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| |
| |
| <section> |
| <h2><a name="Plugins"></a>Plugins</h2> |
| <section> |
| <h3><a name="Introduction"></a>Introduction</h3> |
| <a name="Introduction"></a> |
| |
| <p> |
| 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. |
| </p> |
| |
| <p> |
| In Log4j 2 a plugin is declared by adding a |
| <a href="../log4j-core/apidocs/org/apache/logging/log4j/core/config/plugins/Plugin.html" class="javadoc">@Plugin</a> |
| annotation to the class declaration. During initialization the |
| <a href="../log4j-core/apidocs/org/apache/logging/log4j/core/config/Configuration.html" class="javadoc">Configuration</a> |
| will invoke the |
| <a href="../log4j-core/apidocs/org/apache/logging/log4j/core/config/plugins/util/PluginManager.html" class="javadoc">PluginManager</a> |
| to load the built-in Log4j plugins as well as any custom plugins. The PluginManager locates |
| plugins by looking in five places: |
| </p> |
| |
| <ol style="list-style-type: decimal"> |
| |
| <li>Serialized plugin listing files on the classpath. These files are generated automatically during |
| the build (more details below).</li> |
| |
| <li>(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.</li> |
| |
| <li>A comma-separated list of packages specified by the log4j.plugin.packages |
| system property.</li> |
| |
| <li>Packages passed to the static PluginManager.addPackages method (before Log4j configuration |
| occurs).</li> |
| |
| <li>The <a href="./configuration.html#ConfigurationSyntax">packages</a> declared in your log4j2 |
| configuration file.</li> |
| </ol> |
| |
| <p> |
| 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 |
| <a class="javadoc" href="http://www.osgi.org/javadoc/r5/core/org/osgi/framework/BundleContext.html#getBundles()">getBundles()</a> |
| and |
| <a class="javadoc" href="http://www.osgi.org/javadoc/r5/core/org/osgi/framework/SynchronousBundleListener.html">SynchronousBundleListener</a>. |
| In short, name collisions are even more unpredictable in an OSGi environment. |
| |
| </p> |
| |
| <p> |
| 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 <i>maven-compiler-plugin</i> (version 2.2 or higher) |
| build plugin: |
| </p> |
| |
| <div> |
| <pre class="prettyprint linenums"> |
| <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> |
| </pre></div> |
| |
| <p> |
| 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. |
| </p> |
| </section> |
| <section> |
| <h3><a name="Core"></a>Core</h3> |
| <a name="Core"></a> |
| |
| <p> |
| 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. |
| </p> |
| |
| <p> |
| 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 <a href="extending.html#Plugin_Builders">Extending |
| Log4j with Plugin Builders</a> for more details. |
| </p> |
| <section> |
| <h4><a name="Attribute_Types"></a>Attribute Types</h4> |
| |
| <dl> |
| |
| <dt><a href="../log4j-core/apidocs/org/apache/logging/log4j/core/config/plugins/PluginAttribute.html" class="javadoc">PluginAttribute</a></dt> |
| |
| <dd>The parameter must be convertible from a String using a <a href="#TypeConverters">TypeConverter</a>. |
| Most built-in types are already supported, but custom TypeConverter plugins may also be |
| provided for more type support. Note that PluginBuilderAttribute can be used in builder class |
| fields as an easier way to provide default values.</dd> |
| |
| <dt><a href="../log4j-core/apidocs/org/apache/logging/log4j/core/config/plugins/PluginElement.html" class="javadoc">PluginElement</a></dt> |
| |
| <dd>The parameter may represent a complex object that itself has parameters that can be configured. |
| This also supports injecting an array of elements.</dd> |
| |
| <dt><a href="../log4j-core/apidocs/org/apache/logging/log4j/core/config/plugins/PluginConfiguration.html" class="javadoc">PluginConfiguration</a></dt> |
| |
| <dd>The current Configuration object will be passed to the plugin as a parameter.</dd> |
| |
| <dt><a href="../log4j-core/apidocs/org/apache/logging/log4j/core/config/plugins/PluginNode.html" class="javadoc">PluginNode</a></dt> |
| |
| <dd>The current Node being parsed will be passed to the plugin as a parameter.</dd> |
| |
| <dt><a href="../log4j-core/apidocs/org/apache/logging/log4j/core/config/plugins/PluginValue.html" class="javadoc">PluginValue</a></dt> |
| |
| <dd>The value of the current Node or its attribute named value.</dd> |
| </dl> |
| </section><section> |
| <h4><a name="Constraint_Validators"></a>Constraint Validators</h4> |
