探索Java日志的奥秘：底层日志系统-log4j2

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 factories = new TreeMap();
            // note that the following initial call to ProviderUtil may block until a Provider has been installed when
            // running in an OSGi environment
            if (ProviderUtil.hasProviders()) {
                for (final Provider provider : ProviderUtil.getProviders()) {
                    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 DoubleCheckedLocking
        if (factories == null) {
            LOCK.lock();
            try {
                if (factories == null) {
                    final List list = new ArrayList();
                    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> 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 alreadyStarted = new HashSet();
        for (final LoggerConfig logger : loggers.values()) {
            logger.start();
            alreadyStarted.add(logger);
        }
        for (final Appender appender : appenders.values()) {
            appender.start();
        }
        if (!alreadyStarted.contains(root)) { // LOG4J2-392
            root.start(); // LOG4J2-336
        }
        super.start();
        LOGGER.debug("Started configuration {} OK.", this);
    }

这里面有如下步骤：

1. 获取日志等级的插件
2. 初始化
3. 初始化Advertiser
4. 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，

@Override
    public 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 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 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 copy = new ArrayList(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 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, LOG4J2-2031
            tryLogMessage(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 logger
            handleLogMessageException(e, fqcn, msg);
        }
    }

继续跟进：

@Override
    public 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

@Override
    public void log(final Supplier 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 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.
     */
    @Override
    public 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 deadlock
                    AsyncQueueFullMessageUtil.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：

1. 生成logEvent
2. 将logEvent放入BlockingQueue，就是transfer方法
3. 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 appenders;
        private final BlockingQueue queue;

        public AsyncThread(final List appenders, final BlockingQueue queue) {
            super("AsyncAppender-" + THREAD_SEQUENCE.getAndIncrement());
            this.appenders = appenders;
            this.queue = queue;
            setDaemon(true);
        }

        @Override
        public 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：

1. 阻塞获取logEvent
2. 将logEvent分发出去
3. 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 apps
            logWithVarargTranslator(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 implements
        SequenceReportingEventHandler, LifecycleAware {

    private static final int NOTIFY_PROGRESS_THRESHOLD = 50;
    private Sequence sequenceCallback;
    private int counter;
    private long threadId = -1;

    @Override
    public void setSequenceCallback(final Sequence sequenceCallback) {
        this.sequenceCallback = sequenceCallback;
    }

    @Override
    public 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;
    }

    @Override
    public void onStart() {
        threadId = Thread.currentThread().getId();
    }

    @Override
    public 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  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
{
    /**
     * 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 packages) {
        final String categoryLowerCase = category.toLowerCase();
        final Map>> 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 Bundles
        for (final Map