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.

Important: Security Vulnerability CVE-2021-44832

Summary: Apache Log4j2 vulnerable to RCE via JDBC Appender when attacker controls configuration.

Details

Apache Log4j2 versions 2.0-beta7 through 2.17.0 (excluding security fix releases 2.3.2 and 2.12.4) are vulnerable to a remote code execution (RCE) attack where an attacker with permission to modify the logging configuration file can construct a malicious configuration using a JDBC Appender with a data source referencing a JNDI URI which can execute remote code. This issue is fixed by limiting JNDI data source names to the java protocol in Log4j2 versions 2.17.1, 2.12.4, and 2.3.2.

Mitigation

Upgrade to Log4j 2.3.2 (for Java 6), 2.12.4 (for Java 7), or 2.17.1 (for Java 8 and later)

Reference

Please refer to the Security page for details and mitigation measures for older versions of Log4j.

Important: Security Vulnerabilities CVE-2021-45105, CVE-2021-45046 and CVE-2021-44228

Please refer to the Security page for details and mitigation measures for these security issues.

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:

  1. The Log4j API supports logging Messages instead of just Strings.
  2. The Log4j API supports lambda expressions.
  3. The Log4j API provides many more logging methods than SLF4J.
  4. 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.
  5. The Log4j API provides a LogManager.shutdown() method. The underlying logging implementation must implement the Terminable interface for the method to have effect.
  6. 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 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.

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 Log Builder for more information.

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.

Integrating with Application Servers

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

Cloud Enabled

Version 2.12.0 introduced 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 Logging in the Cloud for details.

Compatible with Log4j 1.x

The Log4j-1.2-api module of Log4j 2 provides compatibility 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 Log4j 2 Compatibility with Log4j 1 for more information.

Documentation

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

Requirements

Log4j 2.13.0 and greater require Java 8. Version 2.4 through 2.12.1 required Java 7 (the Log4j team no longer supports Java 7). Some features require optional dependencies; the documentation for these features will specify the required dependencies.

News

Log4j 2.17.2 has been released to:

  • Over 50 improvements and fixes to the Log4j 1.x support. Continued testing has shown it is a suitable replacement for Log4j 1.x in most cases.
  • Scripting now requires a system property be specified naming the languages the user wishes to allow. The scripting engine will not load if the property isn't set.
  • By default, the only remote protocol allowed for loading configuration files is HTTPS. Users can specify a system property to allow others or prevent remote loading entirely.
  • Variable resolution has been modified so that only properties defined as properties in the configuration file can be recursive. All other Lookups are now non-recursive. This addresses issues users were having resolving lookups specified in property definitions for use in the RoutingAppender and RollingFileAppender due to restrictions put in place in 2.17.1.
  • Many other fixes and improvements.

2.17.2 (for Java 8) is a recommended upgrade.

Log4j 2.19.0 is now available for production. While the normal API for Log4j 2 is not compatible with Log4j 1.x, an adapter is available to allow applications to continue to use the Log4j 1.x API and configuration files. Adapters are also available for Apache Commons Logging, SLF4J, and java.util.logging.

Log4j 2.19.0 is the latest release of Log4j. As of Log4j 2.13.0 Log4j 2 requires Java 8 or greater at runtime. This release contains new features and fixes which can be found in the latest changes report.

Log4j 2.19.0 maintains binary compatibility with previous releases.