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+<html><head><META http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><title>Apache Tomcat 7 (7.0.42) - Class Loader HOW-TO</title><meta name="author" content="Craig R. McClanahan"><meta name="author" content="Yoav Shapira"><style type="text/css" media="print">
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+ </style></head><body bgcolor="#ffffff" text="#000000" link="#525D76" alink="#525D76" vlink="#525D76"><table border="0" width="100%" cellspacing="0"><!--PAGE HEADER--><tr><td><!--PROJECT LOGO--><a href="http://tomcat.apache.org/"><img src="./images/tomcat.gif" align="right" alt="
+ The Apache Tomcat Servlet/JSP Container
+ " border="0"></a></td><td><h1><font face="arial,helvetica,sanserif">Apache Tomcat 7</font></h1><font face="arial,helvetica,sanserif">Version 7.0.42, Jul 2 2013</font></td><td><!--APACHE LOGO--><a href="http://www.apache.org/"><img src="./images/asf-logo.gif" align="right" alt="Apache Logo" border="0"></a></td></tr></table><table border="0" width="100%" cellspacing="4"><!--HEADER SEPARATOR--><tr><td colspan="2"><hr noshade size="1"></td></tr><tr><!--LEFT SIDE NAVIGATION--><td width="20%" valign="top" nowrap class="noPrint"><p><strong>Links</strong></p><ul><li><a href="index.html">Docs Home</a></li><li><a href="http://wiki.apache.org/tomcat/FAQ">FAQ</a></li><li><a href="#comments_section">User Comments</a></li></ul><p><strong>User Guide</strong></p><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="realm-howto.html">6) Realms and AAA</a></li><li><a href="security-manager-howto.html">7) Security Manager</a></li><li><a href="jndi-resources-howto.html">8) JNDI Resources</a></li><li><a href="jndi-datasource-examples-howto.html">9) JDBC DataSources</a></li><li><a href="class-loader-howto.html">10) Classloading</a></li><li><a href="jasper-howto.html">11) JSPs</a></li><li><a href="ssl-howto.html">12) SSL</a></li><li><a href="ssi-howto.html">13) SSI</a></li><li><a href="cgi-howto.html">14) CGI</a></li><li><a href="proxy-howto.html">15) Proxy Support</a></li><li><a href="mbeans-descriptor-howto.html">16) MBean Descriptor</a></li><li><a href="default-servlet.html">17) Default Servlet</a></li><li><a href="cluster-howto.html">18) Clustering</a></li><li><a href="balancer-howto.html">19) Load Balancer</a></li><li><a href="connectors.html">20) Connectors</a></li><li><a href="monitoring.html">21) Monitoring and Management</a></li><li><a href="logging.html">22) Logging</a></li><li><a href="apr.html">23) APR/Native</a></li><li><a href="virtual-hosting-howto.html">24) Virtual Hosting</a></li><li><a href="aio.html">25) Advanced IO</a></li><li><a href="extras.html">26) Additional Components</a></li><li><a href="maven-jars.html">27) Mavenized</a></li><li><a href="security-howto.html">28) Security Considerations</a></li><li><a href="windows-service-howto.html">29) Windows Service</a></li><li><a href="windows-auth-howto.html">30) Windows Authentication</a></li><li><a href="jdbc-pool.html">31) Tomcat's JDBC Pool</a></li><li><a href="web-socket-howto.html">32) WebSocket</a></li></ul><p><strong>Reference</strong></p><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="http://tomcat.apache.org/connectors-doc/">JK 1.2 Documentation</a></li></ul><p><strong>Apache Tomcat Development</strong></p><ul><li><a href="building.html">Building</a></li><li><a href="changelog.html">Changelog</a></li><li><a href="http://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="funcspecs/index.html">Functional Specs.</a></li><li><a href="tribes/introduction.html">Tribes</a></li></ul></td><!--RIGHT SIDE MAIN BODY--><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"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="Table of Contents"><!--()--></a><a name="Table_of_Contents"><strong>Table of Contents</strong></a></font></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></ul>
+</blockquote></td></tr></table><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="Overview"><strong>Overview</strong></a></font></td></tr><tr><td><blockquote>
+
+<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 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 <em>differs</em> 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>
+
+<div align="left"><table cellspacing="4" cellpadding="0" border="0"><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#ffffff" height="1"><pre>
+ Bootstrap
+ |
+ System
+ |
+ Common
+ / \
+ Webapp1 Webapp2 ...
