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<!DOCTYPE document [
  <!ENTITY project SYSTEM "project.xml">
  <!ENTITY le              "&#x02264;" >
]>
<document url="logging.html">

    &project;

  <properties>
    <title>Logging in Tomcat</title>
  </properties>

<body>

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

  <section name="Introduction">
    <p>
      The internal logging for Apache Tomcat uses JULI, a packaged renamed fork
      of <a href="https://commons.apache.org/logging">Apache Commons Logging</a>
      that is hard-coded to use the <code>java.util.logging</code> framework.
      This ensures that Tomcat&apos;s internal logging and any web application
      logging will remain independent, even if a web application uses Apache
      Commons Logging.
    </p>

    <p>
      To configure Tomcat to use an alternative logging framework for its
      internal logging, follow the instructions provided by the alternative
      logging framework for redirecting logging for applications that use
      <code>java.util.logging</code>. Links to some of these instructions are
      provided towards the end of this page. Keep in mind that the alternative
      logging framework will need to be capable of working in an environment
      where different loggers with the same name may exist in different class
      loaders.
    </p>

    <p>
      A web application running on Apache Tomcat can:
    </p>
    <ul>
      <li>
        Use any logging framework of its choice.
      </li>
      <li>
        Use system logging API, <code>java.util.logging</code>.
      </li>
      <li>
        Use the logging API provided by the Java Servlets specification,
        <code>jakarta.servlet.ServletContext.log(...)</code>
      </li>
    </ul>

    <p>
      The logging frameworks used by different web applications are independent.
      See <a href="class-loader-howto.html">class loading</a> for more details.
      The exception to this rule is <code>java.util.logging</code>. If it is
      used directly or indirectly by your logging library then elements of it
      will be shared across web applications because it is loaded by the system
      class loader.
    </p>

    <subsection name="Java logging API &#8212; java.util.logging">

    <p>
      Apache Tomcat has its own implementation of several key elements of
      <code>java.util.logging</code> API. This implementation is called JULI.
      The key component there is a custom LogManager implementation,
      that is aware of different web applications running on Tomcat (and
      their different class loaders). It supports private per-application
      logging configurations. It is also notified by Tomcat when a web application
      is unloaded from memory, so that the references to its classes can be
      cleared, preventing memory leaks.
    </p>

    <p>
      This <code>java.util.logging</code> implementation is enabled by providing
      certain system properties when starting Java. The Apache Tomcat startup
      scripts do this for you, but if you are using different tools to run
      Tomcat (such as jsvc, or running Tomcat from within an IDE), you should
      take care of them by yourself.
    </p>

    <p>
      More details about java.util.logging may be found in the documentation
      for your JDK and on its Javadoc pages for the <code>java.util.logging</code>
      package.
    </p>

    <p>
      More details about Tomcat JULI may be found below.
    </p>

    </subsection>

    <subsection name="Servlets logging API">

    <p>
      The calls to <code>jakarta.servlet.ServletContext.log(...)</code> to write
      log messages are handled by internal Tomcat logging. Such messages are
      logged to the category named
    </p>
      <source>org.apache.catalina.core.ContainerBase.[${engine}].[${host}].[${context}]</source>
    <p>
      This logging is performed according to the Tomcat logging configuration. You
      cannot overwrite it in a web application.
    </p>

    <p>
      The Servlets logging API predates the <code>java.util.logging</code> API
      that is now provided by Java. As such, it does not offer you much options.
      E.g., you cannot control the log levels. It can be noted, though, that
      in Apache Tomcat implementation the calls to <code>ServletContext.log(String)</code>
      or <code>GenericServlet.log(String)</code> are logged at the INFO level.
      The calls to <code>ServletContext.log(String, Throwable)</code> or
      <code>GenericServlet.log(String, Throwable)</code>
      are logged at the SEVERE level.
    </p>

    </subsection>

    <subsection name="Console">

    <p>
      When running Tomcat on unixes, the console output is usually redirected
      to the file named <code>catalina.out</code>. The name is configurable
      using an environment variable. (See the startup scripts).
      Whatever is written to <code>System.err/out</code> will be caught into
      that file. That may include:
    </p>

