<?xml version="1.0"?>
<!--
    Licensed to the Apache Software Foundation (ASF) under one or more
    contributor license agreements.  See the NOTICE file distributed with
    this work for additional information regarding copyright ownership.
    The ASF licenses this file to You under the Apache License, Version 2.0
    (the "License"); you may not use this file except in compliance with
    the License.  You may obtain a copy of the License at

         http://www.apache.org/licenses/LICENSE-2.0

    Unless required by applicable law or agreed to in writing, software
    distributed under the License is distributed on an "AS IS" BASIS,
    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    See the License for the specific language governing permissions and
    limitations under the License.
-->

<document xmlns="http://maven.apache.org/XDOC/2.0"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://maven.apache.org/XDOC/2.0 http://maven.apache.org/xsd/xdoc-2.0.xsd">
  <properties>
    <title>Log4j 2 Web Applications</title>
    <author email="nickwilliams@apache.org">Nick Williams</author>
    <author email="mattsicker@apache.org">Matt Sicker</author>
  </properties>

  <body>
    <section name="Using Log4j 2 in Web Applications">
      <p>
        You must take particular care when using Log4j or any other logging framework within a Java EE web application.
        It's important for logging resources to be properly cleaned up (database connections closed, files closed, etc.)
        when the container shuts down or the web application is undeployed. Because of the nature of class loaders
        within web applications, Log4j resources cannot be cleaned up through normal means. Log4j must be "started" when
        the web application deploys and "shut down" when the web application undeploys. How this works varies depending
        on whether your application is a <a href="#Servlet-3.0">Servlet 3.0 or newer</a> or
        <a href="#Servlet-2.5">Servlet 2.5</a> web application.
      </p>
      <p>
        In either case, you'll need to add the <code>log4j-web</code> module to your deployment as detailed in the
        <a href="../maven-artifacts.html">Maven, Ivy, and Gradle Artifacts</a> manual page.
      </p>
      <p class="big-red">
        <i class="icon-exclamation-sign"/>
        To avoid problems the Log4j shutdown hook will automatically be disabled when the log4j-web jar is included.
      </p>
      <a name="Configuration"/>
      <subsection name="Configuration">
        <p>
          Log4j allows the configuration file to be specified in web.xml using the <code>log4jConfiguration</code>
          context parameter. Log4j will search for configuration files by:
        </p>
        <ol>
          <li>
            If a location is provided it will be searched for as a servlet context resource. For example, if
            <code>log4jConfiguration</code> contains "logging.xml" then Log4j will look for a file with that
            name in the root directory of the web application.
          </li>
          <li>
            If no location is defined Log4j will search for a file that starts with "log4j2" in the WEB-INF directory.
            If more than one file is found, and if a file that starts with "log4j2-<var>name</var>" is present, where
            <var>name</var> is the name of the web application, then it will be used. Otherwise the first file will be
            used.
          </li>
          <li>
            The "normal" search sequence using the classpath and file URLs will be used to locate the configuration file.
          </li>
        </ol>
      </subsection>
      <subsection name="Servlet 3.0 and Newer Web Applications">
        <a name="Servlet-3.0" />
        <p>
          A Servlet 3.0 or newer web application is any <code>&lt;web-app&gt;</code> whose <code>version</code>
          attribute has a value of "3.0" or higher. Of course, the application must also be running in a compatible
          web container. Some examples are: Tomcat 7.0 and higher, GlassFish 3.0 and higher, JBoss 7.0 and higher,
          Oracle WebLogic 12c and higher, and IBM WebSphere 8.0 and higher.
        </p>
        <h4>The Short Story</h4>
        <p>
          Log4j 2 "just works" in Servlet 3.0 and newer web applications. It is capable of automatically starting when
          the application deploys and shutting down when the application undeploys. Thanks to the
          <a class="javadoc" href="http://docs.oracle.com/javaee/6/api/javax/servlet/ServletContainerInitializer.html">ServletContainerInitializer</a>
          API added to Servlet 3.0, the relevant <code>Filter</code> and <code>ServletContextListener</code> classes
          can be registered dynamically on web application startup.
        </p>
        <p>
          <strong class="text-warning"><i class="icon-exclamation-sign"/> Important Note!</strong>
          For performance reasons, containers often ignore certain JARs known not to
          contain TLDs or <code>ServletContainerInitializer</code>s and do not scan them for web-fragments and