+</pre></td><td bgcolor="#023264" width="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./images/void.gif" alt="" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr></table></div>
+
+<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>
+
+</blockquote></td></tr></table><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="Class Loader Definitions"><!--()--></a><a name="Class_Loader_Definitions"><strong>Class Loader Definitions</strong></a></font></td></tr><tr><td><blockquote>
+
+<p>As indicated in the diagram above, Tomcat creates the following class
+loaders as it is initialized:</p>
+<ul>
+<li><p><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>$JAVA_HOME/jre/lib/ext</code>). <em>Note</em>: some JVMs may
+ implement this as more than one class loader, or it may not be visible
+ (as a class loader) at all.</p></li>
+<li><p><strong>System</strong> — This class loader is normally initialized
+ from the contents of the <code>CLASSPATH</code> environment variable. All
+ such classes are visible to both Tomcat internal classes, and to web
+ applications. However, the standard Tomcat startup scripts
+ (<code>$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:
+ </p>
+ <ul>
+ <li><p><em>$CATALINA_HOME/bin/bootstrap.jar</em> — Contains the
+ main() method that is used to initialize the Tomcat server, and the
+ class loader implementation classes it depends on.</p></li>
+ <li><p><em>$CATALINA_BASE/bin/tomcat-juli.jar</em> or
+ <em>$CATALINA_HOME/bin/tomcat-juli.jar</em> — Logging
+ implementation classes. These include enhancement classes to
+ <code>java.util.logging</code> API, known as Tomcat JULI,
+ and a package-renamed copy of Apache Commons Logging library
+ used internally by Tomcat.
+ See <a href="logging.html">logging documentation</a> for more
+ details.</p>
+ <p>If <code>tomcat-juli.jar</code> is present in
+ <em>$CATALINA_BASE/bin</em>, it is used instead of the one in
+ <em>$CATALINA_HOME/bin</em>. It is useful in certain logging
+ configurations</p></li>
+ <li><p><em>$CATALINA_HOME/bin/commons-daemon.jar</em> — The classes
+ from <a href="http://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 <em>bootstrap.jar</em>.</p></li>
+ </ul>
+ </li>
+<li><p><strong>Common</strong> — This class loader contains additional
+ classes that are made visible to both Tomcat internal classes and to all
+ web applications.</p>
+ <p>Normally, application classes should <strong>NOT</strong>
+ be placed here. The locations searched by this class loader are defined by
+ the <code>common.loader</code> property in
+ $CATALINA_BASE/conf/catalina.properties. The default setting will search the
+ following locations in the order they are listed:</p>
+ <ul>
+ <li>unpacked classes and resources in <code>$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>
+ <p>By default, this includes the following:</p>
+ <ul>
+ <li><em>annotations-api.jar</em> — JavaEE 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>ecj-*.jar</em> — Eclipse JDT Java compiler.</li>
+ <li><em>el-api.jar</em> — EL 2.2 API.</li>
+ <li><em>jasper.jar</em> — Tomcat Jasper JSP Compiler and Runtime.</li>
+ <li><em>jasper-el.jar</em> — Tomcat Jasper EL implementation.</li>
+ <li><em>jsp-api.jar</em> — JSP 2.2 API.</li>
+ <li><em>servlet-api.jar</em> — Servlet 3.0 API.</li>
+ <li><em>tomcat-api.jar</em> — Several interfaces defined by Tomcat.</li>
+ <li><em>tomcat-coyote.jar</em> — Tomcat connectors and utility classes.</li>
+ <li><em>tomcat-dbcp.jar</em> — Database connection pool
+ implementation based on package-renamed copy of Apache Commons Pool
+ and Apache 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>
+ <li><em>tomcat-jdbc.jar</em> — An alternative database connection pool
+ implementation, known as Tomcat JDBC pool. See
+ <a href="jdbc-pool.html">documentation</a> for more details.</li>
+ <li><em>tomcat-util.jar</em> — Common classes used by various components of
+ Apache Tomcat.</li>
+ </ul></li>
+<li><p><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, 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.</p></li>
+</ul>
+
+<p>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 <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 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.</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>Common class loader classes (described above)</li>
+</ul>
+
+</blockquote></td></tr></table><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="XML Parsers and Java"><!--()--></a><a name="XML_Parsers_and_Java"><strong>XML Parsers and Java</strong></a></font></td></tr><tr><td><blockquote>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p>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>.</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. The default value of this option is
+<em>$CATALINA_HOME/endorsed</em>. This <em>endorsed</em> directory is not
+created by default.</p>
+
+</blockquote></td></tr></table><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="Running under a security manager"><!--()--></a><a name="Running_under_a_security_manager"><strong>Running under a security manager</strong></a></font></td></tr><tr><td><blockquote>
+
+<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>
+
+</blockquote></td></tr></table></td></tr><tr class="noPrint"><td width="20%" valign="top" nowrap class="noPrint"></td><td width="80%" valign="top" align="left"><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="comments_section" id="comments_section"><strong>Comments</strong></a></font></td></tr><tr><td><blockquote><p class="notice"><strong>Notice: </strong>This comments section collects your suggestions
+ on improving documentation for Apache Tomcat.<br><br>
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