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</style></head><body bgcolor="#ffffff" text="#000000" link="#525D76" alink="#525D76" vlink="#525D76"><table border="0" width="100%" cellspacing="0"><tr><td><a href="https://tomcat.apache.org/"><img src="./images/tomcat.gif" align="right" alt="
 The Apache Tomcat Servlet/JSP Container
 " border="0"></a></td><td><h1>Apache Tomcat 7</h1>Version 7.0.107, Nov 18 2020</td><td><a href="http://www.apache.org/"><img src="./images/asf-logo.svg" align="right" alt="Apache Logo" border="0" style="width: 266px;height: 83px;"></a></td></tr></table><table border="0" width="100%" cellspacing="4"><tr><td colspan="2"><hr noshade size="1"></td></tr><tr><td width="20%" valign="top" nowrap class="noPrint">Links<ul><li><a href="index.html">Docs Home</a></li><li><a href="https://wiki.apache.org/tomcat/FAQ">FAQ</a></li><li><a href="#comments_section">User Comments</a></li></ul>User Guide<ul><li><a href="introduction.html">1) Introduction</a></li><li><a href="setup.html">2) Setup</a></li><li><a href="appdev/index.html">3) First webapp</a></li><li><a href="deployer-howto.html">4) Deployer</a></li><li><a href="manager-howto.html">5) Manager</a></li><li><a href="host-manager-howto.html">6) Host Manager</a></li><li><a href="realm-howto.html">7) Realms and AAA</a></li><li><a href="security-manager-howto.html">8) Security Manager</a></li><li><a href="jndi-resources-howto.html">9) JNDI Resources</a></li><li><a href="jndi-datasource-examples-howto.html">10) JDBC DataSources</a></li><li><a href="class-loader-howto.html">11) Classloading</a></li><li><a href="jasper-howto.html">12) JSPs</a></li><li><a href="ssl-howto.html">13) SSL/TLS</a></li><li><a href="ssi-howto.html">14) SSI</a></li><li><a href="cgi-howto.html">15) CGI</a></li><li><a href="proxy-howto.html">16) Proxy Support</a></li><li><a href="mbeans-descriptors-howto.html">17) MBeans Descriptors</a></li><li><a href="default-servlet.html">18) Default Servlet</a></li><li><a href="cluster-howto.html">19) Clustering</a></li><li><a href="balancer-howto.html">20) Load Balancer</a></li><li><a href="connectors.html">21) Connectors</a></li><li><a href="monitoring.html">22) Monitoring and Management</a></li><li><a href="logging.html">23) Logging</a></li><li><a href="apr.html">24) APR/Native</a></li><li><a href="virtual-hosting-howto.html">25) Virtual Hosting</a></li><li><a href="aio.html">26) Advanced IO</a></li><li><a href="extras.html">27) Additional Components</a></li><li><a href="maven-jars.html">28) Mavenized</a></li><li><a href="security-howto.html">29) Security Considerations</a></li><li><a href="windows-service-howto.html">30) Windows Service</a></li><li><a href="windows-auth-howto.html">31) Windows Authentication</a></li><li><a href="jdbc-pool.html">32) Tomcat's JDBC Pool</a></li><li><a href="web-socket-howto.html">33) WebSocket</a></li></ul>Reference<ul><li><a href="RELEASE-NOTES.txt">Release Notes</a></li><li><a href="config/index.html">Configuration</a></li><li><a href="api/index.html">Tomcat Javadocs</a></li><li><a href="servletapi/index.html">Servlet Javadocs</a></li><li><a href="jspapi/index.html">JSP 2.2 Javadocs</a></li><li><a href="elapi/index.html">EL 2.2 Javadocs</a></li><li><a href="websocketapi/index.html">WebSocket 1.1 Javadocs</a></li><li><a href="annotationapi/index.html">Common Annotations 1.1 Javadocs</a></li><li><a href="https://tomcat.apache.org/connectors-doc/">JK 1.2 Documentation</a></li></ul>Apache Tomcat Development<ul><li><a href="building.html">Building</a></li><li><a href="changelog.html">Changelog</a></li><li><a href="https://wiki.apache.org/tomcat/TomcatVersions">Status</a></li><li><a href="developers.html">Developers</a></li><li><a href="architecture/index.html">Architecture</a></li><li><a href="tribes/introduction.html">Tribes</a></li></ul></td><td width="80%" valign="top" align="left" id="mainBody"><h1>Class Loader HOW-TO</h1><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><a name="Table of Contents"></a><a name="Table_of_Contents">Table of Contents</a></td></tr><tr><td><blockquote>
<ul><li><a href="#Overview">Overview</a></li><li><a href="#Class_Loader_Definitions">Class Loader Definitions</a></li><li><a href="#XML_Parsers_and_Java">XML Parsers and Java</a></li><li><a href="#Running_under_a_security_manager">Running under a security manager</a></li><li><a href="#Advanced_configuration">Advanced configuration</a></li></ul>
</blockquote></td></tr></table><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><a name="Overview">Overview</a></td></tr><tr><td><blockquote>

