前言

log4j2是apache在log4j的基础上，参考logback架构实现的一套新的日志系统（我感觉是apache害怕logback了）。
log4j2的官方文档上写着一些它的优点：

在拥有全部logback特性的情况下，还修复了一些隐藏问题
API 分离：现在log4j2也是门面模式使用日志，默认的日志实现是log4j2，当然你也可以用logback（应该没有人会这么做）
性能提升：log4j2包含下一代基于LMAX Disruptor library的异步logger，在多线程场景下，拥有18倍于log4j和logback的性能
多API支持：log4j2提供Log4j 1.2, SLF4J, Commons Logging and java.util.logging (JUL) 的API支持
避免锁定：使用Log4j2 API的应用程序始终可以选择使用任何符合SLF4J的库作为log4j-to-slf4j适配器的记录器实现
自动重新加载配置：与Logback一样，Log4j 2可以在修改时自动重新加载其配置。与Logback不同，它会在重新配置发生时不会丢失日志事件。
高级过滤：与Logback一样，Log4j 2支持基于Log事件中的上下文数据，标记，正则表达式和其他组件进行过滤。
插件架构： Log4j使用插件模式配置组件。因此，您无需编写代码来创建和配置Appender，Layout，Pattern Converter等。Log4j自动识别插件并在配置引用它们时使用它们。
属性支持：您可以在配置中引用属性，Log4j将直接替换它们，或者Log4j将它们传递给将动态解析它们的底层组件。
Java 8 Lambda支持
自定义日志级别
产生垃圾少：在稳态日志记录期间，Log4j 2 在独立应用程序中是无垃圾的，在Web应用程序中是低垃圾。这减少了垃圾收集器的压力，并且可以提供更好的响应时间性能。
和应用server集成：版本2.10.0引入了一个模块log4j-appserver，以改进与Apache Tomcat和Eclipse Jetty的集成。

Log4j2类图：

这次从四个地方去探索源码：启动，配置，异步，插件化

源码探索

启动

log4j2的关键组件

LogManager

根据配置指定LogContexFactory，初始化对应的LoggerContext

LoggerContext

1、解析配置文件，解析为对应的java对象。
2、通过LoggerRegisty缓存Logger配置
3、Configuration配置信息
4、start方法解析配置文件，转化为对应的java对象
5、通过getLogger获取logger对象

Logger

LogManaer

该组件是Log4J启动的入口，后续的LoggerContext以及Logger都是通过调用LogManager的静态方法获得。我们可以使用下面的代码获取Logger

Logger logger = LogManager.getLogger();

可以看出LogManager是十分关键的组件，因此在这个小节中我们详细分析LogManager的启动流程。
LogManager启动的入口是下面的static代码块：

/*** Scans the classpath to find all logging implementation. Currently, only one will be used but this could be* extended to allow multiple implementations to be used.*/static {// Shortcut binding to force a specific logging implementation.final PropertiesUtil managerProps = PropertiesUtil.getProperties();final String factoryClassName = managerProps.getStringProperty(FACTORY_PROPERTY_NAME);if (factoryClassName != null) {try {factory = LoaderUtil.newCheckedInstanceOf(factoryClassName, LoggerContextFactory.class);} catch (final ClassNotFoundException cnfe) {LOGGER.error("Unable to locate configured LoggerContextFactory {}", factoryClassName);} catch (final Exception ex) {LOGGER.error("Unable to create configured LoggerContextFactory {}", factoryClassName, ex);}}if (factory == null) {final SortedMap<Integer, LoggerContextFactory> factories = new TreeMap<>();// note that the following initial call to ProviderUtil may block until a Provider has been installed when// running in an OSGi environmentif (ProviderUtil.hasProviders()) {for (final Provider provider : ProviderUtil.getProviders()) {final Class<? extends LoggerContextFactory> factoryClass = provider.loadLoggerContextFactory();if (factoryClass != null) {try {factories.put(provider.getPriority(), factoryClass.newInstance());} catch (final Exception e) {LOGGER.error("Unable to create class {} specified in provider URL {}", factoryClass.getName(), provider.getUrl(), e);}}}if (factories.isEmpty()) {LOGGER.error("Log4j2 could not find a logging implementation. "+ "Please add log4j-core to the classpath. Using SimpleLogger to log to the console...");factory = new SimpleLoggerContextFactory();} else if (factories.size() == 1) {factory = factories.get(factories.lastKey());} else {final StringBuilder sb = new StringBuilder("Multiple logging implementations found: \n");for (final Map.Entry<Integer, LoggerContextFactory> entry : factories.entrySet()) {sb.append("Factory: ").append(entry.getValue().getClass().getName());sb.append(", Weighting: ").append(entry.getKey()).append('\n');}factory = factories.get(factories.lastKey());sb.append("Using factory: ").append(factory.getClass().getName());LOGGER.warn(sb.toString());}} else {LOGGER.error("Log4j2 could not find a logging implementation. "+ "Please add log4j-core to the classpath. Using SimpleLogger to log to the console...");factory = new SimpleLoggerContextFactory();}}}

这段静态代码段主要分为下面的几个步骤：

首先根据特定配置文件的配置信息获取loggerContextFactory
如果没有找到对应的Factory的实现类则通过ProviderUtil中的getProviders()方法载入providers，随后通过provider的loadLoggerContextFactory方法载入LoggerContextFactory的实现类
如果provider中没有获取到LoggerContextFactory的实现类或provider为空，则使用SimpleLoggerContextFactory作为LoggerContextFactory。