    <ul>
      <li>Uncaught exceptions printed by <code>java.lang.ThreadGroup.uncaughtException(..)</code></li>
      <li>Thread dumps, if you requested them via a system signal</li>
    </ul>

    <p>
      When running as a service on Windows, the console output is also caught
      and redirected, but the file names are different.
    </p>

    <p>
      The default logging configuration in Apache Tomcat writes the same
      messages to the console and to a log file. This is great when using
      Tomcat for development, but usually is not needed in production.
    </p>

    <p>
      Old applications that still use <code>System.out</code> or <code>System.err</code>
      can be tricked by setting <code>swallowOutput</code> attribute on a
      <a href="config/context.html">Context</a>. If the attribute is set to
      <code>true</code>, the calls to <code>System.out/err</code> during request
      processing will be intercepted, and their output will be fed to the
      logging subsystem using the
      <code>jakarta.servlet.ServletContext.log(...)</code> calls.<br />
      <strong>Note</strong>, that the <code>swallowOutput</code>  feature is
      actually a trick, and it has its limitations.
      It works only with direct calls to <code>System.out/err</code>,
      and only during request processing cycle. It may not work in other
      threads that might be created by the application. It cannot be used to
      intercept logging frameworks that themselves write to the system streams,
      as those start early and may obtain a direct reference to the streams
      before the redirection takes place.
    </p>

    </subsection>

    <subsection name="Access logging">

    <p>
      Access logging is a related but different feature, which is
      implemented as a <code>Valve</code>. It uses self-contained
      logic to write its log files. The essential requirement for
      access logging is to handle a large continuous stream of data
      with low overhead, so it only uses Apache Commons Logging for
      its own debug messages. This implementation approach avoids
      additional overhead and potentially complex configuration.
      Please refer to the <a href="config/valve.html#Access_Logging">Valves</a>
      documentation for more details on its configuration, including
      the various report formats.
    </p>

