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<!DOCTYPE document [
  <!ENTITY project SYSTEM "project.xml">
]>
<document url="loader.html">

  &project;

  <properties>
    <author email="craigmcc@apache.org">Craig R. McClanahan</author>
    <title>The Loader Component</title>
  </properties>

<body>

<section name="Table of Contents">
<toc/>
</section>

<section name="Introduction">

  <p>The <strong>Loader</strong> element represents the <em>web
  application class loader</em> that will be used to load Java
  classes and resources for your web application.  Such
  a class loader must follow the requirements of the Servlet
  Specification, and load classes from the following locations:</p>
  <ul>
  <li>From the <code>/WEB-INF/classes</code> directory inside your
      web application.</li>
  <li>From JAR files in the <code>/WEB-INF/lib</code> directory
      inside your web application.</li>
  <li>From resources made available by Catalina to all web
      applications globally.</li>
  </ul>

  <p>A Loader element MAY be nested inside a <a href="context.html">Context</a>
  component.  If it is not included, a default Loader configuration will be
  created automatically, which is sufficient for most requirements.</p>

  <p>For a more in-depth description of the class loader hierarchy
  that is implemented by Catalina, see <a href="../class-loader-howto.html">the ClassLoader HowTo</a>.</p>

    <p><em>The description below uses the variable name $CATALINA_BASE to refer the
    base directory against which most relative paths are resolved. If you have
    not configured Tomcat for multiple instances by setting a CATALINA_BASE
    directory, then $CATALINA_BASE will be set to the value of $CATALINA_HOME,
    the directory into which you have installed Tomcat.</em></p>

</section>


<section name="Attributes">

  <subsection name="Common Attributes">

    <p>All implementations of <strong>Loader</strong>
    support the following attributes:</p>

    <attributes>

      <attribute name="className" required="false">
        <p>Java class name of the implementation to use.  This class must
        implement the <code>org.apache.catalina.Loader</code> interface.
        If not specified, the standard value (defined below) will be used.</p>
      </attribute>

      <attribute name="delegate" required="false">
        <p>Set to <code>true</code> if you want the class loader to follow
        the standard Java2 delegation model, and attempt to load classes from
        parent class loaders <strong>before</strong> looking inside the web
        application.  Set to <code>false</code> (the default) to have the
        class loader look inside the web application first, before asking
        parent class loaders to find requested classes or resources.</p>
      </attribute>

    </attributes>

  </subsection>


  <subsection name="Standard Implementation">

    <p>The standard implementation of <strong>Loader</strong> is
    <strong>org.apache.catalina.loader.WebappLoader</strong>.
    It supports the following additional attributes (in addition to the
    common attributes listed above):</p>

    <attributes>

      <attribute name="loaderClass" required="false">
        <p>Java class name of the <code>java.lang.ClassLoader</code>
        implementation class to use. Custom implementations must extend
        <code>org.apache.catalina.loader.WebappClassLoaderBase</code>.
        </p>

        <p>If not specified, the default value is
        <code>org.apache.catalina.loader.ParallelWebappClassLoader</code>. The
        default <strong>loaderClass</strong> is parallel capable, which
        means that multiple threads may load difference classes in parallel.
        A non-parallel capable <strong>loaderClass</strong> is available and can
        be used by specifying
        <code>org.apache.catalina.loader.WebappClassLoader</code>.</p>
      </attribute>

    </attributes>

  </subsection>

</section>


<section name="Nested Components">

  <p>No components may be nested inside a <strong>Loader</strong> element.</p>

</section>


<section name="Special Features">

  <subsection name="Logging">

    <p>A loader is associated with the log category based on its classname.</p>

  </subsection>

</section>


</body>


</document>