          initializers. Importantly, Tomcat 7 &lt;7.0.43 ignores all JAR files named log4j*.jar, which prevents this
          feature from working. This has been fixed in Tomcat 7.0.43, Tomcat 8, and later. In Tomcat 7 &lt;7.0.43 you
          will need to change <code>catalina.properties</code> and remove "log4j*.jar" from the <code>jarsToSkip</code>
          property. You may need to do something similar on other containers if they skip scanning Log4j JAR files.
        </p>
        <h4>The Long Story</h4>
        <p>
          The Log4j 2 Web JAR file is a web-fragment configured to order before any other web fragments in your
          application. It contains a <code>ServletContainerInitializer</code>
          (<a class="javadoc" href="../log4j-web/apidocs/org/apache/logging/log4j/web/Log4jServletContainerInitializer.html"
            >Log4jServletContainerInitializer</a>) that the container automatically discovers and initializes. This adds
          the <a class="javadoc" href="../log4j-core/apidocs/org/apache/logging/log4j/web/Log4jServletContextListener.html"
            >Log4jServletContextListener</a> and
          <a class="javadoc" href="../log4j-web/apidocs/org/apache/logging/log4j/web/Log4jServletFilter.html"
            >Log4jServletFilter</a> to the <code>ServletContext</code>. These classes properly initialize
          and deinitialize the Log4j configuration.
        </p>
        <p>
          For some users, automatically starting Log4j is problematic or undesirable. You can easily disable this
          feature using the <code>isLog4jAutoInitializationDisabled</code> context parameter. Simply add it to your
          deployment descriptor with the value "true" to disable auto-initialization. You <em>must</em> define the
          context parameter in <code>web.xml</code>. If you set in programmatically, it will be too late for Log4j
          to detect the setting.
        </p>
          <pre class="prettyprint linenums"><![CDATA[    <context-param>
        <param-name>isLog4jAutoInitializationDisabled</param-name>
        <param-value>true</param-value>
    </context-param>]]></pre>
        <p>
          Once you disable auto-initialization, you must initialize Log4j as you would a
          <a href="#Servlet-2.5">Servlet 2.5 web application</a>. You must do so in a way that this initialization
          happens before any other application code (such as Spring Framework startup code) executes.
        </p>
        <p>
          You can customize the behavior of the listener and filter using the <code>log4jContextName</code>,
          <code>log4jConfiguration</code>, and/or <code>isLog4jContextSelectorNamed</code> context parameters. Read more
          about this in the <a href="#ContextParams">Context Parameters</a> section below. You <em>must not</em>
          manually configure the <code>Log4jServletContextListener</code> or <code>Log4jServletFilter</code> in your
          deployment descriptor (<code>web.xml</code>) or in another initializer or listener in a Servlet 3.0 or newer
          application <em>unless you disable auto-initialization</em> with
          <code>isLog4jAutoInitializationDisabled</code>. Doing so will result in startup errors and unspecified
          erroneous behavior.
        </p>
      </subsection>
      <subsection name="Servlet 2.5 Web Applications">
        <a name="Servlet-2.5" />
        <p>
          A Servlet 2.5 web application is any <code>&lt;web-app&gt;</code> whose <code>version</code> attribute has a
          value of "2.5." The <code>version</code> attribute is the only thing that matters; even if the web application
          is running in a Servlet 3.0 or newer container, it is a Servlet 2.5 web application if the
          <code>version</code> attribute is "2.5." Note that Log4j 2 does not support Servlet 2.4 and older web
          applications.
        </p>
        <p>
          If you are using Log4j in a Servlet 2.5 web application, or if you have disabled auto-initialization with
          the <code>isLog4jAutoInitializationDisabled</code> context parameter, you <em>must</em> configure the
          <a class="javadoc" href="../log4j-web/apidocs/org/apache/logging/log4j/web/Log4jServletContextListener.html"
            >Log4jServletContextListener</a> and
          <a class="javadoc" href="../log4j-web/apidocs/org/apache/logging/log4j/web/Log4jServletFilter.html"
            >Log4jServletFilter</a> in the deployment descriptor or programmatically. The filter should match all
          requests of any type. The listener should be the very first listener defined in your application, and the
          filter should be the very first filter defined and mapped in your application. This is easily accomplished
          using the following <code>web.xml</code> code:
        </p>
          <pre class="prettyprint linenums"><![CDATA[    <listener>
        <listener-class>org.apache.logging.log4j.web.Log4jServletContextListener</listener-class>
    </listener>