根据配置文件载入LoggerContextFactory

// Shortcut binding to force a specific logging implementation.final PropertiesUtil managerProps = PropertiesUtil.getProperties();final String factoryClassName = managerProps.getStringProperty(FACTORY_PROPERTY_NAME);if (factoryClassName != null) {try {factory = LoaderUtil.newCheckedInstanceOf(factoryClassName, LoggerContextFactory.class);} catch (final ClassNotFoundException cnfe) {LOGGER.error("Unable to locate configured LoggerContextFactory {}", factoryClassName);} catch (final Exception ex) {LOGGER.error("Unable to create configured LoggerContextFactory {}", factoryClassName, ex);}}

在这段逻辑中，LogManager优先通过配置文件”log4j2.component.properties”通过配置项”log4j2.loggerContextFactory”来获取LoggerContextFactory，如果用户做了对应的配置，通过newCheckedInstanceOf方法实例化LoggerContextFactory的对象，最终的实现方式为：

public static <T> T newInstanceOf(final Class<T> clazz)throws InstantiationException, IllegalAccessException, InvocationTargetException {try {return clazz.getConstructor().newInstance();} catch (final NoSuchMethodException ignored) {// FIXME: looking at the code for Class.newInstance(), this seems to do the same thing as abovereturn clazz.newInstance();}}

在默认情况下，不存在初始的默认配置文件log4j2.component.properties，因此需要从其他途径获取LoggerContextFactory。

通过Provider实例化LoggerContextFactory对象

代码：

if (factory == null) {final SortedMap<Integer, LoggerContextFactory> factories = new TreeMap<>();// note that the following initial call to ProviderUtil may block until a Provider has been installed when// running in an OSGi environmentif (ProviderUtil.hasProviders()) {for (final Provider provider : ProviderUtil.getProviders()) {final Class<? extends LoggerContextFactory> factoryClass = provider.loadLoggerContextFactory();if (factoryClass != null) {try {factories.put(provider.getPriority(), factoryClass.newInstance());} catch (final Exception e) {LOGGER.error("Unable to create class {} specified in provider URL {}", factoryClass.getName(), provider.getUrl(), e);}}}if (factories.isEmpty()) {LOGGER.error("Log4j2 could not find a logging implementation. "+ "Please add log4j-core to the classpath. Using SimpleLogger to log to the console...");factory = new SimpleLoggerContextFactory();} else if (factories.size() == 1) {factory = factories.get(factories.lastKey());} else {final StringBuilder sb = new StringBuilder("Multiple logging implementations found: \n");for (final Map.Entry<Integer, LoggerContextFactory> entry : factories.entrySet()) {sb.append("Factory: ").append(entry.getValue().getClass().getName());sb.append(", Weighting: ").append(entry.getKey()).append('\n');}factory = factories.get(factories.lastKey());sb.append("Using factory: ").append(factory.getClass().getName());LOGGER.warn(sb.toString());}} else {LOGGER.error("Log4j2 could not find a logging implementation. "+ "Please add log4j-core to the classpath. Using SimpleLogger to log to the console...");factory = new SimpleLoggerContextFactory();}}

这里比较有意思的是hasProviders和getProviders都会通过线程安全的方式去懒加载ProviderUtil这个对象。跟进lazyInit方法：

protected static void lazyInit() {//noinspection DoubleCheckedLockingif (INSTANCE == null) {try {STARTUP_LOCK.lockInterruptibly();if (INSTANCE == null) {INSTANCE = new ProviderUtil();}} catch (final InterruptedException e) {LOGGER.fatal("Interrupted before Log4j Providers could be loaded.", e);Thread.currentThread().interrupt();} finally {STARTUP_LOCK.unlock();}}}

再看构造方法：

private ProviderUtil() {for (final LoaderUtil.UrlResource resource : LoaderUtil.findUrlResources(PROVIDER_RESOURCE)) {loadProvider(resource.getUrl(), resource.getClassLoader());}}

这里的懒加载其实就是懒加载Provider对象。在创建新的providerUtil实例的过程中就会直接实例化provider对象，其过程是先通过getClassLoaders方法获取provider的类加载器，然后通过loadProviders(classLoader);加载类。在providerUtil实例化的最后，会统一查找”META-INF/log4j-provider.properties”文件中对应的provider的url，会考虑从远程加载provider。而loadProviders方法就是在ProviderUtil的PROVIDERS列表中添加对一个的provider。可以看到默认的provider是org.apache.logging.log4j.core.impl.Log4jContextFactory

LoggerContextFactory = org.apache.logging.log4j.core.impl.Log4jContextFactory
Log4jAPIVersion = 2.1.0
FactoryPriority= 10

很有意思的是这里懒加载加上了锁，而且使用的是
lockInterruptibly这个方法。lockInterruptibly和lock的区别如下：

lock 与 lockInterruptibly比较区别在于：

lock 优先考虑获取锁，待获取锁成功后，才响应中断。
lockInterruptibly 优先考虑响应中断，而不是响应锁的普通获取或重入获取。
ReentrantLock.lockInterruptibly允许在等待时由其它线程调用等待线程的
Thread.interrupt 方法来中断等待线程的等待而直接返回，这时不用获取锁，而会抛出一个InterruptedException。 ReentrantLock.lock方法不允许Thread.interrupt中断,即使检测到Thread.isInterrupted,一样会继续尝试获取锁，失败则继续休眠。只是在最后获取锁成功后再把当前线程置为interrupted状态,然后再中断线程。

上面有一句注释值得注意：

/*** Guards the ProviderUtil singleton instance from lazy initialization. This is primarily used for OSGi support.** @since 2.1*/protected static final Lock STARTUP_LOCK = new ReentrantLock();// STARTUP_LOCK guards INSTANCE for lazy initialization; this allows the OSGi Activator to pause the startup and// wait for a Provider to be installed. See LOG4J2-373private static volatile ProviderUtil INSTANCE;

