Apache Log4j 3.x

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:

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.

While the Log4j 2.x/3.x API will provide the best performance, Log4j 3.x provides support for the Log4j 1.2, SLF4J, Commons Logging and java.util.logging (JUL) APIs.

Applications coded to the Log4j 3.x API have the option to use an SLF4J-compliant library as their logger implementation with the log4j-to-slf4j adapter or use java.util.logging with the log4j-to-jul adapter. The binding between the Log4j API and a Log4j implementation uses java.util.ServiceLoader making it easy to create alternate implementations.

Log4j 3.x can automatically reload its configuration upon modification. Unlike Logback, it will do so without losing log events while reconfiguration is taking place.

Log4j 3.x 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.

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.

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.

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.

In Log4j 3.x, custom log levels can easily be defined in code or in configuration. No subclassing is required.

In addition to using one of the many log methods in the Log4j API, log events can be constructed using a builder. See Log Builder for more information.

During steady state logging, Log4j 3.x 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.

Application code written using the Log4j 2.x API should be binary compatible with Log4j 3.x. Custom plugins written for Log4j 2.x may need minor changes when compiling with Log4j 3.x as some package names have changed.

The Log4j-1.2-api module of Log4j 3.x provides compatiblity for applications using the Log4j 1 logging methods. Log4j 3.x also provides experimental support for Log4j 1.x configuration files. See Log4j 3.x Compatiblity with Log4j 1 for more information.