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<html lang="en"><head><META http-equiv="Content-Type" content="text/html; charset=UTF-8"><link href="./images/docs-stylesheet.css" rel="stylesheet" type="text/css"><title>Apache Tomcat 8 (8.0.53) - Class Loader HOW-TO</title><meta name="author" content="Craig R. McClanahan"><meta name="author" content="Yoav Shapira"><script type="application/javascript" data-comments-identifier="tomcat-8.0-doc/class-loader-howto">
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</script></head><body><div id="wrapper"><header><div id="header"><div><div><div class="logo noPrint"><a href="https://tomcat.apache.org/"><img alt="Tomcat Home" src="./images/tomcat.png"></a></div><div style="height: 1px;"></div><div class="asfLogo noPrint"><a href="https://www.apache.org/" target="_blank"><img src="./images/asf-logo.svg" alt="The Apache Software Foundation" style="width: 266px; height: 83px;"></a></div><h1>Apache Tomcat 8</h1><div class="versionInfo">
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Version 8.0.53,
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<time datetime="2018-06-29">Jun 29 2018</time></div><div style="height: 1px;"></div><div style="clear: left;"></div></div></div></div></header><div id="middle"><div><div id="mainLeft" class="noprint"><div><nav><div><h2>Links</h2><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></div><div><h2>User Guide</h2><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><li><a href="rewrite.html">34) Rewrite</a></li></ul></div><div><h2>Reference</h2><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 2.3 Javadocs</a></li><li><a href="elapi/index.html">EL 3.0 Javadocs</a></li><li><a href="websocketapi/index.html">WebSocket 1.1 Javadocs</a></li><li><a href="https://tomcat.apache.org/connectors-doc/">JK 1.2 Documentation</a></li></ul></div><div><h2>Apache Tomcat Development</h2><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="funcspecs/index.html">Functional Specs.</a></li><li><a href="tribes/introduction.html">Tribes</a></li></ul></div></nav></div></div><div id="mainRight"><div id="content"><h2>Class Loader HOW-TO</h2><h3 id="Table_of_Contents">Table of Contents</h3><div class="text">
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<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>
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</div><h3 id="Overview">Overview</h3><div class="text">
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<p>Like many server applications, Tomcat installs a variety of class loaders
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(that is, classes that implement <code>java.lang.ClassLoader</code>) to allow
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different portions of the container, and the web applications running on the
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container, to have access to different repositories of available classes and
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resources. This mechanism is used to provide the functionality defined in the
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Servlet Specification, version 2.4 — in particular, Sections 9.4
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and 9.6.</p>
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<p>In a Java environment, class loaders are
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arranged in a parent-child tree. Normally, when a class loader is asked to
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load a particular class or resource, it delegates the request to a parent
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class loader first, and then looks in its own repositories only if the parent
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class loader(s) cannot find the requested class or resource. Note, that the
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model for web application class loaders <em>differs</em> slightly from this,
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as discussed below, but the main principles are the same.</p>
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<p>When Tomcat is started, it creates a set of class loaders that are
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organized into the following parent-child relationships, where the parent
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class loader is above the child class loader:</p>
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<div class="codeBox"><pre><code> Bootstrap
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System
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Common
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/ \
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Webapp1 Webapp2 ...</code></pre></div>
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<p>The characteristics of each of these class loaders, including the source
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of classes and resources that they make visible, are discussed in detail in
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the following section.</p>
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</div><h3 id="Class_Loader_Definitions">Class Loader Definitions</h3><div class="text">
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<p>As indicated in the diagram above, Tomcat creates the following class
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loaders as it is initialized:</p>
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<ul>
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<li><p><strong>Bootstrap</strong> — This class loader contains the basic
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runtime classes provided by the Java Virtual Machine, plus any classes from
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JAR files present in the System Extensions directory
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(<code>$JAVA_HOME/jre/lib/ext</code>). <em>Note</em>: some JVMs may
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implement this as more than one class loader, or it may not be visible
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(as a class loader) at all.</p></li>
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<li><p><strong>System</strong> — This class loader is normally initialized
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from the contents of the <code>CLASSPATH</code> environment variable. All
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such classes are visible to both Tomcat internal classes, and to web
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applications. However, the standard Tomcat startup scripts
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(<code>$CATALINA_HOME/bin/catalina.sh</code> or
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<code>%CATALINA_HOME%\bin\catalina.bat</code>) totally ignore the contents
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of the <code>CLASSPATH</code> environment variable itself, and instead
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build the System class loader from the following repositories:
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</p>
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<ul>
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<li><p><em>$CATALINA_HOME/bin/bootstrap.jar</em> — Contains the
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main() method that is used to initialize the Tomcat server, and the
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class loader implementation classes it depends on.</p></li>
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<li><p><em>$CATALINA_BASE/bin/tomcat-juli.jar</em> or
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<em>$CATALINA_HOME/bin/tomcat-juli.jar</em> — Logging
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implementation classes. These include enhancement classes to
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<code>java.util.logging</code> API, known as Tomcat JULI,
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and a package-renamed copy of Apache Commons Logging library
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used internally by Tomcat.