原来这里是为了让osgi可以阻止启动。
再回到logManager：
可以看到在加载完Provider之后，会做factory的绑定：

if (factories.isEmpty()) {LOGGER.error("Log4j2 could not find a logging implementation. "+ "Please add log4j-core to the classpath. Using SimpleLogger to log to the console...");factory = new SimpleLoggerContextFactory();} else if (factories.size() == 1) {factory = factories.get(factories.lastKey());} else {final StringBuilder sb = new StringBuilder("Multiple logging implementations found: \n");for (final Map.Entry<Integer, LoggerContextFactory> entry : factories.entrySet()) {sb.append("Factory: ").append(entry.getValue().getClass().getName());sb.append(", Weighting: ").append(entry.getKey()).append('\n');}factory = factories.get(factories.lastKey());sb.append("Using factory: ").append(factory.getClass().getName());LOGGER.warn(sb.toString());}

到这里，logmanager的启动流程就结束了。

配置

在不使用slf4j的情况下，我们获取logger的方式是这样的：

Logger logger = logManager.getLogger(xx.class)

跟进getLogger方法：

    public static Logger getLogger(final Class<?> clazz) {final Class<?> cls = callerClass(clazz);return getContext(cls.getClassLoader(), false).getLogger(toLoggerName(cls));}

这里有一个getContext方法，跟进，

public static LoggerContext getContext(final ClassLoader loader, final boolean currentContext) {try {return factory.getContext(FQCN, loader, null, currentContext);} catch (final IllegalStateException ex) {LOGGER.warn(ex.getMessage() + " Using SimpleLogger");return new SimpleLoggerContextFactory().getContext(FQCN, loader, null, currentContext);}}

上文提到factory的具体实现是Log4jContextFactory，跟进getContext
方法：

public LoggerContext getContext(final String fqcn, final ClassLoader loader, final Object externalContext,final boolean currentContext) {final LoggerContext ctx = selector.getContext(fqcn, loader, currentContext);if (externalContext != null && ctx.getExternalContext() == null) {ctx.setExternalContext(externalContext);}if (ctx.getState() == LifeCycle.State.INITIALIZED) {ctx.start();}return ctx;}

直接看start：

public void start() {LOGGER.debug("Starting LoggerContext[name={}, {}]...", getName(), this);if (PropertiesUtil.getProperties().getBooleanProperty("log4j.LoggerContext.stacktrace.on.start", false)) {LOGGER.debug("Stack trace to locate invoker",new Exception("Not a real error, showing stack trace to locate invoker"));}if (configLock.tryLock()) {try {if (this.isInitialized() || this.isStopped()) {this.setStarting();reconfigure();if (this.configuration.isShutdownHookEnabled()) {setUpShutdownHook();}this.setStarted();}} finally {configLock.unlock();}}LOGGER.debug("LoggerContext[name={}, {}] started OK.", getName(), this);}

发现其中的核心方法是reconfigure方法，继续跟进：

private void reconfigure(final URI configURI) {final ClassLoader cl = ClassLoader.class.isInstance(externalContext) ? (ClassLoader) externalContext : null;LOGGER.debug("Reconfiguration started for context[name={}] at URI {} ({}) with optional ClassLoader: {}",contextName, configURI, this, cl);final Configuration instance = ConfigurationFactory.getInstance().getConfiguration(this, contextName, configURI, cl);if (instance == null) {LOGGER.error("Reconfiguration failed: No configuration found for '{}' at '{}' in '{}'", contextName, configURI, cl);} else {setConfiguration(instance);/** instance.start(); Configuration old = setConfiguration(instance); updateLoggers(); if (old != null) {* old.stop(); }*/final String location = configuration == null ? "?" : String.valueOf(configuration.getConfigurationSource());LOGGER.debug("Reconfiguration complete for context[name={}] at URI {} ({}) with optional ClassLoader: {}",contextName, location, this, cl);}}

可以看到每一个configuration都是从ConfigurationFactory拿出来的，我们先看看这个类的getInstance看看：

public static ConfigurationFactory getInstance() {// volatile works in Java 1.6+, so double-checked locking also works properly//noinspection DoubleCheckedLockingif (factories == null) {LOCK.lock();try {if (factories == null) {final List<ConfigurationFactory> list = new ArrayList<ConfigurationFactory>();final String factoryClass = PropertiesUtil.getProperties().getStringProperty(CONFIGURATION_FACTORY_PROPERTY);if (factoryClass != null) {addFactory(list, factoryClass);}final PluginManager manager = new PluginManager(CATEGORY);manager.collectPlugins();final Map<String, PluginType<?>> plugins = manager.getPlugins();final List<Class<? extends ConfigurationFactory>> ordered =new ArrayList<Class<? extends ConfigurationFactory>>(plugins.size());for (final PluginType<?> type : plugins.values()) {try {ordered.add(type.getPluginClass().asSubclass(ConfigurationFactory.class));} catch (final Exception ex) {LOGGER.warn("Unable to add class {}", type.getPluginClass(), ex);}}Collections.sort(ordered, OrderComparator.getInstance());for (final Class<? extends ConfigurationFactory> clazz : ordered) {addFactory(list, clazz);}// see above comments about double-checked locking//noinspection NonThreadSafeLazyInitializationfactories = Collections.unmodifiableList(list);}} finally {LOCK.unlock();}}LOGGER.debug("Using configurationFactory {}", configFactory);return configFactory;}