    <filter>
        <filter-name>log4jServletFilter</filter-name>
        <filter-class>org.apache.logging.log4j.web.Log4jServletFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>log4jServletFilter</filter-name>
        <url-pattern>/*</url-pattern>
        <dispatcher>REQUEST</dispatcher>
        <dispatcher>FORWARD</dispatcher>
        <dispatcher>INCLUDE</dispatcher>
        <dispatcher>ERROR</dispatcher>
        <dispatcher>ASYNC</dispatcher><!-- Servlet 3.0 w/ disabled auto-initialization only; not supported in 2.5 -->
    </filter-mapping>]]></pre>
        <p>
          You can customize the behavior of the listener and filter using the <code>log4jContextName</code>,
          <code>log4jConfiguration</code>, and/or <code>isLog4jContextSelectorNamed</code> context parameters. Read more
          about this in the <a href="#ContextParams">Context Parameters</a> section below.
        </p>
      </subsection>
      <subsection name="Context Parameters">
        <a name="ContextParams" />
        <p>
          By default, Log4j 2 uses the <code>ServletContext</code>'s
          <a href="http://docs.oracle.com/javaee/6/api/javax/servlet/ServletContext.html#getServletContextName()">context name</a>
          as the <code>LoggerContext</code> name and uses the standard pattern for locating the Log4j configuration
          file. There are three context parameters that you can use to control this behavior. The first,
          <code>isLog4jContextSelectorNamed</code>, specifies whether the context should be selected using the
          <a class="javadoc" href="../log4j-core/apidocs/org/apache/logging/log4j/core/selector/JndiContextSelector.html"
            >JndiContextSelector</a>. If <code>isLog4jContextSelectorNamed</code> is not specified or is anything other
          than <code>true</code>, it is assumed to be <code>false</code>.
        </p>
        <p>
          If <code>isLog4jContextSelectorNamed</code> is <code>true</code>, <code>log4jContextName</code> must be
          specified or <code>display-name</code> must be specified in <code>web.xml</code>; otherwise, the application
          will fail to start with an exception. <code>log4jConfiguration</code>
          <em>should</em> also be specified in this case, and must be a valid URI for the configuration file; however,
          this parameter is not required.
        </p>
        <p>
          If <code>isLog4jContextSelectorNamed</code> is not <code>true</code>, <code>log4jConfiguration</code> may
          optionally be specified and must be a valid URI or path to a configuration file or start with "classpath:" to
          denote a configuration file that can be found on the classpath. Without this parameter, Log4j will use the
          standard mechanisms for locating the configuration file.
        </p>
        <p>
          When specifying these context parameters, you must specify them in the deployment descriptor
          (<code>web.xml</code>) even in a Servlet 3.0 or never application. If you add them to the
          <code>ServletContext</code> within a listener, Log4j will initialize before the context parameters are
          available and they will have no effect. Here are some sample uses of these context parameters.
        </p>
        <h4>Set the Logging Context Name to "myApplication"</h4>
        <pre class="prettyprint linenums"><![CDATA[    <context-param>
        <param-name>log4jContextName</param-name>
        <param-value>myApplication</param-value>
    </context-param>]]></pre>
        <h4>Set the Configuration Path/File/URI to "/etc/myApp/myLogging.xml"</h4>
        <pre class="prettyprint linenums"><![CDATA[    <context-param>
        <param-name>log4jConfiguration</param-name>
        <param-value>file:///etc/myApp/myLogging.xml</param-value>
    </context-param>]]></pre>
        <h4>Use the <code>JndiContextSelector</code></h4>
        <pre class="prettyprint linenums"><![CDATA[    <context-param>
        <param-name>isLog4jContextSelectorNamed</param-name>
        <param-value>true</param-value>
    </context-param>
    <context-param>
        <param-name>log4jContextName</param-name>
        <param-value>appWithJndiSelector</param-value>
    </context-param>
    <context-param>
        <param-name>log4jConfiguration</param-name>
        <param-value>file:///D:/conf/myLogging.xml</param-value>
    </context-param>]]></pre>
        <p>
          Note that in this case you must also set the "Log4jContextSelector" system property to
          "<kbd>org.apache.logging.log4j.core.selector.JndiContextSelector</kbd>".
        </p>
      </subsection>
      <subsection name="Using Web Application Information During the Configuration">
        <a name="WebLookup" />
        <p>
          You may want to use information about the web application during configuration. For example, you could embed
