前言

log4j2 使用插件机制加载各种组件：appender, logger .etc，本文简要分析 log4j2 插件机制实现

Plugin Annotation（注解）

Plugin 注解提供了一种便捷的方法将一个类声明成 log4j2 的插件，比如

@Plugin(name = "Console", category = "Core", elementType = "appender", printObject = true)
public final class ConsoleAppender extendsAbstractOutputStreamAppender<OutputStreamManager> {...
}

• name，name of the plugin

• category, category to place the plugin under

• name of the corresponding category of elements this plugin belongs under

Plugin Registry

单例

PluginRegistry 类用来保存插件信息，暴露了一些方法从配置文件中加载（内置）插件，使用了单例设计模式

private static volatile PluginRegistry INSTANCE;private static final Object INSTANCE_LOCK = new Object();private PluginRegistry() {
}public static PluginRegistry getInstance() {PluginRegistry result = INSTANCE;if (result == null) {synchronized(INSTANCE_LOCK) {result = INSTANCE;if (result == null) {INSTANCE = result = new PluginRegistry();}}}
}

线程安全的数据结构

PluginRegistry 使用了一些 Java 多线程编程的最佳实践：

• 使用 AtomicReference 类的 CAS（compare and set）操作，避免在多线程环境下 插件配置 被多次加载

• 使用 ConcurrentMapHash 类替代 HashMap 提供线程安全的 map

PluginType 类用来描述插件，比如插件对应的 class

    /*** Contains plugins found in Log4j2Plugins.dat cache files in the main CLASSPATH.*/private final AtomicReference<Map<String, List<PluginType<?>>>> pluginsByCategoryRef =new AtomicReference<>();/*** Contains plugins found in Log4j2Plugins.dat cache files in OSGi Bundles.*/private final ConcurrentMap<Long, Map<String, List<PluginType<?>>>> pluginsByCategoryByBundleId =new ConcurrentHashMap<>();/*** Contains plugins found by searching for annotated classes at runtime.*/private final ConcurrentMap<String, Map<String, List<PluginType<?>>>> pluginsByCategoryByPackage =new ConcurrentHashMap<>();

log4j2Plugins.dat 是插件描述文件，内部插件描述文件位于：

log4j-core-2.5.jarMETA-INForg.apache.logging.log4j.core.config.pluginsLog4j2Plugins.dat

加载插件

PluginRegistry 中和加载扫描插件相关的方法

• loadFromMainClassLoader，加载内部插件

• loadFromBundle，osgi相关

• loadFromPackage，加载指定 package（包）中的插件

加载内部插件

    public Map<String, List<PluginType<?>>> loadFromMainClassLoader() {final Map<String, List<PluginType<?>>> existing = pluginsByCategoryRef.get();// 如果 get 方法返回非空，说明已经通过 main class loader 加载过插件配置if (existing != null) {// already loadedreturn existing;}// 从配置文件加载 插件配置final Map<String, List<PluginType<?>>> newPluginsByCategory =    decodeCacheFiles(Loader.getClassLoader());// CASif (pluginsByCategoryRef.compareAndSet(null, newPluginsByCategory)) {return newPluginsByCategory;}return pluginsByCategoryRef.get();}

加载指定包中插件

public Map<String, List<PluginType<?>>> loadFromPackage(final String pkg) {// 参数校验if (Strings.isBlank(pkg)) {return Collections.emptyMap();}// 如果 pkg 已经被加载过直接返回Map<String, List<PluginType<?>>> existing = pluginByCategoryByPackage.get(pkg);if (existing != null) {return existing;}// 加载 pkg// 线程安全的 put if absent 操作（类似 CAS）existing = pluginsByCategoryByPackage.putIfAbsent(pkg, newPluginsByCategory);if (existing != null) {return existing;}return newPluginsByCategory;
}

PluginManager

PluginManager 类用来加载和管理所有的插件，每一个 category（类别）都有一个对应的 PluginManager

public PluginManager(final String category) {this.category = category;
}

collectPlugins 方法用于加载插件（描述信息）

public void collectPlugins(final List<String> packages) {...
}

总结