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= Apache Log4j 2

Apache Log4j 2 is an upgrade to Log4j that provides significant improvements over its predecessor, Log4j 1.x, and
provides many of the improvements available in Logback while fixing some inherent problems in Logback's architecture.

== Features

=== API Separation

The API for Log4j is separate from the implementation making it clear for application developers which classes and
methods they can use while ensuring forward compatibility. This allows the Log4j team to improve the implementation
safely and in a compatible manner.


The Log4j API is a logging facade that may, of course, be used with the Log4j implementation, but may also be used
in front of other logging implementations such as Logback. The Log4j API has several advantages over SLF4J:

. The Log4j API supports logging [Messages](manual/messages.html) instead of just Strings.
. The Log4j API supports lambda expressions.
. The Log4j API provides many more logging methods than SLF4J.
. In addition to the "parameterized logging" format supported by SLF4J, the Log4j API also supports events using
the java.text.MessageFormat syntax as well printf-style messages.
. The Log4j API provides a LogManager.shutdown() method. The underlying logging implementation must implement the
Terminable interface for the method to have effect.
. Other constructs such as Markers, log Levels, and ThreadContext (aka MDC) are fully supported.

=== Improved Performance

Log4j 2 contains next-generation Asynchronous Loggers based on the LMAX Disruptor library. In multi-threaded scenarios
Asynchronous Loggers have 18 times higher throughput and orders of magnitude lower latency than Log4j 1.x and Logback.
See link:manual/async.html#Performance[Asynchronous Logging Performance] for details. Otherwise, Log4j 2 significantly
outperforms Log4j 1.x, Logback and java.util.logging, especially in multi-threaded applications.
See link:performance.html[Performance] for more information.

=== Support for multiple APIs

While the Log4j 2 API will provide the best performance, Log4j 2 provides support for the Log4j 1.2, SLF4J, Commons
Logging and java.util.logging (JUL) APIs.

=== Avoid lock-in

Applications coded to the Log4j 2 API always have the option to use any SLF4J-compliant library as their logger
implementation with the log4j-to-slf4j adapter.

=== Automatic Reloading of Configurations

Like Logback, Log4j 2 can automatically reload its configuration upon modification. Unlike Logback, it will do so
without losing log events while reconfiguration is taking place.

=== Advanced Filtering

Like Logback, Log4j 2 supports filtering based on context data, markers, regular expressions, and other components in
the Log event. Filtering can be specified to apply to all events before being passed to Loggers or as they pass through
Appenders. In addition, filters can also be associated with Loggers. Unlike Logback, you can use a common Filter class
in any of these circumstances.

=== Plugin Architecture

Log4j uses the plugin pattern to configure components. As such, you do not need to write code to create and configure an
Appender, Layout, Pattern Converter, and so on. Log4j automatically recognizes plugins and uses them when a
configuration references them.

=== Property Support

You can reference properties in a configuration, Log4j will directly replace them, or Log4j will pass them to an
underlying component that will dynamically resolve them. Properties come from values defined in the configuration file,
system properties, environment variables, the ThreadContext Map, and data present in the event. Users can further
customize the property providers by adding their own link:manual/lookups.html[Lookup] Plugin.

=== Java 8 Lambda Support

Previously, if a log message was expensive to construct, you would often explicitly check if the requested log level is
enabled before constructing the message. Client code running on Java 8 can benefit from Log4j's
link:manual/api.html#LambdaSupport[lambda support]. Since Log4j will not evaluate a lambda expression if the requested log
level is not enabled, the same effect can be achieved with less code.

=== Custom Log Levels

In Log4j 2, link:manual/customloglevels.html[custom log levels] can easily be defined in code or in configuration. No
subclassing is required.

=== Log Builder API
In addition to using one of the many log methods in the Log4j API, log events can be constructed using a builder. See
link:manual/logbuilder.html[Log Builder] for more information.

=== Garbage-free

During steady state logging, Log4j 2 is link:manual/garbagefree.html[garbage-free] in stand-alone applications, and low
garbage in web applications. This reduces pressure on the garbage collector and can give better response time performance.

=== Integrating with Application Servers

Version 2.10.0 introduces a the module log4j-appserver to improve integration with Apache Tomcat and Eclipse Jetty.

=== Cloud Enabled

Version 2.12.0 introduces support for accessing Docker container information via a Lookup and for accessing
and updating the Log4j configuration through Spring Cloud Configuration. This support was enhanced in
version 2.13.0 to add support for accessing Spring Boot properties as well as Kubernetes information.
See link:manual/cloud.html[Logging in the Cloud] for details.

=== Compatible with Log4j 1.x

The Log4j-1.2-api module of Log4j 2 provides compatiblity for applications using the Log4j 1 logging methods. As
of Log4j 2.13.0 Log4j 2 also provides experimental support for Log4j 1.x configuration files. See
link:manual/compatiblity.html[Log4j 2 Compatiblity with Log4j 1] for more information.

== Documentation

The Log4j 2 User's Guide is available on this link:manual/index.html[site] or as a downloadable
link:log4j-users-guide.pdf[PDF].

== Requirements

Log4j 2.13.0 and greater require Java 8. Version 2.4 through 2.12.1 required Java 7 and versions 2.0-alpha1 to 2.3
required Java 6. Some features require optional dependencies; the documentation for these features specifies the
dependencies.

== News

Log4j {Log4jReleaseVersion} is now available for production. The API for Log4j 2 is not compatible with Log4j 1.x, however an adapter is
available to allow applications to continue to use the Log4j 1.x API. Adapters are also available for Apache Commons
Logging, SLF4J, and java.util.logging.

Log4j {Log4jReleaseVersion} is the latest release of Log4j and contains several bug fixes that were found after the release of Log4j 2.6.
The list of fixes can be found in the latest link:changes-report.html#a{Log4jReleaseVersion}[changes report].

Note that subsequent to the release of Log4j 2.6 a minor source incompatibility with prior release was found due to the
addition of new methods to the Logger interface. If you have code that does:

[source,java]
----
logger.error(null, "This is the log message", throwable);
----

or similar with any log level you will get a compiler error saying the reference is ambiguous. To correct this either
do:

[source,java]
----
logger.error("This is the log message", throwable);
----

or

[source,java]
----
logger.error((Marker) null, "This is the log message", throwable);
----

Log4j {Log4jReleaseVersion} maintains binary compatibility with previous releases.