这里可以看到ConfigurationFactory中利用了PluginManager来进行初始化，PluginManager会将ConfigurationFactory的子类加载进来，默认使用的XmlConfigurationFactory，JsonConfigurationFactory，YamlConfigurationFactory这三个子类，这里插件化加载暂时按下不表。
回到reconfigure这个方法，我们看到获取ConfigurationFactory实例之后会去调用getConfiguration方法：

public Configuration getConfiguration(final String name, final URI configLocation, final ClassLoader loader) {if (!isActive()) {return null;}if (loader == null) {return getConfiguration(name, configLocation);}if (isClassLoaderUri(configLocation)) {final String path = extractClassLoaderUriPath(configLocation);final ConfigurationSource source = getInputFromResource(path, loader);if (source != null) {final Configuration configuration = getConfiguration(source);if (configuration != null) {return configuration;}}}return getConfiguration(name, configLocation);}

跟进getConfiguration，这里值得注意的是有很多个getConfiguration，注意甄别，如果不确定的话可以通过debug的方式来确定。

public Configuration getConfiguration(final String name, final URI configLocation) {if (configLocation == null) {final String config = this.substitutor.replace(PropertiesUtil.getProperties().getStringProperty(CONFIGURATION_FILE_PROPERTY));if (config != null) {ConfigurationSource source = null;try {source = getInputFromUri(FileUtils.getCorrectedFilePathUri(config));} catch (final Exception ex) {// Ignore the error and try as a String.LOGGER.catching(Level.DEBUG, ex);}if (source == null) {final ClassLoader loader = LoaderUtil.getThreadContextClassLoader();source = getInputFromString(config, loader);}if (source != null) {for (final ConfigurationFactory factory : factories) {final String[] types = factory.getSupportedTypes();if (types != null) {for (final String type : types) {if (type.equals("*") || config.endsWith(type)) {final Configuration c = factory.getConfiguration(source);if (c != null) {return c;}}}}}}}} else {for (final ConfigurationFactory factory : factories) {final String[] types = factory.getSupportedTypes();if (types != null) {for (final String type : types) {if (type.equals("*") || configLocation.toString().endsWith(type)) {final Configuration config = factory.getConfiguration(name, configLocation);if (config != null) {return config;}}}}}}Configuration config = getConfiguration(true, name);if (config == null) {config = getConfiguration(true, null);if (config == null) {config = getConfiguration(false, name);if (config == null) {config = getConfiguration(false, null);}}}if (config != null) {return config;}LOGGER.error("No log4j2 configuration file found. Using default configuration: logging only errors to the console.");return new DefaultConfiguration();}

这里就会根据之前加载进来的factory进行配置的获取，具体的不再解析。
回到reconfigure，之后的步骤就是setConfiguration，入参就是刚才获取的config

private synchronized Configuration setConfiguration(final Configuration config) {Assert.requireNonNull(config, "No Configuration was provided");final Configuration prev = this.config;config.addListener(this);final ConcurrentMap<String, String> map = config.getComponent(Configuration.CONTEXT_PROPERTIES);try { // LOG4J2-719 network access may throw android.os.NetworkOnMainThreadExceptionmap.putIfAbsent("hostName", NetUtils.getLocalHostname());} catch (final Exception ex) {LOGGER.debug("Ignoring {}, setting hostName to 'unknown'", ex.toString());map.putIfAbsent("hostName", "unknown");}map.putIfAbsent("contextName", name);config.start();this.config = config;updateLoggers();if (prev != null) {prev.removeListener(this);prev.stop();}firePropertyChangeEvent(new PropertyChangeEvent(this, PROPERTY_CONFIG, prev, config));try {Server.reregisterMBeansAfterReconfigure();} catch (final Throwable t) {// LOG4J2-716: Android has no java.lang.managementLOGGER.error("Could not reconfigure JMX", t);}return prev;}

这个方法最重要的步骤就是config.start,这才是真正做配置解析的

public void start() {LOGGER.debug("Starting configuration {}", this);this.setStarting();pluginManager.collectPlugins(pluginPackages);final PluginManager levelPlugins = new PluginManager(Level.CATEGORY);levelPlugins.collectPlugins(pluginPackages);final Map<String, PluginType<?>> plugins = levelPlugins.getPlugins();if (plugins != null) {for (final PluginType<?> type : plugins.values()) {try {// Cause the class to be initialized if it isn't already.Loader.initializeClass(type.getPluginClass().getName(), type.getPluginClass().getClassLoader());} catch (final Exception e) {LOGGER.error("Unable to initialize {} due to {}", type.getPluginClass().getName(), e.getClass().getSimpleName(), e);}}}setup();setupAdvertisement();doConfigure();final Set<LoggerConfig> alreadyStarted = new HashSet<LoggerConfig>();for (final LoggerConfig logger : loggers.values()) {logger.start();alreadyStarted.add(logger);}for (final Appender appender : appenders.values()) {appender.start();}if (!alreadyStarted.contains(root)) { // LOG4J2-392root.start(); // LOG4J2-336}super.start();LOGGER.debug("Started configuration {} OK.", this);}

这里面有如下步骤：

获取日志等级的插件
初始化
初始化Advertiser
配置

先看一下初始化，也就是setup这个方法，setup是一个需要被复写的方法，我们以XMLConfiguration作为例子，

@Overridepublic void setup() {if (rootElement == null) {LOGGER.error("No logging configuration");return;}constructHierarchy(rootNode, rootElement);if (status.size() > 0) {for (final Status s : status) {LOGGER.error("Error processing element {}: {}", s.name, s.errorType);}return;}rootElement = null;}

