| <html><head><META http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><title>Apache Tomcat 6.0 (6.0.41) - 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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| " border="0"></a></td><td><h1><font face="arial,helvetica,sanserif">Apache Tomcat 6.0</font></h1><font face="arial,helvetica,sanserif">Version 6.0.41, May 19 2014</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="noshade" size="1"></td></tr><tr><!--LEFT SIDE NAVIGATION--><td width="20%" valign="top" nowrap="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></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></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">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></ul></td><!--RIGHT SIDE MAIN BODY--><td width="80%" valign="top" align="left" id="mainBody"><h1>Apache Tomcat 6.0</h1><h2>Class Loader HOW-TO</h2><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 6 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> and | |
| <em>$CATALINA_HOME/bin/tomcat-juli.jar</em> — Logging | |
| implementation classes. These include enhancement classes to | |
| <code>java.util.logging</code> API, known as Tomcat JULI, | |
| and a package-renamed copy of Apache Commons Logging library | |
| used internally by Tomcat. | |
| See <a href="logging.html">logging documentation</a> for more | |
| details.</p></li> | |
| <li><p><em>$CATALINA_HOME/bin/commons-daemon.jar</em> — The classes | |
| from <a href="http://commons.apache.org/daemon/">Apache Commons | |
| Daemon</a> project.</p></li> | |
| </ul> | |
| <p>The <em>tomcat-juli.jar</em> and <em>commons-daemon.jar</em> JARs in | |
| <em>$CATALINA_HOME/bin</em> are not present in the <code>CLASSPATH</code> | |
| built by <code>catalina.bat</code>|<code>.sh</code> scripts, but are | |
| referenced from the manifest file of <em>bootstrap.jar</em>. | |
| </p> | |
| <p>If <em>$CATALINA_BASE</em> and <em>$CATALINA_HOME</em> do differ and | |
| <em>$CATALINA_BASE/bin/tomcat-juli.jar</em> does exist, the startup scripts | |
| will add it to <code>CLASSPATH</code> before <em>bootstrap.jar</em>, so | |
| that Java will look into <em>$CATALINA_BASE/bin/tomcat-juli.jar</em> for | |
| classes before it will look into <em>$CATALINA_HOME/bin/tomcat-juli.jar</em> | |
| referenced by <em>bootstrap.jar</em>. It should work in most cases but, | |
| if you are using such configuration, it might be recommended to remove | |
| <em>tomcat-juli.jar</em> from <em>$CATALINA_HOME/bin</em> so that only | |
| one copy of the file is present on the classpath. The next version of | |
| Tomcat, Tomcat 7, takes different approach here. | |
| </p></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 2.1 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 2.1 API.</li> | |
| <li><em>servlet-api.jar</em> — Servlet 2.5 API.</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> | |
| </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 2 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 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 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 6 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><!--FOOTER SEPARATOR--><tr><td colspan="2"><hr noshade="noshade" size="1"></td></tr><!--PAGE FOOTER--><tr><td colspan="2"><div align="center"><font color="#525D76" size="-1"><em> | |
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