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See <a href="logging.html">logging documentation</a> for more
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details.</p>
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<p>If <code>tomcat-juli.jar</code> is present in
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<em>$CATALINA_BASE/bin</em>, it is used instead of the one in
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<em>$CATALINA_HOME/bin</em>. It is useful in certain logging
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configurations</p></li>
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<li><p><em>$CATALINA_HOME/bin/commons-daemon.jar</em> — The classes
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from <a href="https://commons.apache.org/daemon/">Apache Commons
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Daemon</a> project.
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This JAR file is not present in the <code>CLASSPATH</code> built by
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<code>catalina.bat</code>|<code>.sh</code> scripts, but is referenced
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from the manifest file of <em>bootstrap.jar</em>.</p></li>
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</ul>
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</li>
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<li><p><strong>Common</strong> — This class loader contains additional
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classes that are made visible to both Tomcat internal classes and to all
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web applications.</p>
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<p>Normally, application classes should <strong>NOT</strong>
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be placed here. The locations searched by this class loader are defined by
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the <code>common.loader</code> property in
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$CATALINA_BASE/conf/catalina.properties. The default setting will search the
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following locations in the order they are listed:</p>
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<ul>
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<li>unpacked classes and resources in <code>$CATALINA_BASE/lib</code></li>
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<li>JAR files in <code>$CATALINA_BASE/lib</code></li>
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<li>unpacked classes and resources in <code>$CATALINA_HOME/lib</code></li>
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<li>JAR files in <code>$CATALINA_HOME/lib</code></li>
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</ul>
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<p>By default, this includes the following:</p>
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<ul>
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<li><em>annotations-api.jar</em> — JavaEE annotations classes.</li>
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<li><em>catalina.jar</em> — Implementation of the Catalina servlet
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container portion of Tomcat.</li>
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<li><em>catalina-ant.jar</em> — Tomcat Catalina Ant tasks.</li>
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<li><em>catalina-ha.jar</em> — High availability package.</li>
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<li><em>catalina-storeconfig.jar</em> — Generation of XML
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configuration files from current state</li>
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<li><em>catalina-tribes.jar</em> — Group communication package.</li>
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<li><em>ecj-*.jar</em> — Eclipse JDT Java compiler.</li>
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<li><em>el-api.jar</em> — EL 3.0 API.</li>
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<li><em>jasper.jar</em> — Tomcat Jasper JSP Compiler and Runtime.</li>
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<li><em>jasper-el.jar</em> — Tomcat Jasper EL implementation.</li>
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<li><em>jsp-api.jar</em> — JSP 2.3 API.</li>
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<li><em>servlet-api.jar</em> — Servlet 3.1 API.</li>
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<li><em>tomcat-api.jar</em> — Several interfaces defined by Tomcat.</li>
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<li><em>tomcat-coyote.jar</em> — Tomcat connectors and utility classes.</li>
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<li><em>tomcat-dbcp.jar</em> — Database connection pool
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implementation based on package-renamed copy of Apache Commons Pool
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and Apache Commons DBCP.</li>
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<li><em>tomcat-i18n-**.jar</em> — Optional JARs containing resource bundles
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for other languages. As default bundles are also included in each
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individual JAR, they can be safely removed if no internationalization
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of messages is needed.</li>
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<li><em>tomcat-jdbc.jar</em> — An alternative database connection pool
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implementation, known as Tomcat JDBC pool. See
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<a href="jdbc-pool.html">documentation</a> for more details.</li>
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<li><em>tomcat-util.jar</em> — Common classes used by various components of
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Apache Tomcat.</li>
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<li><em>tomcat-websocket.jar</em> — WebSocket 1.1 implementation</li>
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<li><em>websocket-api.jar</em> — WebSocket 1.1 API</li>
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</ul></li>
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<li><p><strong>WebappX</strong> — A class loader is created for each web
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application that is deployed in a single Tomcat instance. All unpacked
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classes and resources in the <code>/WEB-INF/classes</code> directory of
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your web application, plus classes and resources in JAR files
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under the <code>/WEB-INF/lib</code> directory of your web application,
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are made visible to this web application, but not to other ones.</p></li>
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</ul>
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<p>As mentioned above, the web application class loader diverges from the
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default Java delegation model (in accordance with the recommendations in the
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Servlet Specification, version 2.4, section 9.7.2 Web Application Classloader).