发现这里面有一个比较重要的方法constructHierarchy，跟进：

private void constructHierarchy(final Node node, final Element element) {processAttributes(node, element);final StringBuilder buffer = new StringBuilder();final NodeList list = element.getChildNodes();final List<Node> children = node.getChildren();for (int i = 0; i < list.getLength(); i++) {final org.w3c.dom.Node w3cNode = list.item(i);if (w3cNode instanceof Element) {final Element child = (Element) w3cNode;final String name = getType(child);final PluginType<?> type = pluginManager.getPluginType(name);final Node childNode = new Node(node, name, type);constructHierarchy(childNode, child);if (type == null) {final String value = childNode.getValue();if (!childNode.hasChildren() && value != null) {node.getAttributes().put(name, value);} else {status.add(new Status(name, element, ErrorType.CLASS_NOT_FOUND));}} else {children.add(childNode);}} else if (w3cNode instanceof Text) {final Text data = (Text) w3cNode;buffer.append(data.getData());}}final String text = buffer.toString().trim();if (text.length() > 0 || (!node.hasChildren() && !node.isRoot())) {node.setValue(text);}}

发现这个就是一个树遍历的过程。诚然，配置文件是以xml的形式给出的，xml的结构就是一个树形结构。回到start方法，跟进doConfiguration：

protected void doConfigure() {if (rootNode.hasChildren() && rootNode.getChildren().get(0).getName().equalsIgnoreCase("Properties")) {final Node first = rootNode.getChildren().get(0);createConfiguration(first, null);if (first.getObject() != null) {subst.setVariableResolver((StrLookup) first.getObject());}} else {final Map<String, String> map = this.getComponent(CONTEXT_PROPERTIES);final StrLookup lookup = map == null ? null : new MapLookup(map);subst.setVariableResolver(new Interpolator(lookup, pluginPackages));}boolean setLoggers = false;boolean setRoot = false;for (final Node child : rootNode.getChildren()) {if (child.getName().equalsIgnoreCase("Properties")) {if (tempLookup == subst.getVariableResolver()) {LOGGER.error("Properties declaration must be the first element in the configuration");}continue;}createConfiguration(child, null);if (child.getObject() == null) {continue;}if (child.getName().equalsIgnoreCase("Appenders")) {appenders = child.getObject();} else if (child.isInstanceOf(Filter.class)) {addFilter(child.getObject(Filter.class));} else if (child.getName().equalsIgnoreCase("Loggers")) {final Loggers l = child.getObject();loggers = l.getMap();setLoggers = true;if (l.getRoot() != null) {root = l.getRoot();setRoot = true;}} else if (child.getName().equalsIgnoreCase("CustomLevels")) {customLevels = child.getObject(CustomLevels.class).getCustomLevels();} else if (child.isInstanceOf(CustomLevelConfig.class)) {final List<CustomLevelConfig> copy = new ArrayList<CustomLevelConfig>(customLevels);copy.add(child.getObject(CustomLevelConfig.class));customLevels = copy;} else {LOGGER.error("Unknown object \"{}\" of type {} is ignored.", child.getName(),child.getObject().getClass().getName());}}if (!setLoggers) {LOGGER.warn("No Loggers were configured, using default. Is the Loggers element missing?");setToDefault();return;} else if (!setRoot) {LOGGER.warn("No Root logger was configured, creating default ERROR-level Root logger with Console appender");setToDefault();// return; // LOG4J2-219: creating default root=ok, but don't exclude configured Loggers}for (final Map.Entry<String, LoggerConfig> entry : loggers.entrySet()) {final LoggerConfig l = entry.getValue();for (final AppenderRef ref : l.getAppenderRefs()) {final Appender app = appenders.get(ref.getRef());if (app != null) {l.addAppender(app, ref.getLevel(), ref.getFilter());} else {LOGGER.error("Unable to locate appender {} for logger {}", ref.getRef(), l.getName());}}}setParents();}

发现就是对刚刚获取的configuration进行解析，然后塞进正确的地方。回到start方法，可以看到昨晚配置之后就是开启logger和appender了。

异步

AsyncAppender

log4j2突出于其他日志的优势，异步日志实现。我们先从日志打印看进去。找到Logger，随便找一个log日志的方法。

public void debug(final Marker marker, final Message msg) {logIfEnabled(FQCN, Level.DEBUG, marker, msg, msg != null ? msg.getThrowable() : null);}

一路跟进

@PerformanceSensitive// NOTE: This is a hot method. Current implementation compiles to 29 bytes of byte code.// This is within the 35 byte MaxInlineSize threshold. Modify with care!private void logMessageTrackRecursion(final String fqcn,final Level level,final Marker marker,final Message msg,final Throwable throwable) {try {incrementRecursionDepth(); // LOG4J2-1518, LOG4J2-2031tryLogMessage(fqcn, level, marker, msg, throwable);} finally {decrementRecursionDepth();}}

可以看出这个在打日志之前做了调用次数的记录。跟进tryLogMessage，

@PerformanceSensitive// NOTE: This is a hot method. Current implementation compiles to 26 bytes of byte code.// This is within the 35 byte MaxInlineSize threshold. Modify with care!private void tryLogMessage(final String fqcn,final Level level,final Marker marker,final Message msg,final Throwable throwable) {try {logMessage(fqcn, level, marker, msg, throwable);} catch (final Exception e) {// LOG4J2-1990 Log4j2 suppresses all exceptions that occur once application called the loggerhandleLogMessageException(e, fqcn, msg);}}

继续跟进：

@Overridepublic void logMessage(final String fqcn, final Level level, final Marker marker, final Message message,final Throwable t) {final Message msg = message == null ? new SimpleMessage(Strings.EMPTY) : message;final ReliabilityStrategy strategy = privateConfig.loggerConfig.getReliabilityStrategy();strategy.log(this, getName(), fqcn, marker, level, msg, t);}