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&nbsp;&mdash; 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:

<div class="codeBox"><pre><code> Bootstrap
 |
 System
 |
 Common
 / \
 Webapp1 Webapp2 ...</code></pre></div>

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.

</blockquote></td></tr></table><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><a name="Class Loader Definitions"></a><a name="Class_Loader_Definitions">Class Loader Definitions</a></td></tr><tr><td><blockquote>

As indicated in the diagram above, Tomcat creates the following class
loaders as it is initialized:
<ul>
<li>Bootstrap &mdash; 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>$JAVA_HOME/jre/lib/ext</code>). Note: some JVMs may
 implement this as more than one class loader, or it may not be visible
 (as a class loader) at all.</li>
<li>System &mdash; This class loader is normally initialized
 from the contents of the <code>CLASSPATH</code> environment variable. All
 such classes are visible to both Tomcat internal classes, and to web
 applications. However, the standard Tomcat startup scripts
 (<code>$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>$CATALINA_HOME/bin/bootstrap.jar &mdash; Contains the
 main() method that is used to initialize the Tomcat server, and the
 class loader implementation classes it depends on.</li>
 <li>$CATALINA_BASE/bin/tomcat-juli.jar or
 $CATALINA_HOME/bin/tomcat-juli.jar &mdash; Logging
 implementation classes. These include enhancement classes to
 <code>java.util.logging</code> API, known as Tomcat JULI,
 and a package-renamed copy of Apache Commons Logging library
 used internally by Tomcat.
 See <a href="logging.html">logging documentation</a> for more
 details.
 If <code>tomcat-juli.jar</code> is present in
 $CATALINA_BASE/bin, it is used instead of the one in
 $CATALINA_HOME/bin. It is useful in certain logging
 configurations</li>
 <li>$CATALINA_HOME/bin/commons-daemon.jar &mdash; The classes
 from <a href="https://commons.apache.org/daemon/">Apache Commons
 Daemon</a> project.
 This JAR file is not present in the <code>CLASSPATH</code> built by
 <code>catalina.bat</code>|<code>.sh</code> scripts, but is referenced
 from the manifest file of bootstrap.jar.</li>
 </ul>
 </li>
<li>Common &mdash; 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 <code>common.loader</code> property in
 $CATALINA_BASE/conf/catalina.properties. The default setting will search the
 following locations in the order they are listed:
 <ul>
 <li>unpacked classes and resources in <code>$CATALINA_BASE/lib</code></li>
 <li>JAR files in <code>$CATALINA_BASE/lib</code></li>
 <li>unpacked classes and resources in <code>$CATALINA_HOME/lib</code></li>
 <li>JAR files in <code>$CATALINA_HOME/lib</code></li>
 </ul>
 By default, this includes the following:
 <ul>
 <li>annotations-api.jar &mdash; JavaEE annotations classes.</li>
 <li>catalina.jar &mdash; Implementation of the Catalina servlet
 container portion of Tomcat.</li>
 <li>catalina-ant.jar &mdash; Tomcat Catalina Ant tasks.</li>
 <li>catalina-ha.jar &mdash; High availability package.</li>
 <li>catalina-tribes.jar &mdash; Group communication package.</li>
 <li>ecj-*.jar &mdash; Eclipse JDT Java compiler.</li>
 <li>el-api.jar &mdash; EL 2.2 API.</li>
 <li>jasper.jar &mdash; Tomcat Jasper JSP Compiler and Runtime.</li>
 <li>jasper-el.jar &mdash; Tomcat Jasper EL implementation.</li>
 <li>jsp-api.jar &mdash; JSP 2.2 API.</li>
 <li>servlet-api.jar &mdash; Servlet 3.0 API.</li>
 <li>tomcat-api.jar &mdash; Several interfaces defined by Tomcat.</li>
 <li>tomcat-coyote.jar &mdash; Tomcat connectors and utility classes.</li>
 <li>tomcat-dbcp.jar &mdash; Database connection pool
 implementation based on package-renamed copy of Apache Commons Pool
 and Apache Commons DBCP 1.x.</li>
 <li>tomcat-i18n-**.jar &mdash; 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>
 <li>tomcat-jdbc.jar &mdash; An alternative database connection pool
 implementation, known as Tomcat JDBC pool. See
 <a href="jdbc-pool.html">documentation</a> for more details.</li>
 <li>tomcat-util.jar &mdash; Common classes used by various components of
 Apache Tomcat.</li>
 <li>tomcat7-websocket.jar &mdash; WebSocket 1.1 implementation</li>
 <li>websocket-api.jar &mdash; WebSocket 1.1 API</li>
 </ul></li>
<li>WebappX &mdash; 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, plus classes and resources in JAR files
 under the <code>/WEB-INF/lib</code> directory of your web application,
 are made visible to this web application, but not to other ones.</li>
</ul>