    </subsection>

  </section>

  <section name="Using java.util.logging (default)">

  <p>
    The default implementation of java.util.logging provided in the JDK is too
    limited to be useful. The key limitation is the inability to have per-web
    application logging, as the configuration is per-VM. As a result, Tomcat
    will, in the default configuration, replace the default LogManager
    implementation with a container friendly implementation called JULI, which
    addresses these shortcomings.
  </p>
  <p>
    JULI supports the same configuration mechanisms as the standard JDK
    <code>java.util.logging</code>, using either a programmatic approach, or
    properties files. The main difference is that per-classloader properties
    files can be set (which enables easy redeployment friendly webapp
    configuration), and the properties files support extended constructs which
    allows more freedom for defining handlers and assigning them to loggers.
  </p>
  <p>
    JULI is enabled by default, and supports per classloader configuration, in
    addition to the regular global java.util.logging configuration. This means
    that logging can be configured at the following layers:
  </p>
    <ul>
      <li>Globally. That is usually done in the
        <code>${catalina.base}/conf/logging.properties</code> file.
        The file is specified by the <code>java.util.logging.config.file</code>
        System property which is set by the startup scripts.
        If it is not readable or is not configured, the default is to use the
        <code>${java.home}/lib/logging.properties</code> file in the JRE.
      </li>
      <li>In the web application. The file will be
        <code>WEB-INF/classes/logging.properties</code>
      </li>
    </ul>
  <p>
    The default <code>logging.properties</code> in the JRE specifies a
    <code>ConsoleHandler</code> that routes logging to System.err.
    The default <code>conf/logging.properties</code> in Apache Tomcat also
    adds several <code>AsyncFileHandler</code>s that write to files.
  </p>
  <p>
    A handler's log level threshold is <code>INFO</code> by default and can be set using
    <code>SEVERE</code>, <code>WARNING</code>, <code>INFO</code>, <code>CONFIG</code>,
    <code>FINE</code>, <code>FINER</code>, <code>FINEST</code> or <code>ALL</code>.
    You can also target specific packages to collect logging from and specify
    a level.
  </p>
  <p>
    To enable debug logging for part of Tomcat&apos;s internals, you should
    configure both the appropriate logger(s) and the appropriate handler(s) to
    use the <code>FINEST</code> or <code>ALL</code> level. e.g.:
  </p>
  <source>org.apache.catalina.session.level=ALL
java.util.logging.ConsoleHandler.level=ALL</source>
  <p>
    When enabling debug logging it is recommended that it is enabled for the
    narrowest possible scope as debug logging can generate large amounts of
    information.
  </p>
  <p>
    The configuration used by JULI is the same as the one supported by plain
    <code>java.util.logging</code>, but uses a few extensions to allow better
    flexibility in configuring loggers and handlers. The main differences are:
  </p>
  <ul>
      <li>A prefix may be added to handler names, so that multiple handlers of a
      single class may be instantiated. A prefix is a String which starts with a
      digit, and ends with '.'. For example, <code>22foobar.</code> is a valid
      prefix.</li>
      <li>System property replacement is performed for property values which
      contain <code>${systemPropertyName}</code>.</li>
      <li>If using a class loader that implements the
      <code>org.apache.juli.WebappProperties</code> interface (Tomcat&apos;s
      web application class loader does) then property replacement is also
      performed for <code>${classloader.webappName}</code>,
      <code>${classloader.hostName}</code> and
      <code>${classloader.serviceName}</code> which are replaced with the
      web application name, the host name and the service name respectively.
      </li>
      <li>By default, loggers will not delegate to their parent if they have
      associated handlers. This may be changed per logger using the
      <code>loggerName.useParentHandlers</code> property, which accepts a
      boolean value.</li>
      <li>The root logger can define its set of handlers using the
      <code>.handlers</code> property.</li>
      <li> By default the log files will be kept on the file system for
      <code>90</code> days. This may be changed per handler using the
      <code>handlerName.maxDays</code> property. If the specified value for the
      property is <code>&le;0</code> then the log files will be kept on the
      file system forever, otherwise they will be kept the specified maximum
      days.</li>
  </ul>
  <p>
    There are several additional implementation classes, that can be used
    together with the ones provided by Java. The notable ones are
    <code>org.apache.juli.FileHandler</code> and <code>org.apache.juli.AsyncFileHandler</code>.
  </p>
  <p>
    <code>org.apache.juli.FileHandler</code> supports buffering of the
      logs. The buffering is not enabled by default. To configure it, use the
      <code>bufferSize</code> property of a handler. The value of <code>0</code>
      uses system default buffering (typically an 8K buffer will be used). A
      value of <code>&lt;0</code> forces a writer flush upon each log write. A
      value <code>&gt;0</code> uses a BufferedOutputStream with the defined
      value but note that the system default buffering will also be
      applied.
  </p>
  <p>
    <code>org.apache.juli.AsyncFileHandler</code> is a subclass of <code>FileHandler</code>
      that queues the log messages and writes them asynchronously to the log files.
      Its additional behaviour can be configured by setting some
      <a href="config/systemprops.html#Logging">system properties</a>.
  </p>
  <p>
    Example logging.properties file to be placed in $CATALINA_BASE/conf:
  </p>
  <source><![CDATA[handlers = 1catalina.org.apache.juli.AsyncFileHandler, \
           2localhost.org.apache.juli.AsyncFileHandler, \
           3manager.org.apache.juli.AsyncFileHandler, \
           java.util.logging.ConsoleHandler

.handlers = 1catalina.org.apache.juli.AsyncFileHandler, java.util.logging.ConsoleHandler

############################################################
# Handler specific properties.
# Describes specific configuration info for Handlers.
############################################################

1catalina.org.apache.juli.AsyncFileHandler.level = ALL
1catalina.org.apache.juli.AsyncFileHandler.directory = ${catalina.base}/logs
1catalina.org.apache.juli.AsyncFileHandler.prefix = catalina.
1catalina.org.apache.juli.AsyncFileHandler.maxDays = 90
1catalina.org.apache.juli.AsyncFileHandler.encoding = UTF-8

