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1<html><head><META http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><title>Apache Tomcat 6.0 - Class Loader HOW-TO</title><meta value="Craig R. McClanahan" name="author"><meta value="craigmcc@apache.org" name="email"><meta value="Yoav Shapira" name="author"><meta value="yoavs@apache.org" name="email"></head><body vlink="#525D76" alink="#525D76" link="#525D76" text="#000000" bgcolor="#ffffff"><table cellspacing="0" width="100%" border="0"><!--PAGE HEADER--><tr><td><!--PROJECT LOGO--><a href="http://tomcat.apache.org/"><img border="0" alt="
2      The Apache Tomcat Servlet/JSP Container
3    " align="right" src="./images/tomcat.gif"></a></td><td><font face="arial,helvetica,sanserif"><h1>Apache Tomcat 6.0</h1></font></td><td><!--APACHE LOGO--><a href="http://www.apache.org/"><img border="0" alt="Apache Logo" align="right" src="./images/asf-logo.gif"></a></td></tr></table><table cellspacing="4" width="100%" border="0"><!--HEADER SEPARATOR--><tr><td colspan="2"><hr size="1" noshade></td></tr><tr><!--LEFT SIDE NAVIGATION--><td nowrap="true" valign="top" width="20%"><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 align="left" valign="top" width="80%"><table cellspacing="4" width="100%" border="0"><tr><td valign="top" align="left"><h1>Apache Tomcat 6.0</h1><h2>Class Loader HOW-TO</h2></td><td nowrap="true" valign="top" align="right"><small><a href="printer/class-loader-howto.html"><img alt="Printer Friendly Version" border="0" src="./images/printer.gif"><br>print-friendly<br>version
4                    </a></small></td></tr></table><table cellpadding="2" cellspacing="0" border="0"><tr><td bgcolor="#525D76"><font face="arial,helvetica.sanserif" color="#ffffff"><a name="Overview"><strong>Overview</strong></a></font></td></tr><tr><td><blockquote>
5
6<p>Like many server applications, Tomcat 6 installs a variety of class loaders
7(that is, classes that implement <code>java.lang.ClassLoader</code>) to allow
8different portions of the container, and the web applications running on the
9container, to have access to different repositories of available classes and
10resources.  This mechanism is used to provide the functionality defined in the
11Servlet Specification, version 2.4 -- in particular, Sections 9.4 and 9.6.</p>
12
13<p>In a J2SE 2 (that is, J2SE 1.2 or later) environment, class loaders are
14arranged in a parent-child tree.  Normally, when a class loader is asked to
15load a particular class or resource, it delegates the request to a parent
16class loader first, and then looks in its own repositories only if the parent
17class loader(s) cannot find the requested class or resource.  The model for
18web application class loaders differs slightly from this, as discussed below,
19but the main principles are the same.</p>
20
21<p>When Tomcat 6 is started, it creates a set of class loaders that are
22organized into the following parent-child relationships, where the parent
23class loader is above the child class loader:</p>
24
25<div align="left"><table border="0" cellpadding="0" cellspacing="4"><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./images/void.gif"></td></tr><tr><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./images/void.gif"></td><td height="1" bgcolor="#ffffff"><pre>
26      Bootstrap
27          |
28       System
29          |
30       Common
31       /     \
32  Webapp1   Webapp2 ...
33</pre></td><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./images/void.gif"></td></tr><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./images/void.gif"></td></tr></table></div>
34
35<p>The characteristics of each of these class loaders, including the source
36of classes and resources that they make visible, are discussed in detail in
37the following section.</p>
38
39</blockquote></td></tr></table><table cellpadding="2" cellspacing="0" border="0"><tr><td bgcolor="#525D76"><font face="arial,helvetica.sanserif" color="#ffffff"><a name="Class Loader Definitions"><strong>Class Loader Definitions</strong></a></font></td></tr><tr><td><blockquote>
40
41<p>As indicated in the diagram above, Tomcat 6 creates the following class
42loaders as it is initialized:</p>
43<ul>
44<li><strong>Bootstrap</strong> - This class loader contains the basic runtime
45    classes provided by the Java Virtual Machine, plus any classes from JAR
46    files present in the System Extensions directory
47    (<code>$JAVA_HOME/jre/lib/ext</code>).  <em>NOTE</em> - Some JVMs may
48    implement this as more than one class loader, or it may not be visible
49    (as a class loader) at all.</li>
50<li><strong>System</strong> - This class loader is normally initialized from
51    the contents of the <code>CLASSPATH</code> environment variable.  All such
52    classes are visible to both Tomcat internal classes, and to web
53    applications.  However, the standard Tomcat 6 startup scripts
54    (<code>$CATALINA_HOME/bin/catalina.sh</code> or
55    <code>%CATALINA_HOME%\bin\catalina.bat</code>) totally ignore the contents
56    of the <code>CLASSPATH</code> environment variable itself, and instead
57    build the System class loader from the following repositories:
58    <ul>
59    <li><em>$CATALINA_HOME/bin/bootstrap.jar</em> - Contains the main() method
60        that is used to initialize the Tomcat 6 server, and the class loader
61        implementation classes it depends on.</li>
62    <li><em>$CATALINA_HOME/bin/tomcat-juli.jar</em> - Package renamed Jakarta commons
63        logging API, and java.util.logging LogManager.</li>
64    </ul></li>
65<li><strong>Common</strong> - This class loader contains additional classes
66    that are made visible to both Tomcat internal classes and to all web
67    applications.  Normally, application classes should <strong>NOT</strong>