          the web application's context path in the name of a Rolling File Appender. See WebLookup in
          <a href="./lookups.html#WebLookup">Lookups</a> for more information.
        </p>
      </subsection>
      <subsection name="JavaServer Pages Logging">
        <a name="JspLogging" />
        <p>
          You may use Log4j 2 within JSPs just as you would within any other Java code. Simple obtain a
          <code>Logger</code> and call its methods to log events. However, this requires you to use Java code within
          your JSPs, and some development teams rightly are not comfortable with doing this. If you have a dedicated
          user interface development team that is not familiar with using Java, you may even have Java code disabled in
          your JSPs.
        </p>
        <p>
          For this reason, Log4j 2 provides a JSP Tag Library that enables you to log events without using any Java
          code. To read more about using this tag library, <a href="../log4j-taglib/index.html">read the Log4j Tag
          Library documentation.</a>
        </p>
        <p>
          <strong class="text-warning"><i class="icon-exclamation-sign"/> Important Note!</strong>
          As noted above, containers often ignore certain JARs known not to
          contain TLDs and do not scan them for TLD files. Importantly, Tomcat 7 &lt;7.0.43 ignores all JAR files named
          log4j*.jar, which prevents the JSP tag library from being automatically discovered. This does not affect
          Tomcat 6.x and has been fixed in Tomcat 7.0.43, Tomcat 8, and later. In Tomcat 7 &lt;7.0.43 you
          will need to change <code>catalina.properties</code> and remove "log4j*.jar" from the <code>jarsToSkip</code>
          property. You may need to do something similar on other containers if they skip scanning Log4j JAR files.
        </p>
      </subsection>
      <subsection name="Asynchronous Requests and Threads">
        <a name="Async" />
        <p>
          The handling of asynchronous requests is tricky, and regardless of Servlet container version or configuration
          Log4j cannot handle everything automatically. When standard requests, forwards, includes, and error resources
          are processed, the <code>Log4jServletFilter</code> binds the <code>LoggerContext</code> to the thread handling
          the request. After request processing completes, the filter unbinds the <code>LoggerContext</code> from the
          thread.
        </p>
        <p>
          Similarly, when an internal request is dispatched using a <code>javax.servlet.AsyncContext</code>, the
          <code>Log4jServletFilter</code> also binds the <code>LoggerContext</code> to the thread handling the request
          and unbinds it when request processing completes. However, this only happens for requests <em>dispatched</em>
          through the <code>AsyncContext</code>. There are other asynchronous activities that can take place other than
          internal dispatched requests.
        </p>
        <p>
          For example, after starting an <code>AsyncContext</code> you could start up a separate thread to process the
          request in the background, possibly writing the response with the <code>ServletOutputStream</code>. Filters
          cannot intercept the execution of this thread. Filters also cannot intercept threads that you start in
          the background during non-asynchronous requests. This is true whether you use a brand new thread or a thread
          borrowed from a thread pool. So what can you do for these special threads?
        </p>
        <p>
          You may not need to do anything. If you didn't use the <code>isLog4jContextSelectorNamed</code> context
          parameter, there is no need to bind the <code>LoggerContext</code> to the thread. Log4j can safely locate the
          <code>LoggerContext</code> on its own. In these cases, the filter provides only very modest performance
          gains, and only when creating new <code>Logger</code>s. However, if you <em>did</em> specify the
          <code>isLog4jContextSelectorNamed</code> context parameter with the value "true", you will need to manually
          bind the <code>LoggerContext</code> to asynchronous threads. Otherwise, Log4j will not be able to locate it.
        </p>
        <p>
          Thankfully, Log4j provides a simple mechanism for binding the <code>LoggerContext</code> to asynchronous
          threads in these special circumstances. The simplest way to do this is to wrap the <code>Runnable</code>
          instance that is passed to the <code>AsyncContext.start()</code> method.
        </p>
        <pre class="prettyprint linenums"><![CDATA[
import java.io.IOException;
import javax.servlet.AsyncContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.web.WebLoggerContextUtils;

