Introduction

Log4j 1.x allowed for extension by requiring class attributes on most of the configuration declarations. In the case of some elements, notably the PatternLayout, the only way to add new pattern converters was to extend the PatternLayout class and add them via code. One goal of Log4j 2 is to make extending it extremely easy through the use of plugins.

In Log4j 2 a plugin is declared by adding a @Plugin annotation to the class declaration. During initialization the Configuration will invoke the PluginManager to load the built-in Log4j plugins as well as any custom plugins. The PluginManager locates plugins by looking in five places:

1. Serialized plugin listing files on the classpath. These files are generated automatically during the build (more details below).
2. (OSGi only) Serialized plugin listing files in each active OSGi bundle. A BundleListener is added on activation to continue checking new bundles after log4j-core has started.
3. (Deprecated) A comma-separated list of packages specified by the log4j.plugin.packages system property.
4. (Deprecated) Packages passed to the static PluginManager.addPackages method (before Log4j configuration occurs).
5. (Deprecated) The packages declared in your log4j2 configuration file.

If multiple Plugins specify the same (case-insensitive) name, then the load order above determines which one will be used. For example, to override the File plugin which is provided by the built-in FileAppender class, you would need to place your plugin in a JAR file in the CLASSPATH ahead oflog4j-core.jar. This is not recommended; plugin name collisions will cause a warning to be emitted. Note that in an OSGi environment, the order that bundles are scanned for plugins generally follows the same order that bundles were installed into the framework. See getBundles() and SynchronousBundleListener. In short, name collisions are even more unpredictable in an OSGi environment.

Serialized plugin listing files are generated by an annotation processor contained in the log4j-core artifact which will automatically scan your code for Log4j 2 plugins and output a metadata file in your processed classes. There is nothing extra that needs to be done to enable this; the Java compiler will automatically pick up the annotation processor on the class path unless you explicitly disable it. In that case, it would be important to add another compiler pass to your build process that only handles annotation processing using the Log4j 2 annotation processor class, org.apache.logging.log4j.core.config.plugins.processor.PluginProcessor. To do this using Apache Maven, add the following execution to your maven-compiler-plugin (version 2.2 or higher) build plugin:

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-compiler-plugin</artifactId>
  <version>3.1</version>
  <executions>
    <execution>
      <id>log4j-plugin-processor</id>
      <goals>
        <goal>compile</goal>
      </goals>
      <phase>process-classes</phase>
      <configuration>
        <proc>only</proc>
        <annotationProcessors>
          <annotationProcessor>org.apache.logging.log4j.core.config.plugins.processor.PluginProcessor</annotationProcessor>
        </annotationProcessors>
      </configuration>
    </execution>
  </executions>
</plugin>
          

As the configuration is processed the appropriate plugins will be automatically configured and initialized. Log4j 2 utilizes a few different categories of plugins which are described in the following sections.

Core

Core plugins are those that are directly represented by an element in a configuration file, such as an Appender, Layout, Logger or Filter. Custom plugins that conform to the rules laid out in the next paragraph may simply be referenced in the configuration, provided they are appropriate configured to be loaded by the PluginManager.

Every Core plugin must declare a static method annotated with @PluginFactory or @PluginBuilderFactory. The former is used for static factory methods that provide all options as method parameters, and the latter is used to construct a new Builder<T> class whose fields are used for injecting attributes and child nodes. To allow the Configuration to pass the correct parameters to the method, every parameter to the method must be annotated as one of the following attribute types. Each attribute or element annotation must include the name that must be present in the configuration in order to match the configuration item to its respective parameter. For plugin builders, the names of the fields will be used by default if no name is specified in the annotation. There are dozens of plugins in Log4j Core that can be used as examples for more complex scenarios including hierarchical builder classes (e.g., see FileAppender). See Extending Log4j with Plugin Builders for more details.

Converters

Converters are used by PatternLayout to render the elements identified by the conversion pattern. Every converter must specify its category as "Converter" on the @Plugin annotation, have a static newInstance method that accepts an array of Strings as its only parameter and returns an instance of the Converter, and must have a @ConverterKeys annotation present that contains the array of converter patterns that will cause the Converter to be selected. Converters that are meant to handle LogEvents must extend the LogEventPatternConverter class and must implement a format method that accepts a LogEvent and a StringBuilder as arguments. The Converter should append the result of its operation to the StringBuilder.

A second type of Converter is the FileConverter - which must have "FileConverter" specified in the category attribute of the @Plugin annotation. While similar to a LogEventPatternConverter, instead of a single format method these Converters will have two variations; one that takes an Object and one that takes an array of Objects instead of the LogEvent. Both append to the provided StringBuilder in the same fashion as a LogEventPatternConverter. These Converters are typically used by the RollingFileAppender to construct the name of the file to log to.

If multiple Converters specify the same ConverterKeys, then the load order above determines which one will be used. For example, to override the %date converter which is provided by the built-in DatePatternConverter class, you would need to place your plugin in a JAR file in the CLASSPATH ahead oflog4j-core.jar. This is not recommended; pattern ConverterKeys collisions will cause a warning to be emitted. Try to use unique ConverterKeys for your custom pattern converters.

KeyProviders

Some components within Log4j may provide the ability to perform data encryption. These components require a secret key to perform the encryption. Applications may provide the key by creating a class that implements the SecretKeyProvider interface.

Lookups

Lookups are perhaps the simplest plugins of all. They must declare their type as "Lookup" on the plugin annotation and must implement the StrLookup interface. They will have two methods; a lookup method that accepts a String key and returns a String value and a second lookup method that accepts both a LogEvent and a String key and returns a String. Lookups may be referenced by specifying ${name:key} where name is the name specified in the Plugin annotation and key is the name of the item to locate.

TypeConverters

TypeConverters are a sort of meta-plugin used for converting strings into other types in a plugin factory method parameter. Other plugins can already be injected via the @PluginElement annotation; now, any type supported by the type conversion system can be used in a @PluginAttribute parameter. Conversion of enum types are supported on demand and do not require custom TypeConverter classes. A large number of built-in Java classes are already supported; see TypeConverters for a more exhaustive listing.

Unlike other plugins, the plugin name of a TypeConverter is purely cosmetic. Appropriate type converters are looked up via the Type interface rather than via Class<?> objects only. Do note that TypeConverter plugins must have a default constructor. When multiple converters match for a type, the first will be returned. If any extends from Comparable<TypeConverter<?>>, it will be used for determining the order.