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 Java endorsed feature can be
used up to Java 8.
Last, any JAR file that contains Servlet API classes will be explicitly
ignored by the classloader &mdash; 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:
<ul>
<li>Bootstrap classes of your JVM</li>
<li>/WEB-INF/classes of your web application</li>
<li>/WEB-INF/lib/*.jar of your web application</li>
<li>System class loader classes (described above)</li>
<li>Common class loader classes (described above)</li>
</ul>

If the web application class loader is
<a href="config/loader.html">configured</a> with
<code>&lt;Loader delegate="true"/&gt;</code>
then the order becomes:
<ul>
<li>Bootstrap classes of your JVM</li>
<li>System class loader classes (described above)</li>
<li>Common class loader classes (described above)</li>
<li>/WEB-INF/classes of your web application</li>
<li>/WEB-INF/lib/*.jar of your web application</li>
</ul>

</blockquote></td></tr></table><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><a name="XML Parsers and Java"></a><a name="XML_Parsers_and_Java">XML Parsers and Java</a></td></tr><tr><td><blockquote>

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:
<a href="http://docs.oracle.com/javase/1.5.0/docs/guide/standards/index.html">
http://docs.oracle.com/javase/1.5.0/docs/guide/standards/index.html</a>.

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. The default value of this option is
$CATALINA_HOME/endorsed. This endorsed directory is not
created by default. Note that the endorsed feature is no longer supported
with Java 9 and the above system property will only be set if either the
directory $CATALINA_HOME/endorsed exists, or the variable
<code>JAVA_ENDORSED_DIRS</code> has been set.

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.

</blockquote></td></tr></table><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><a name="Running under a security manager"></a><a name="Running_under_a_security_manager">Running under a security manager</a></td></tr><tr><td><blockquote>

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.

</blockquote></td></tr></table><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><a name="Advanced configuration"></a><a name="Advanced_configuration">Advanced configuration</a></td></tr><tr><td><blockquote>

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
<code>server.loader</code> and/or <code>shared.loader</code> properties in
<code>conf/catalina.properties</code>.

<div class="codeBox"><pre><code>
 Bootstrap
 |
 System
 |
 Common
 / \
Server Shared
 / \
 Webapp1 Webapp2 ...</code></pre></div>

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.

</blockquote></td></tr></table></td></tr><tr><td colspan="2"><hr noshade size="1"></td></tr><tr><td colspan="2"><div align="center">
 Copyright &copy; 1999-2020, Apache Software Foundation
 </div></td></tr></table></body></html>

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