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159 lines
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<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="
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The Apache Tomcat Servlet/JSP Container
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" 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>
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<p>Like many server applications, Tomcat 6 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 and 9.6.</p>
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<p>In a J2SE 2 (that is, J2SE 1.2 or later) 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. The model for
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web application class loaders differs slightly from this, as discussed below,
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but the main principles are the same.</p>
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<p>When Tomcat 6 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 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>
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Bootstrap
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System
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Common
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/ \
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Webapp1 Webapp2 ...
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</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>
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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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</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>
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<p>As indicated in the diagram above, Tomcat 6 creates the following class
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loaders as it is initialized:</p>
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<ul>
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<li><strong>Bootstrap</strong> - This class loader contains the basic runtime
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classes provided by the Java Virtual Machine, plus any classes from JAR
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files present in the System Extensions directory
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(<code>$JAVA_HOME/jre/lib/ext</code>). <em>NOTE</em> - Some JVMs may
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implement this as more than one class loader, or it may not be visible
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(as a class loader) at all.</li>
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<li><strong>System</strong> - This class loader is normally initialized from
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the contents of the <code>CLASSPATH</code> environment variable. All such
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classes are visible to both Tomcat internal classes, and to web
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applications. However, the standard Tomcat 5 startup scripts
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(<code>$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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<ul>
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<li><em>$CATALINA_HOME/bin/bootstrap.jar</em> - Contains the main() method
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that is used to initialize the Tomcat 6 server, and the class loader
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implementation classes it depends on.</li>
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<li><em>$CATALINA_HOME/bin/tomcat-juli.jar</em> - Package renamed Jakarta commons
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logging API, and java.util.logging LogManager.</li>
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</ul></li>
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<li><strong>Common</strong> - This class loader contains additional classes
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that are made visible to both Tomcat internal classes and to all web
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applications. Normally, application classes should <strong>NOT</strong>
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be placed here. All unpacked classes and resources in
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<code>$CATALINA_HOME/lib</code>, as well as classes and
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resources in JAR files are made visible through this
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class loader. By default, that includes the following:
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<ul>
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<li><em>annotations-api.jar</em> - JEE 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 6.</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-tribes.jar</em> - Group communication package.</li>
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<li><em>el-api.jar</em> - EL 2.1 API.</li>
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<li><em>jasper.jar</em> - Jasper 2 Compiler and Runtime.</li>
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<li><em>jasper-el.jar</em> - Jasper 2 EL implementation.</li>
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<li><em>jasper-jdt.jar</em> - Eclipse JDT 3.2 Java compiler.</li>
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<li><em>jsp-api.jar</em> - JSP 2.1 API.</li>
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<li><em>servlet-api.jar</em> - Servlet 2.5 API.</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> - package renamed database connection
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pool based on 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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</ul></li>
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<li><strong>WebappX</strong> - A class loader is created for each web
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application that is deployed in a single Tomcat 6 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 archive, 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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archive, are made visible to the containing web application, but to
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no others.</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 2 delegation model (in accordance with the recommendations in the
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Servlet Specification, version 2.3, 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 overriden. For some classes (such as
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the XML parser components in J2SE 1.4+), the J2SE 1.4 endorsed feature can be
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used.
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Last, any JAR containing servlet API classes will be ignored by the
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classloader.
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All other class loaders in Tomcat 6 follow the 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>System class loader classses (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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<li><em>$CATALINA_HOME/lib</em></li>
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<li><em>$CATALINA_HOME/lib/*.jar</em></li>
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</ul>
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</blockquote></td></tr></table><table border="0" cellspacing="0" cellpadding="2"><tr><td bgcolor="#525D76"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="XML Parsers and JSE 5"><strong>XML Parsers and JSE 5</strong></a></font></td></tr><tr><td><blockquote>
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<p>Among many other changes, the JSE 5 release packages the JAXP APIs, and
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a version of Xerces, inside the JRE. This has impacts on applications that
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wish to use their own XML parser.</p>
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<p>In previous versions of Tomcat, you could simply replace the XML parser
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in the <code>$CATALINA_HOME/common/lib</code> 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 on JSE 5, because the usual class loader delegation
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process will always choose the implementation inside the JDK in preference
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to this one.</p>
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<p>JDK 1.5 supports a mechanism called the "Endorsed Standards Override
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Mechanism" to allow replacement of APIs created outside of the JCP (i.e.
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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://java.sun.com/j2se/1.5/docs/guide/standards/index.html">
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http://java.sun.com/j2se/1.5/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=$CATALINA_HOME/endorsed</code> in the
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command line that starts the container.</p>
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</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>
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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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</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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Copyright © 1999-2006, Apache Software Foundation
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</em></font></div></td></tr></table></body></html>
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