这里可以看到在实际打日志的时候，会从config中获取打日志的策略，跟踪ReliabilityStrategy的创建，发现默认的实现类为DefaultReliabilityStrategy，跟进看实际打日志的方法

@Overridepublic void log(final Supplier<LoggerConfig> reconfigured, final String loggerName, final String fqcn, final Marker marker, final Level level,final Message data, final Throwable t) {loggerConfig.log(loggerName, fqcn, marker, level, data, t);}

这里实际打日志的方法居然是交给一个config去实现的。。。感觉有点奇怪。。跟进看看

@PerformanceSensitive("allocation")public void log(final String loggerName, final String fqcn, final Marker marker, final Level level,final Message data, final Throwable t) {List<Property> props = null;if (!propertiesRequireLookup) {props = properties;} else {if (properties != null) {props = new ArrayList<>(properties.size());final LogEvent event = Log4jLogEvent.newBuilder().setMessage(data).setMarker(marker).setLevel(level).setLoggerName(loggerName).setLoggerFqcn(fqcn).setThrown(t).build();for (int i = 0; i < properties.size(); i++) {final Property prop = properties.get(i);final String value = prop.isValueNeedsLookup() // since LOG4J2-1575? config.getStrSubstitutor().replace(event, prop.getValue()) //: prop.getValue();props.add(Property.createProperty(prop.getName(), value));}}}final LogEvent logEvent = logEventFactory.createEvent(loggerName, marker, fqcn, level, data, props, t);try {log(logEvent, LoggerConfigPredicate.ALL);} finally {// LOG4J2-1583 prevent scrambled logs when logging calls are nested (logging in toString())ReusableLogEventFactory.release(logEvent);}}

可以清楚的看到try之前是在创建LogEvent，try里面做的才是真正的log（好tm累），一路跟进。

private void processLogEvent(final LogEvent event, LoggerConfigPredicate predicate) {event.setIncludeLocation(isIncludeLocation());if (predicate.allow(this)) {callAppenders(event);}logParent(event, predicate);}

接下来就是callAppender了，我们直接开始看AsyncAppender的append方法：

/*** Actual writing occurs here.** @param logEvent The LogEvent.*/@Overridepublic void append(final LogEvent logEvent) {if (!isStarted()) {throw new IllegalStateException("AsyncAppender " + getName() + " is not active");}final Log4jLogEvent memento = Log4jLogEvent.createMemento(logEvent, includeLocation);InternalAsyncUtil.makeMessageImmutable(logEvent.getMessage());if (!transfer(memento)) {if (blocking) {if (AbstractLogger.getRecursionDepth() > 1) { // LOG4J2-1518, LOG4J2-2031// If queue is full AND we are in a recursive call, call appender directly to prevent deadlockAsyncQueueFullMessageUtil.logWarningToStatusLogger();logMessageInCurrentThread(logEvent);} else {// delegate to the event router (which may discard, enqueue and block, or log in current thread)final EventRoute route = asyncQueueFullPolicy.getRoute(thread.getId(), memento.getLevel());route.logMessage(this, memento);}} else {error("Appender " + getName() + " is unable to write primary appenders. queue is full");logToErrorAppenderIfNecessary(false, memento);}}}

这里主要的步骤就是：

生成logEvent
将logEvent放入BlockingQueue，就是transfer方法
如果BlockingQueue满了则启用相应的策略

同样的，这里也有一个线程用来做异步消费的事情

private class AsyncThread extends Log4jThread {private volatile boolean shutdown = false;private final List<AppenderControl> appenders;private final BlockingQueue<LogEvent> queue;public AsyncThread(final List<AppenderControl> appenders, final BlockingQueue<LogEvent> queue) {super("AsyncAppender-" + THREAD_SEQUENCE.getAndIncrement());this.appenders = appenders;this.queue = queue;setDaemon(true);}@Overridepublic void run() {while (!shutdown) {LogEvent event;try {event = queue.take();if (event == SHUTDOWN_LOG_EVENT) {shutdown = true;continue;}} catch (final InterruptedException ex) {break; // LOG4J2-830}event.setEndOfBatch(queue.isEmpty());final boolean success = callAppenders(event);if (!success && errorAppender != null) {try {errorAppender.callAppender(event);} catch (final Exception ex) {// Silently accept the error.}}}// Process any remaining items in the queue.LOGGER.trace("AsyncAppender.AsyncThread shutting down. Processing remaining {} queue events.",queue.size());int count = 0;int ignored = 0;while (!queue.isEmpty()) {try {final LogEvent event = queue.take();if (event instanceof Log4jLogEvent) {final Log4jLogEvent logEvent = (Log4jLogEvent) event;logEvent.setEndOfBatch(queue.isEmpty());callAppenders(logEvent);count++;} else {ignored++;LOGGER.trace("Ignoring event of class {}", event.getClass().getName());}} catch (final InterruptedException ex) {// May have been interrupted to shut down.// Here we ignore interrupts and try to process all remaining events.}}LOGGER.trace("AsyncAppender.AsyncThread stopped. Queue has {} events remaining. "+ "Processed {} and ignored {} events since shutdown started.", queue.size(), count, ignored);}/*** Calls {@link AppenderControl#callAppender(LogEvent) callAppender} on all registered {@code AppenderControl}* objects, and returns {@code true} if at least one appender call was successful, {@code false} otherwise. Any* exceptions are silently ignored.** @param event the event to forward to the registered appenders* @return {@code true} if at least one appender call succeeded, {@code false} otherwise*/boolean callAppenders(final LogEvent event) {boolean success = false;for (final AppenderControl control : appenders) {try {control.callAppender(event);success = true;} catch (final Exception ex) {// If no appender is successful the error appender will get it.}}return success;}public void shutdown() {shutdown = true;if (queue.isEmpty()) {queue.offer(SHUTDOWN_LOG_EVENT);}if (getState() == State.TIMED_WAITING || getState() == State.WAITING) {this.interrupt(); // LOG4J2-1422: if underlying appender is stuck in wait/sleep/join/park call}}}

