howto.html">Security Manager HOW-TO</a>
	for further information.</p>

	</section>


	</body>

	</document>