public class TestAsyncServlet extends HttpServlet {

    @Override
    protected void doGet(final HttpServletRequest req, final HttpServletResponse resp) throws ServletException, IOException {
        final AsyncContext asyncContext = req.startAsync();
        asyncContext.start(WebLoggerContextUtils.wrapExecutionContext(this.getServletContext(), new Runnable() {
            @Override
            public void run() {
                final Logger logger = LogManager.getLogger(TestAsyncServlet.class);
                logger.info("Hello, servlet!");
            }
        }));
    }

    @Override
    protected void doPost(final HttpServletRequest req, final HttpServletResponse resp) throws ServletException, IOException {
        final AsyncContext asyncContext = req.startAsync();
        asyncContext.start(new Runnable() {
            @Override
            public void run() {
                final Log4jWebSupport webSupport =
                    WebLoggerContextUtils.getWebLifeCycle(TestAsyncServlet.this.getServletContext());
                webSupport.setLoggerContext();
                // do stuff
                webSupport.clearLoggerContext();
            }
        });
    }
}
        ]]></pre>
        <p>
          This can be slightly more convenient when using Java 1.8 and lambda functions as demonstrated below.
        </p>
        <pre class="prettyprint linenums"><![CDATA[
import java.io.IOException;
import javax.servlet.AsyncContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.web.WebLoggerContextUtils;

public class TestAsyncServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        final AsyncContext asyncContext = req.startAsync();
        asyncContext.start(WebLoggerContextUtils.wrapExecutionContext(this.getServletContext(), () -> {
            final Logger logger = LogManager.getLogger(TestAsyncServlet.class);
            logger.info("Hello, servlet!");
        }));
    }
}
        ]]></pre>
        <p>
          Alternatively, you can obtain the
          <a class="javadoc" href="../log4j-web/apidocs/org/apache/logging/log4j/web/Log4jWebLifeCycle.html">Log4jWebLifeCycle</a>
          instance from the <code>ServletContext</code> attributes, call its <code>setLoggerContext</code> method as
          the very first line of code in your asynchronous thread, and call its <code>clearLoggerContext</code> method
          as the very last line of code in your asynchronous thread. The following code demonstrates this. It uses the
          container thread pool to execute asynchronous request processing, passing an anonymous inner
          <code>Runnable</code> to the <code>start</code> method.
        </p>
        <pre class="prettyprint linenums"><![CDATA[
import java.io.IOException;
import javax.servlet.AsyncContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.web.Log4jWebLifeCycle;
import org.apache.logging.log4j.web.WebLoggerContextUtils;

public class TestAsyncServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
         final AsyncContext asyncContext = req.startAsync();
        asyncContext.start(new Runnable() {
            @Override
            public void run() {
                final Log4jWebLifeCycle webLifeCycle =
                    WebLoggerContextUtils.getWebLifeCycle(TestAsyncServlet.this.getServletContext());
                webLifeCycle.setLoggerContext();
                try {
                    final Logger logger = LogManager.getLogger(TestAsyncServlet.class);
                    logger.info("Hello, servlet!");
                } finally {
                    webLifeCycle.clearLoggerContext();
                }
            }
        });
   }
}
        ]]></pre>
        <p>
          Note that you <em>must</em> call <code>clearLoggerContext</code> once your thread is finished
          processing. Failing to do so will result in memory leaks. If using a thread pool, it can even disrupt the
          logging of other web applications in your container. For that reason, the example here shows clearing the
          context in a <code>finally</code> block, which will always execute.
        </p>
      </subsection>
      <subsection name="Using the Servlet Appender">
        <p>
          Log4j provides a Servlet Appender that uses the servlet context as the log target. For example:
        </p>
        <pre class="prettyprint linenums"><![CDATA[
<Configuration status="WARN" name="ServletTest">

    <Appenders>
        <Servlet name="Servlet">
            <PatternLayout pattern="%m%n%ex{none}"/>
        </Servlet>
    </Appenders>

    <Loggers>
        <Root level="debug">
            <AppenderRef ref="Servlet"/>
        </Root>
    </Loggers>

</Configuration>]]></pre>
        <p>
          To avoid double logging of exceptions to the servlet context, you must use <code>%ex{none}</code> in your 
          <code>PatternLayout</code> as shown in the example. The exception will be omitted from the message text but 
          it is passed to the servlet context as the actual Throwable object. 
        </p>
      </subsection>
    </section>
  </body>

</document>