| |
| <p> |
| Plugin factory fields and parameters can be automatically validated at runtime using constraint validators |
| inspired by the <a class="externalLink" href="http://beanvalidation.org/">Bean Validation spec</a>. The following annotations |
| are bundled in Log4j, but custom |
| <a href="../log4j-core/apidocs/org/apache/logging/log4j/core/config/plugins/validation/ConstraintValidator.html" class="javadoc">ConstraintValidators</a> |
| can be created as well. |
| </p> |
| |
| <dl> |
| |
| <dt><a href="../log4j-core/apidocs/org/apache/logging/log4j/core/config/plugins/validation/constraints/Required.html" class="javadoc">Required</a></dt> |
| |
| <dd>This annotation validates that a value is non-empty. This covers a check for null as well |
| as several other scenarios: empty CharSequence objects, empty arrays, empty Collection |
| instances, and empty Map instances.</dd> |
| |
| <dt><a href="../log4j-core/apidocs/org/apache/logging/log4j/core/config/plugins/validation/constraints/ValidHost.html" class="javadoc">ValidHost</a></dt> |
| |
| <dd>This annotation validates that a value corresponds to a valid hostname. This uses the same validation as |
| <a class="javadoc" href="http://docs.oracle.com/javase/8/docs/api/java/net/InetAddress.html#getByName-java.lang.String-">InetAddress::getByName</a>.</dd> |
| |
| <dt><a href="../log4j-core/apidocs/org/apache/logging/log4j/core/config/plugins/validation/constraints/ValidPort.html" class="javadoc">ValidPort</a></dt> |
| |
| <dd>This annotation validates that a value corresponds to a valid port number between 0 and 65535.</dd> |
| </dl> |
| </section></section> |
| <section> |
| <h3><a name="Converters"></a>Converters</h3> |
| <a name="Converters"></a> |
| |
| <p> |
| Converters are used by |
| <a href="../log4j-core/apidocs/org/apache/logging/log4j/core/layout/PatternLayout.html" class="javadoc">PatternLayout</a> |
| 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 |
| <a href="../log4j-core/apidocs/org/apache/logging/log4j/core/layout/LogEventPatternConverter.html" class="javadoc">LogEventPatternConverter</a> |
| 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. |
| </p> |
| |
| <p> |
| 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. |
| </p> |
| |
| <p> |
| 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. |
| </p> |
| </section> |
| <section> |
| <h3><a name="KeyProviders"></a>KeyProviders</h3> |
| <a name="KeyProviders"></a> |
| |
| <p> |
| 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 |
| <a href="../log4j-core/apidocs/org/apache/logging/log4j/core/util/SecretKeyProvider.html" class="javadoc">SecretKeyProvider</a> |
| interface. |
| </p> |
| </section> |
| <section> |
| <h3><a name="Lookups"></a>Lookups</h3> |
| <a name="Lookups"></a> |
| |
| <p> |
| Lookups are perhaps the simplest plugins of all. They must declare their type as "Lookup" on the |
| plugin annotation and must implement the |
| <a href="../log4j-core/apidocs/org/apache/logging/log4j/core/lookup/StrLookup.html" class="javadoc">StrLookup</a> |
| 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 ${<var>name</var>:key} where <var>name</var> is the name specified in the Plugin annotation and |
| key is the name of the item to locate. |
| </p> |
| </section> |
| <section> |
| <h3><a name="TypeConverters"></a>TypeConverters</h3> |
| <a name="TypeConverters"></a> |
| |
| <p> |
| <a href="../log4j-core/apidocs/org/apache/logging/log4j/core/config/plugins/convert/TypeConverter.html" class="javadoc">TypeConverter</a>s |
| 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 |
| <a href="../log4j-core/apidocs/org/apache/logging/log4j/core/config/plugins/convert/TypeConverters.html" class="javadoc">TypeConverters</a> |
| for a more exhaustive listing. |
| </p> |
| |
| <p> |
| 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. |
| </p> |
| </section> |
| </section> |
| <section> |
| <h2><a name="Developer_Notes"></a>Developer Notes</h2> |
| <a name="DeveloperNotes"></a> |
| |
| <p> |
| If a plugin class implements |
| <a class="javadoc" href="http://docs.oracle.com/javase/6/docs/api/java/util/Collection.html">Collection</a> or |
| <a class="javadoc" href="http://docs.oracle.com/javase/6/docs/api/java/util/Map.html">Map</a>, then no |
| factory method is used. Instead, the class is instantiated using the default constructor, and all child |
| configuration nodes are added to the Collection or Map. |
| </p> |
| </section> |
| |
| |
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