直接看run方法：

阻塞获取logEvent
将logEvent分发出去
如果线程要退出了，将blockingQueue里面的event消费完在退出。

AsyncLogger

直接从AsyncLogger的logMessage看进去：

public void logMessage(final String fqcn, final Level level, final Marker marker, final Message message,final Throwable thrown) {if (loggerDisruptor.isUseThreadLocals()) {logWithThreadLocalTranslator(fqcn, level, marker, message, thrown);} else {// LOG4J2-1172: avoid storing non-JDK classes in ThreadLocals to avoid memory leaks in web appslogWithVarargTranslator(fqcn, level, marker, message, thrown);}}

跟进logWithThreadLocalTranslator，

private void logWithThreadLocalTranslator(final String fqcn, final Level level, final Marker marker,final Message message, final Throwable thrown) {// Implementation note: this method is tuned for performance. MODIFY WITH CARE!final RingBufferLogEventTranslator translator = getCachedTranslator();initTranslator(translator, fqcn, level, marker, message, thrown);initTranslatorThreadValues(translator);publish(translator);}

这里的逻辑很简单，就是将日志相关的信息转换成RingBufferLogEvent（RingBuffer是Disruptor的无所队列），然后将其发布到RingBuffer中。发布到RingBuffer中，那肯定也有消费逻辑。这时候有两种方式可以找到这个消费的逻辑。

找disruptor被使用的地方，然后查看，但是这样做会很容易迷惑
按照Log4j2的尿性，这种Logger都有对应的start方法，我们可以从start方法入手寻找

在start方法中，我们找到了一段代码：

final RingBufferLogEventHandler[] handlers = {new RingBufferLogEventHandler()};disruptor.handleEventsWith(handlers);

直接看看这个RingBufferLogEventHandler的实现：

public class RingBufferLogEventHandler implementsSequenceReportingEventHandler<RingBufferLogEvent>, LifecycleAware {private static final int NOTIFY_PROGRESS_THRESHOLD = 50;private Sequence sequenceCallback;private int counter;private long threadId = -1;@Overridepublic void setSequenceCallback(final Sequence sequenceCallback) {this.sequenceCallback = sequenceCallback;}@Overridepublic void onEvent(final RingBufferLogEvent event, final long sequence,final boolean endOfBatch) throws Exception {event.execute(endOfBatch);event.clear();// notify the BatchEventProcessor that the sequence has progressed.// Without this callback the sequence would not be progressed// until the batch has completely finished.if (++counter > NOTIFY_PROGRESS_THRESHOLD) {sequenceCallback.set(sequence);counter = 0;}}/*** Returns the thread ID of the background consumer thread, or {@code -1} if the background thread has not started* yet.* @return the thread ID of the background consumer thread, or {@code -1}*/public long getThreadId() {return threadId;}@Overridepublic void onStart() {threadId = Thread.currentThread().getId();}@Overridepublic void onShutdown() {}
}

顺着接口找上去，发现一个接口：

/*** Callback interface to be implemented for processing events as they become available in the {@link RingBuffer}** @param <T> event implementation storing the data for sharing during exchange or parallel coordination of an event.* @see BatchEventProcessor#setExceptionHandler(ExceptionHandler) if you want to handle exceptions propagated out of the handler.*/
public interface EventHandler<T>
{/*** Called when a publisher has published an event to the {@link RingBuffer}** @param event      published to the {@link RingBuffer}* @param sequence   of the event being processed* @param endOfBatch flag to indicate if this is the last event in a batch from the {@link RingBuffer}* @throws Exception if the EventHandler would like the exception handled further up the chain.*/void onEvent(T event, long sequence, boolean endOfBatch) throws Exception;
}

通过注释可以发现，这个onEvent就是处理逻辑，回到RingBufferLogEventHandler的onEvent方法，发现里面有一个execute方法，跟进：

public void execute(final boolean endOfBatch) {this.endOfBatch = endOfBatch;asyncLogger.actualAsyncLog(this);}

这个方法就是实际打日志了，AsyncLogger看起来还是比较简单的，只是使用了一个Disruptor。

插件化

之前在很多代码里面都可以看到

final PluginManager manager = new PluginManager(CATEGORY);
manager.collectPlugins(pluginPackages);

其实整个log4j2为了获得更好的扩展性，将自己的很多组件都做成了插件，然后在配置的时候去加载plugin。
跟进collectPlugins。

 public void collectPlugins(final List<String> packages) {final String categoryLowerCase = category.toLowerCase();final Map<String, PluginType<?>> newPlugins = new LinkedHashMap<>();// First, iterate the Log4j2Plugin.dat files found in the main CLASSPATHMap<String, List<PluginType<?>>> builtInPlugins = PluginRegistry.getInstance().loadFromMainClassLoader();if (builtInPlugins.isEmpty()) {// If we didn't find any plugins above, someone must have messed with the log4j-core.jar.// Search the standard package in the hopes we can find our core plugins.builtInPlugins = PluginRegistry.getInstance().loadFromPackage(LOG4J_PACKAGES);}mergeByName(newPlugins, builtInPlugins.get(categoryLowerCase));// Next, iterate any Log4j2Plugin.dat files from OSGi Bundlesfor (final Map<String, List<PluginType<?>>> pluginsByCategory : PluginRegistry.getInstance().getPluginsByCategoryByBundleId().values()) {mergeByName(newPlugins, pluginsByCategory.get(categoryLowerCase));}// Next iterate any packages passed to the static addPackage method.for (final String pkg : PACKAGES) {mergeByName(newPlugins, PluginRegistry.getInstance().loadFromPackage(pkg).get(categoryLowerCase));}// Finally iterate any packages provided in the configuration (note these can be changed at runtime).if (packages != null) {for (final String pkg : packages) {mergeByName(newPlugins, PluginRegistry.getInstance().loadFromPackage(pkg).get(categoryLowerCase));}}LOGGER.debug("PluginManager '{}' found {} plugins", category, newPlugins.size());plugins = newPlugins;}

