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