159 lines
12 KiB
HTML
159 lines
12 KiB
HTML
<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 name="author" value="Craig R. McClanahan"><meta name="email" value="craigmcc@apache.org"><meta name="author" value="Yoav Shapira"><meta name="email" value="yoavs@apache.org"></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><font face="arial,helvetica,sanserif"><h1>Apache Tomcat 6.0</h1></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><!--RIGHT SIDE MAIN BODY--><td width="80%" valign="top" align="left"><table border="0" width="100%" cellspacing="4"><tr><td align="left" valign="top"><h1>Apache Tomcat 6.0</h1><h2>Class Loader HOW-TO</h2></td><td align="right" valign="top" nowrap="true"><img src="./../images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"></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 6 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 J2SE 2 (that is, J2SE 1.2 or later) 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. The model for
|
|
web application class loaders differs slightly from this, as discussed below,
|
|
but the main principles are the same.</p>
|
|
|
|
<p>When Tomcat 6 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" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./../images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./../images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr><tr><td bgcolor="#023264" width="1"><img src="./../images/void.gif" 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" 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" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" height="1"><img src="./../images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"></td><td bgcolor="#023264" width="1" height="1"><img src="./../images/void.gif" 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"><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><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>$JAVA_HOME/jre/lib/ext</code>). <em>NOTE</em> - Some JVMs may
|
|
implement this as more than one class loader, or it may not be visible
|
|
(as a class loader) at all.</li>
|
|
<li><strong>System</strong> - This class loader is normally initialized from
|
|
the contents of the <code>CLASSPATH</code> environment variable. All such
|
|
classes are visible to both Tomcat internal classes, and to web
|
|
applications. However, the standard Tomcat 5 startup scripts
|
|
(<code>$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:
|
|
<ul>
|
|
<li><em>$CATALINA_HOME/bin/bootstrap.jar</em> - Contains the main() method
|
|
that is used to initialize the Tomcat 6 server, and the class loader
|
|
implementation classes it depends on.</li>
|
|
<li><em>$CATALINA_HOME/bin/tomcat-juli.jar</em> - Package renamed Jakarta commons
|
|
logging API, and java.util.logging LogManager.</li>
|
|
</ul></li>
|
|
<li><strong>Common</strong> - This class loader contains additional classes
|
|
that are made visible to both Tomcat internal classes and to all web
|
|
applications. Normally, application classes should <strong>NOT</strong>
|
|
be placed here. All unpacked classes and resources in
|
|
<code>$CATALINA_HOME/lib</code>, as well as classes and
|
|
resources in JAR files are made visible through this
|
|
class loader. By default, that includes the following:
|
|
<ul>
|
|
<li><em>annotations-api.jar</em> - JEE annotations classes.</li>
|
|
<li><em>catalina.jar</em> - Implementation of the Catalina servlet
|
|
container portion of Tomcat 6.</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>el-api.jar</em> - EL 2.1 API.</li>
|
|
<li><em>jasper.jar</em> - Jasper 2 Compiler and Runtime.</li>
|
|
<li><em>jasper-el.jar</em> - Jasper 2 EL implementation.</li>
|
|
<li><em>jasper-jdt.jar</em> - Eclipse JDT 3.2 Java compiler.</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> - package renamed database connection
|
|
pool based on 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><strong>WebappX</strong> - A class loader is created for each web
|
|
application that is deployed in a single Tomcat 6 instance. All unpacked
|
|
classes and resources in the <code>/WEB-INF/classes</code> directory of
|
|
your web application archive, plus classes and resources in JAR files
|
|
under the <code>/WEB-INF/lib</code> directory of your web application
|
|
archive, are made visible to the containing web application, but to
|
|
no others.</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 2.3, 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 containing servlet API classes will be ignored by the
|
|
classloader.
|
|
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 classses (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><em>$CATALINA_HOME/lib</em></li>
|
|
<li><em>$CATALINA_HOME/lib/*.jar</em></li>
|
|
</ul>
|
|
|
|
</blockquote></td></tr></table><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="XML Parsers and JSE 5"><strong>XML Parsers and JSE 5</strong></a></font></td></tr><tr><td><blockquote>
|
|
|
|
<p>Among many other changes, the JSE 5 release packages the JAXP APIs, and
|
|
a version of Xerces, inside the JRE. This has impacts on applications that
|
|
wish to use their own XML parser.</p>
|
|
|
|
<p>In previous versions of Tomcat, you could simply replace the XML parser
|
|
in the <code>$CATALINA_HOME/common/lib</code> directory to change the parser
|
|
used by all web applications. However, this technique will not be effective
|
|
when you are running on JSE 5, because the usual class loader delegation
|
|
process will always choose the implementation inside the JDK in preference
|
|
to this one.</p>
|
|
|
|
<p>JDK 1.5 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://java.sun.com/j2se/1.5/docs/guide/standards/index.html">
|
|
http://java.sun.com/j2se/1.5/docs/guide/standards/index.html</a>.</p>
|
|
|
|
<p>Tomcat utilizes this mechanism by including the system property setting
|
|
<code>-Djava.endorsed.dirs=$CATALINA_HOME/endorsed</code> in the
|
|
command line that starts the container.</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"><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>
|
|
Copyright © 1999-2006, Apache Software Foundation
|
|
</em></font></div></td></tr></table></body></html> |