处理逻辑如下：

从Log4j2Plugin.dat中加载所有的内置的plugin
然后将OSGi Bundles中的Log4j2Plugin.dat中的plugin加载进来
再加载传入的package路径中的plugin
最后加载配置中的plugin

逻辑还是比较简单的，但是我在看源码的时候发现了一个很有意思的东西，就是在加载log4j2 core插件的时候，也就是

PluginRegistry.getInstance().loadFromMainClassLoader()

这个方法，跟进到decodeCacheFiles：

private Map<String, List<PluginType<?>>> decodeCacheFiles(final ClassLoader loader) {final long startTime = System.nanoTime();final PluginCache cache = new PluginCache();try {final Enumeration<URL> resources = loader.getResources(PluginProcessor.PLUGIN_CACHE_FILE);if (resources == null) {LOGGER.info("Plugin preloads not available from class loader {}", loader);} else {cache.loadCacheFiles(resources);}} catch (final IOException ioe) {LOGGER.warn("Unable to preload plugins", ioe);}final Map<String, List<PluginType<?>>> newPluginsByCategory = new HashMap<>();int pluginCount = 0;for (final Map.Entry<String, Map<String, PluginEntry>> outer : cache.getAllCategories().entrySet()) {final String categoryLowerCase = outer.getKey();final List<PluginType<?>> types = new ArrayList<>(outer.getValue().size());newPluginsByCategory.put(categoryLowerCase, types);for (final Map.Entry<String, PluginEntry> inner : outer.getValue().entrySet()) {final PluginEntry entry = inner.getValue();final String className = entry.getClassName();try {final Class<?> clazz = loader.loadClass(className);final PluginType<?> type = new PluginType<>(entry, clazz, entry.getName());types.add(type);++pluginCount;} catch (final ClassNotFoundException e) {LOGGER.info("Plugin [{}] could not be loaded due to missing classes.", className, e);} catch (final LinkageError e) {LOGGER.info("Plugin [{}] could not be loaded due to linkage error.", className, e);}}}final long endTime = System.nanoTime();final DecimalFormat numFormat = new DecimalFormat("#0.000000");final double seconds = (endTime - startTime) * 1e-9;LOGGER.debug("Took {} seconds to load {} plugins from {}",numFormat.format(seconds), pluginCount, loader);return newPluginsByCategory;}

可以发现加载时候是从一个文件（PLUGIN_CACHE_FILE）获取所有要获取的plugin。看到这里的时候我有一个疑惑就是，为什么不用反射的方式直接去扫描，而是要从文件中加载进来，而且文件是写死的，很不容易扩展啊。然后我找了一下PLUGIN_CACHE_FILE这个静态变量的用处，发现了PluginProcessor这个类，这里用到了注解处理器。

/*** Annotation processor for pre-scanning Log4j 2 plugins.*/
@SupportedAnnotationTypes("org.apache.logging.log4j.core.config.plugins.*")
public class PluginProcessor extends AbstractProcessor {// TODO: this could be made more abstract to allow for compile-time and run-time plugin processing/*** The location of the plugin cache data file. This file is written to by this processor, and read from by* {@link org.apache.logging.log4j.core.config.plugins.util.PluginManager}.*/public static final String PLUGIN_CACHE_FILE ="META-INF/org/apache/logging/log4j/core/config/plugins/Log4j2Plugins.dat";private final PluginCache pluginCache = new PluginCache();@Overridepublic boolean process(final Set<? extends TypeElement> annotations, final RoundEnvironment roundEnv) {System.out.println("Processing annotations");try {final Set<? extends Element> elements = roundEnv.getElementsAnnotatedWith(Plugin.class);if (elements.isEmpty()) {System.out.println("No elements to process");return false;}collectPlugins(elements);writeCacheFile(elements.toArray(new Element[elements.size()]));System.out.println("Annotations processed");return true;} catch (final IOException e) {e.printStackTrace();error(e.getMessage());return false;} catch (final Exception ex) {ex.printStackTrace();error(ex.getMessage());return false;}}
}

（不太重要的方法省略）
我们可以看到在process方法中，PluginProcessor会先收集所有的Plugin，然后在写入文件。这样做的好处就是可以省去反射时候的开销。
然后我又看了一下Plugin这个注解，发现它的RetentionPolicy是RUNTIME，一般来说PluginProcessor是搭配RetentionPolicy.SOURCE，CLASS使用的，而且既然你把自己的Plugin扫描之后写在文件中了，RetentionPolicy就没有必要是RUNTIME了吧，这个是一个很奇怪的地方。

小结

总算是把Log4j2的代码看完了，发现它的设计理念很值得借鉴，为了灵活性，所有的东西都设计成插件式。互联网技术日益发展，各种中间件层出不穷，而作为工程师的我们更需要做的是去思考代码与代码之间的关系，毫无疑问的是，解耦是最具有美感的关系。

原文链接
本文为云栖社区原创内容，未经允许不得转载。

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探索Java日志的奥秘：底层日志系统-log4j2

前言

源码探索

启动

LogManaer

配置

异步

AsyncAppender

AsyncLogger

插件化

小结

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