68    be placed here.  All unpacked classes and resources in
69    <code>$CATALINA_HOME/lib</code>, as well as classes and
70    resources in JAR files are made visible through this
71    class loader.  By default, that includes the following:
72    <ul>
73    <li><em>annotations-api.jar</em> - JEE annotations classes.</li>
74    <li><em>catalina.jar</em> - Implementation of the Catalina servlet
75        container portion of Tomcat 6.</li>
76    <li><em>catalina-ant.jar</em> - Tomcat Catalina Ant tasks.</li>
77    <li><em>catalina-ha.jar</em> - High availability package.</li>
78    <li><em>catalina-tribes.jar</em> - Group communication package.</li>
79    <li><em>el-api.jar</em> - EL 2.1 API.</li>
80    <li><em>jasper.jar</em> - Jasper 2 Compiler and Runtime.</li>
81    <li><em>jasper-el.jar</em> - Jasper 2 EL implementation.</li>
82    <li><em>jasper-jdt.jar</em> - Eclipse JDT 3.2 Java compiler.</li>
83    <li><em>jsp-api.jar</em> - JSP 2.1 API.</li>
84    <li><em>servlet-api.jar</em> - Servlet 2.5 API.</li>
85    <li><em>tomcat-coyote.jar</em> - Tomcat connectors and utility classes.</li>
86    <li><em>tomcat-dbcp.jar</em> - package renamed database connection
87        pool based on Commons DBCP.</li>
88    <li><em>tomcat-i18n-**.jar</em> - Optional JARs containing resource bundles
89        for other languages. As default bundles are also included in each
90        individual JAR, they can be safely removed if no internationalization
91        of messages is needed.</li>
92    </ul></li>
93<li><strong>WebappX</strong> - A class loader is created for each web
94    application that is deployed in a single Tomcat 6 instance.  All unpacked
95    classes and resources in the <code>/WEB-INF/classes</code> directory of
96    your web application archive, plus classes and resources in JAR files
97    under the <code>/WEB-INF/lib</code> directory of your web application
98    archive, are made visible to the containing web application, but to
99    no others.</li>
100</ul>
101
102<p>As mentioned above, the web application class loader diverges from the
103default Java 2 delegation model (in accordance with the recommendations in the
104Servlet Specification, version 2.3, section 9.7.2 Web Application Classloader). 
105When a request to load a
106class from the web application's <em>WebappX</em> class loader is processed,
107this class loader will look in the local repositories <strong>first</strong>,
108instead of delegating before looking.  There are exceptions. Classes which are
109part of the JRE base classes cannot be overriden. For some classes (such as
110the XML parser components in J2SE 1.4+), the J2SE 1.4 endorsed feature can be
111used.
112Last, any JAR containing servlet API classes will be ignored by the
113classloader.
114All other class loaders in Tomcat 6 follow the usual delegation pattern.</p>
115
116<p>Therefore, from the perspective of a web application, class or resource
117loading looks in the following repositories, in this order:</p>
118<ul>
119<li>Bootstrap classes of your JVM</li>
120<li>System class loader classses (described above)</li>
121<li><em>/WEB-INF/classes</em> of your web application</li>
122<li><em>/WEB-INF/lib/*.jar</em> of your web application</li>
123<li><em>$CATALINA_HOME/lib</em></li>
124<li><em>$CATALINA_HOME/lib/*.jar</em></li>
125</ul>
126
127</blockquote></td></tr></table><table cellpadding="2" cellspacing="0" border="0"><tr><td bgcolor="#525D76"><font face="arial,helvetica.sanserif" color="#ffffff"><a name="XML Parsers and JSE 5"><strong>XML Parsers and JSE 5</strong></a></font></td></tr><tr><td><blockquote>
128
129<p>Among many other changes, the JSE 5 release packages the JAXP APIs, and
130a version of Xerces, inside the JRE.  This has impacts on applications that
131wish to use their own XML parser.</p>
132
133<p>In previous versions of Tomcat, you could simply replace the XML parser
134in the <code>$CATALINA_HOME/common/lib</code> directory to change the parser
135used by all web applications.  However, this technique will not be effective
136when you are running on JSE 5, because the usual class loader delegation
137process will always choose the implementation inside the JDK in preference
138to this one.</p>
139
140<p>JDK 1.5 supports a mechanism called the "Endorsed Standards Override
141Mechanism" to allow replacement of APIs created outside of the JCP (i.e.
142DOM and SAX from W3C).  It can also be used to update the XML parser
143implementation.  For more information, see:
144<a href="http://java.sun.com/j2se/1.5/docs/guide/standards/index.html">
145http://java.sun.com/j2se/1.5/docs/guide/standards/index.html</a>.</p>
146
147<p>Tomcat utilizes this mechanism by including the system property setting
148<code>-Djava.endorsed.dirs=$JAVA_ENDORSED_DIRS</code> in the
149command line that starts the container.</p>
150
151</blockquote></td></tr></table><table cellpadding="2" cellspacing="0" border="0"><tr><td bgcolor="#525D76"><font face="arial,helvetica.sanserif" color="#ffffff"><a name="Running under a security manager"><strong>Running under a security manager</strong></a></font></td></tr><tr><td><blockquote>
152
153<p>When running under a security manager the locations from which classes
154are permitted to be loaded will also depend on the contents of your policy
155file. See <a href="security-manager-howto.html">Security Manager HOW-TO</a>
156for further information.</p>
157
158</blockquote></td></tr></table></td></tr><!--FOOTER SEPARATOR--><tr><td colspan="2"><hr size="1" noshade></td></tr><!--PAGE FOOTER--><tr><td colspan="2"><div align="center"><font size="-1" color="#525D76"><em>
159        Copyright &copy; 1999-2008, Apache Software Foundation
160        </em></font></div></td></tr></table></body></html>
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