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
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
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
- 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.
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.
The Log4j 2 User's Guide is available on this site or as a downloadable
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
Log4j 2.6 is now available for production use. 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.
Note that subsequent to the release a minor source incompatibility 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);
logger.error((Marker) null, "This is the log message", throwable);
Log4j 2.6 maintains binary compatibility with previous releases.