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When a request to load a
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class from the web application's <em>WebappX</em> class loader is processed,
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this class loader will look in the local repositories <strong>first</strong>,
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instead of delegating before looking. There are exceptions. Classes which are
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part of the JRE base classes cannot be overridden. For some classes (such as
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the XML parser components in J2SE 1.4+), the Java endorsed feature can be
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used up to Java 8.
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Lastly, the web application class loader will always delegate first for JavaEE
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API classes for the specifications implemented by Tomcat
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(Servlet, JSP, EL, WebSocket). All other class loaders in Tomcat follow the
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usual delegation pattern.</p>
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<p>Therefore, from the perspective of a web application, class or resource
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loading looks in the following repositories, in this order:</p>
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<ul>
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<li>Bootstrap classes of your JVM</li>
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<li><em>/WEB-INF/classes</em> of your web application</li>
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<li><em>/WEB-INF/lib/*.jar</em> of your web application</li>
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<li>System class loader classes (described above)</li>
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<li>Common class loader classes (described above)</li>
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</ul>
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<p>If the web application class loader is
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<a href="config/loader.html">configured</a> with
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<code><Loader delegate="true"/></code>
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then the order becomes:</p>
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<ul>
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<li>Bootstrap classes of your JVM</li>
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<li>System class loader classes (described above)</li>
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<li>Common class loader classes (described above)</li>
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<li><em>/WEB-INF/classes</em> of your web application</li>
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<li><em>/WEB-INF/lib/*.jar</em> of your web application</li>
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</ul>
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</div><h3 id="XML_Parsers_and_Java">XML Parsers and Java</h3><div class="text">
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<p>Starting with Java 1.4 a copy of JAXP APIs and an XML parser are packed
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inside the JRE. This has impacts on applications that wish to use their own
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XML parser.</p>
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<p>In old versions of Tomcat, you could simply replace the XML parser
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in the Tomcat libraries directory to change the parser
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used by all web applications. However, this technique will not be effective
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when you are running modern versions of Java, because the usual class loader
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delegation process will always choose the implementation inside the JDK in
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preference to this one.</p>
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<p>Java supports a mechanism called the "Endorsed Standards Override
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Mechanism" to allow replacement of APIs created outside of the JCP
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(i.e. DOM and SAX from W3C). It can also be used to update the XML parser
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implementation. For more information, see:
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<a href="http://docs.oracle.com/javase/1.5.0/docs/guide/standards/index.html">
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http://docs.oracle.com/javase/1.5.0/docs/guide/standards/index.html</a>.</p>
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<p>Tomcat utilizes this mechanism by including the system property setting
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<code>-Djava.endorsed.dirs=$JAVA_ENDORSED_DIRS</code> in the
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command line that starts the container. The default value of this option is
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<em>$CATALINA_HOME/endorsed</em>. This <em>endorsed</em> directory is not
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created by default. Note that the endorsed feature is no longer supported
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with Java 9 and the above system property will only be set if either the
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directory <em>$CATALINA_HOME/endorsed</em> exists, or the variable
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<code>JAVA_ENDORSED_DIRS</code> has been set.
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</p>
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<p>Note that overriding any JRE component carries risk. If the overriding
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component does not provide a 100% compatible API (e.g. the API provided by
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Xerces is not 100% compatible with the XML API provided by the JRE) then there
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is a risk that Tomcat and/or the deployed application will experience errors.</p>
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</div><h3 id="Running_under_a_security_manager">Running under a security manager</h3><div class="text">
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<p>When running under a security manager the locations from which classes
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are permitted to be loaded will also depend on the contents of your policy
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file. See <a href="security-manager-howto.html">Security Manager HOW-TO</a>
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for further information.</p>
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</div><div class="noprint"><h3 id="comments_section">
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Comments
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</h3><div class="text"><p class="notice"><strong>Notice: </strong>This comments section collects your suggestions
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on improving documentation for Apache Tomcat.<br><br>
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If you have trouble and need help, read
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<a href="https://tomcat.apache.org/findhelp.html">Find Help</a> page
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and ask your question on the tomcat-users
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<a href="https://tomcat.apache.org/lists.html">mailing list</a>.
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Do not ask such questions here. This is not a Q&A section.<br><br>
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The Apache Comments System is explained <a href="./comments.html">here</a>.
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Comments may be removed by our moderators if they are either
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implemented or considered invalid/off-topic.
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</p><div id="comments_thread"></div></div></div></div></div></div></div><footer><div id="footer">
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Copyright © 1999-2018, The Apache Software Foundation
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