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    <properties>
        <title>Logging Separation</title>
        <author email="rgoers@apache.org">Ralph Goers</author>
    </properties>

    <body>
      <section name="Logging Separation">
        <p>
          There are many well known use cases where applications may share an environment with other applications
          and each has a need to have its own, separate logging environment. This purpose of this section is to
          discuss some of these cases and ways to accomplish this.
        </p>
        <subsection name="Use Cases">
          <a name="UseCases"/>
          <p>
            This section describes some of the use cases where Log4j could be used and what its desired behavior
            might be.
          </p>
          <h4>Standalone Application</h4>
            <p>
              Standalone applications are usually relatively simple. They typically have one bundled executable
              that requires only a single logging configuration.
            </p>
          <h4>Web Applications</h4>
            <p>
              A typical web application will be packaged as a WAR file and will include all of its dependencies in
              WEB-INF/lib and will have its configuration file located in the class path or in a location
              configured in the web.xml. Be sure to follow the <a href="webapp.html">instructions to initialize Log4j 2
              in a web application</a>.
            </p>
          <h4>Java EE Applications</h4>
            <p>
              A Java EE application will consist of one or more WAR files and possible some EJBs, typically all
              packaged in an EAR file. Usually, it is desirable to have a single configuration that applies to
              all the components in the EAR. The logging classes will generally be placed in a location shared
              across all the components and the configuration needs to also be shareable.  Be sure to follow the
              <a href="webapp.html">instructions to initialize Log4j 2 in a web application</a>.
            </p>
          <h4>"Shared" Web Applications and REST Service Containers</h4>
            <p>
              In this scenario there are multiple WAR files deployed into a single container. Each of the applications
              should use the same logging configuration and share the same logging implementation across each of the
              web applications. When writing to files and streams each of the applications should share them to avoid
              the issues that can occur when multiple components try to write to the same file(s) through different
              File objects, channels, etc.
            </p>
          <h4>OSGi Applications</h4>
            <p>
              An OSGi container physically separates each JAR into its own ClassLoader, thus enforcing modularity of
              JARs as well as providing standardized ways for JARs to share code based on version numbers. Suffice to
              say, the OSGi framework is beyond the scope of this manual. There are some differences when using Log4j
              in an OSGi container. By default, each JAR bundle is scanned for its own Log4j configuration file.
              Similar to the web application paradigm, every bundle has its own LoggerContext. As this may be
              undesirable when a global Log4j configuration is wanted, then the
              <a href="extending.html#ContextSelector">ContextSelector</a> should be overridden with
              <code>BasicContextSelector</code> or <code>JndiContextSelector</code>.
            </p>
        </subsection>
        <subsection name="Approaches">
          <a name="Approaches"/>
          <h4>The Simple Approach</h4>
            <p>
              The simplest approach for separating logging within applications is to package each application with
              its own copy of Log4j and to use the BasicContextSelector. While this works for standalone applications
              and may work for web applications and possibly Java EE applications, it does not work at all in the
              last case.  However, when this approach does work it should be used as it is ultimately the simplest
              and most straightforward way of implementing logging.
            </p>

          <h4>Using Context Selectors</h4>
            <p>
              There are a few patterns for achieving the desired state of logging separation using ContextSelectors:
            </p>
              <ol>
                <li>Place the logging jars in the container's classpath and set the system property
                  <tt>log4j2.contextSelector</tt> to <code>org.apache.logging.log4j.core.selector.BasicContextSelector</code>.
                  This will create a single LoggerContext using a single configuration that will be shared across all
                  applications.</li>
                <li>
                  Place the logging jars in the container's classpath and use the default ClassLoaderContextSelector.
                  Follow the <a href="webapp.html">instructions to initialize Log4j 2 in a web application</a>. Each
                  application can be configured to share the same configuration used at the container or can be
                  individually configured. If status logging is set to debug in the configuration there will be output
                  from when logging is initialized in the container and then again in each web application.
                </li>
                <li>
                  Follow the <a href="webapp.html">instructions to initialize Log4j 2 in a web application</a> and set
                  the system property or servlet context parameter <tt>log4j2.contextSelector</tt> to
                  <kbd>org.apache.logging.log4j.core.selector.JndiContextSelector</kbd>. This will cause the container
                  to use JNDI to locate each web application's <code>LoggerContext</code>. Be sure to set the
                  <code>isLog4jContextSelectorNamed</code> context parameter to <kbd>true</kbd> and also set the
                  <code>log4jContextName</code> and <code>log4jConfiguration</code> context parameters.
                  Note that the JndiContextSelector will not work unless <code>log4j2.enableJndiContextSelector=true</code> is set as a
                  system property or environment variable. See the
                  <a href="./configuration.html#enableJndiContextSelector">enableJndiContextSelector</a> system property.
                </li>
              </ol>
            <p>
              The exact method for setting system properties depends on the container. For Tomcat, edit
              <code>$CATALINA_HOME/conf/catalina.properties</code>. Consult the documentation for other web containers.
            </p>
        </subsection>
      </section>
    </body>
</document>
