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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.

Some of the features and improvements in Log4j 2 are:

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.
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 Asynchronous Logging Performance for details. Otherwise, Log4j 2 significantly outperforms Log4j 1.x, Logback and java.util.logging, especially in multi-threaded applications. See 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 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 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, custom log levels can easily be defined in code or in configuration. No subclassing is required.
Garbage-free
During steady state logging, Log4j 2 is 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.

Documentation

The Log4j 2 User's Guide is available on this site or as a downloadable PDF.

Requirements

Log4j 2.4 and greater requires Java 7, 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 2.6.2 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 and SLF4J.

Log4j 2.6.2 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 changes report.

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

              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:

              logger.error("This is the log message", throwable);
            

or

              logger.error((Marker) null, "This is the log message", throwable);
            

Log4j 2.6.2 maintains binary compatibility with previous releases.