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

    <body>
      <section name="Welcome to Log4j 2!">
        <subsection name="Introduction">
          <p>Almost every large application includes its own logging or tracing
          API. In conformance with this rule, the E.U.  <a
          href="http://www.semper.org">SEMPER</a> project decided to write its
          own tracing API. This was in early 1996. After countless enhancements,
          several incarnations and much work that API has evolved to become
          log4j, a popular logging package for Java. The package is distributed
          under the <a href="../LICENSE">Apache Software License</a>, a
          fully-fledged open source license certified by the <a
          href="http://www.opensource.org">open source</a> initiative. The
          latest log4j version, including full-source code, class files and
          documentation can be found at <a
          href="http://logging.apache.org/log4j/2.x/index.html"><b>http://logging.apache.org/log4j/2.x/index.html</b></a>.
          </p>

          <p>Inserting log statements into code is a low-tech method for
          debugging it. It may also be the only way because debuggers are not
          always available or applicable. This is usually the case for
          multithreaded applications and distributed applications at large.</p>

          <p>Experience indicates that logging was an important component of the
          development cycle. It offers several advantages. It provides precise
          <em>context</em> about a run of the application. Once inserted into
          the code, the generation of logging output requires no human
          intervention.  Moreover, log output can be saved in persistent medium
          to be studied at a later time. In addition to its use in the
          development cycle, a sufficiently rich logging package can also be
          viewed as an auditing tool.</p>

          <p>As Brian W. Kernighan and Rob Pike put it in their truly excellent
          book <i>"The Practice of Programming":</i></p>

          <div class="well">
            As personal choice, we tend not to use debuggers beyond getting a
            stack trace or the value of a variable or two. One reason is that it
            is easy to get lost in details of complicated data structures and
            control flow; we find stepping through a program less productive
            than thinking harder and adding output statements and self-checking
            code at critical places. Clicking over statements takes longer than
            scanning the output of judiciously-placed displays. It takes less
            time to decide where to put print statements than to single-step to
            the critical section of code, even assuming we know where that
            is. More important, debugging statements stay with the program;
            debugging sessions are transient.
          </div>

          <p>Logging does have its drawbacks. It can slow down an
          application. If too verbose, it can cause scrolling blindness. To
          alleviate these concerns, log4j is designed to be reliable, fast and
          extensible. Since logging is rarely the main focus of an application,
          the log4j API strives to be simple to understand and to use.</p>
        </subsection>
        <subsection name="Log4j 2">
          <p>
          Log4j 1.x has been widely adopted and used in many applications. However,
          through the years development on it has slowed down. It has become more
          difficult to maintain due to its need to be compliant with very old versions
          of Java.  Its alternative, SLF4J/Logback made many needed improvements to the
          framework. So why bother with Log4j 2? Here are a few of the reasons.
          </p>
          <ol>
            <li>Log4j 2 is designed to be usable as an audit logging framework. Both Log4j
              1.x and Logback will lose events while reconfiguring. Log4j 2 will not. in
              Logback exceptions in Appenders are never visible to the application. In
              Log4j 2 Appenders can be configured to allow the exception to percolate
              to the application</li>
            <li>Log4j 2 contains next-generation lock-free <a href="async.html">Asynchronous Loggers</a> based
              on the <a href="https://lmax-exchange.github.io/disruptor/">LMAX Disruptor library</a>.
              In multi-threaded scenarios Asynchronous Loggers have 10 times higher throughput and
              orders of magnitude lower latency than Log4j 1.x and Logback.</li>
            <li>Log4j 2 uses a <a href="plugins.html">Plugin system</a> that makes it extremely easy to
              <a href="extending.html">extend the framework</a> by adding new <a href="appenders.html">Appenders</a>,
              <a href="filters.html">Filters</a>, <a href="layouts.html">Layouts</a>, <a href="lookups.html">Lookups</a>,
              and Pattern Converters without requiring any changes to Log4j.</li>
            <li>Due to the Plugin system configuration is simpler. Entries in the configuration
              do not require a class name to be specified.</li>
            <li>Support for <a href="customloglevels.html">custom log levels</a>.
              Custom log levels can be defined in code or in configuration.</li>
            <li>Support for <a href="api.html#LambdaSupport">lambda expressions</a>.
              Client code running on Java 8 can use lambda expressions to lazily construct a log message only if
              the requested log level is enabled. Explicit level checks are not needed, resulting in cleaner code.</li>
            <li>Support for <a href="messages.html">Message objects</a>. Messages allow support for interesting and
              complex constructs to be passed through the logging system and be efficiently
              manipulated. Users are free to create their own
              <code><a href="../log4j-api/apidocs/org/apache/logging/log4j/message/Message.html">Message</a></code>
              types and write custom <a href="layouts.html">Layouts</a>, <a href="filters.html">Filters</a> and
              <a href="lookups.html">Lookups</a> to manipulate them.</li>
            <li>Log4j 1.x supports Filters on Appenders. Logback added TurboFilters to allow
              filtering of events before they are processed by a Logger. Log4j 2 supports
              Filters that can be configured to process events before they are handled by
              a Logger, as they are processed by a Logger or on an Appender.</li>
            <li>Many Logback Appenders do not accept a Layout and will only send data in a
              fixed format. Most Log4j 2 Appenders accept a Layout, allowing the data to
              be transported in any format desired.</li>
            <li>Layouts in Log4j 1.x and Logback return a String. This resulted in the problems
              discussed at <a href="http://logback.qos.ch/manual/encoders.html">Logback Encoders</a>.
              Log4j 2 takes the simpler approach that Layouts always return a byte array. This has
              the advantage that it means they can be used in virtually any Appender, not just
              the ones that write to an OutputStream.</li>
            <!-- and coming up: ByteBuffers, too! -->
            <li>The <a href="appenders.html#SyslogAppender">Syslog Appender</a> supports both TCP and UDP as well as
              support for the BSD syslog
              and the <a href="http://tools.ietf.org/html/rfc5424">RFC 5424</a> formats.</li>
            <li>Log4j 2 takes advantage of Java 5 concurrency support and performs locking
              at the lowest level possible. Log4j 1.x has known deadlock issues. Many of these
              are fixed in Logback but many Logback classes still require synchronization at
              a fairly high level.</li>
            <li>It is an Apache Software Foundation project following the community and support
              model used by all ASF projects. If you want to contribute or gain the right to
              commit changes just follow the path outlined at
              <a href="http://jakarta.apache.org/site/contributing.html">Contributing</a></li>
          </ol>
        </subsection>
      </section>
    </body>
</document>