2localhost.org.apache.juli.AsyncFileHandler.level = ALL
2localhost.org.apache.juli.AsyncFileHandler.directory = ${catalina.base}/logs
2localhost.org.apache.juli.AsyncFileHandler.prefix = localhost.
2localhost.org.apache.juli.AsyncFileHandler.maxDays = 90
2localhost.org.apache.juli.AsyncFileHandler.encoding = UTF-8

3manager.org.apache.juli.AsyncFileHandler.level = ALL
3manager.org.apache.juli.AsyncFileHandler.directory = ${catalina.base}/logs
3manager.org.apache.juli.AsyncFileHandler.prefix = manager.
3manager.org.apache.juli.AsyncFileHandler.bufferSize = 16384
3manager.org.apache.juli.AsyncFileHandler.maxDays = 90
3manager.org.apache.juli.AsyncFileHandler.encoding = UTF-8

java.util.logging.ConsoleHandler.level = ALL
java.util.logging.ConsoleHandler.formatter = org.apache.juli.OneLineFormatter
java.util.logging.ConsoleHandler.encoding = UTF-8

############################################################
# Facility specific properties.
# Provides extra control for each logger.
############################################################

org.apache.catalina.core.ContainerBase.[Catalina].[localhost].level = INFO
org.apache.catalina.core.ContainerBase.[Catalina].[localhost].handlers = \
   2localhost.org.apache.juli.AsyncFileHandler

org.apache.catalina.core.ContainerBase.[Catalina].[localhost].[/manager].level = INFO
org.apache.catalina.core.ContainerBase.[Catalina].[localhost].[/manager].handlers = \
   3manager.org.apache.juli.AsyncFileHandler

# For example, set the org.apache.catalina.util.LifecycleBase logger to log
# each component that extends LifecycleBase changing state:
#org.apache.catalina.util.LifecycleBase.level = FINE]]></source>

    <p>
      Example logging.properties for the servlet-examples web application to be
      placed in WEB-INF/classes inside the web application:
    </p>
    <source><![CDATA[handlers = org.apache.juli.AsyncFileHandler, java.util.logging.ConsoleHandler

############################################################
# Handler specific properties.
# Describes specific configuration info for Handlers.
############################################################

org.apache.juli.AsyncFileHandler.level = ALL
org.apache.juli.AsyncFileHandler.directory = ${catalina.base}/logs
org.apache.juli.AsyncFileHandler.prefix = ${classloader.webappName}.
org.apache.juli.AsyncFileHandler.encoding = UTF-8

java.util.logging.ConsoleHandler.level = ALL
java.util.logging.ConsoleHandler.formatter = org.apache.juli.OneLineFormatter
java.util.logging.ConsoleHandler.encoding = UTF-8]]></source>


    <subsection name="Documentation references">
      <p>See the following resources for additional information:</p>
      <ul>
        <li>Apache Tomcat Javadoc for the
          <a href="api/org/apache/juli/package-summary.html"><code>org.apache.juli</code></a>
          package.
        </li>
        <li>Oracle Java 17 Javadoc for the
          <a href="https://docs.oracle.com/en/java/javase/17/docs/api/java.logging/java/util/logging/package-summary.html"><code>java.util.logging</code></a>
          package.
        </li>
      </ul>
    </subsection>

    <subsection name="Considerations for production usage">
      <p>You may want to take note of the following:</p>
      <ul>
        <li>Consider removing <code>ConsoleHandler</code> from configuration. By
        default (thanks to the <code>.handlers</code> setting) logging goes both
        to a <code>AsyncFileHandler</code> and to a <code>ConsoleHandler</code>. The
        output of the latter one is usually captured into a file, such as
        <code>catalina.out</code>. Thus you end up with two copies of the same
        messages.</li>
        <li>Consider removing <code>AsyncFileHandler</code>s for the applications
        that you do not use. E.g., the one for <code>host-manager</code>.</li>
        <li>Consider configuring an
        <a href="config/valve.html#Access_Logging">Access log</a>.</li>
      </ul>
    </subsection>

  </section>

  <section name="Using Log4j">

    <p>The Log4j project provides <a
    href="https://logging.apache.org/log4j/2.x/jakarta.html#replace-tomcat"
    >instructions for using Log4J for Tomcat's internal logging</a>.</p>

  </section>
</body>
</document>
