log4j2异步日志解读（一）AsyncAppender

log4j、logback、log4j2 历史和关系，我们就在这里不展开讲了。直接上干货，log4j2突出于其他日志的优势，异步日志实现。

看一个东西，首先看官网文档 ，因为前面文章已经讲解了disruptor源码，本文主要展开说说异步日志AsyncAppender和AsyncLogger（基于disruptor实现）。

AsyncLogger笔者下文展开讲。

一、AsyncAppender

我们先来看看AsyncApperder核心，就是logger将数据通过append方法放入到阻塞队列中，随后后台线程从队列中取出数据然后进行后续的操作。

那这样看来，就很简单了，一个append()方法，一个后台线程执行就是我们要看的核心代码了。围绕我们要看的类AsyncAppender，来看看类关系图。

一、放入队列

主要实现就是logger将数据通过append方法放入到阻塞队列中。

//AsyncAppender.java
    /**
     * 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");
        }
        //创建Log4jLogEvent的对象memento
        final Log4jLogEvent memento = Log4jLogEvent.createMemento(logEvent, includeLocation);
        InternalAsyncUtil.makeMessageImmutable(logEvent.getMessage());
        //transfer(memento)将event放入队列
        //默认ArrayBlockingQueueFactory 大小1024
        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);
            }
        }
    }

    private boolean transfer(final LogEvent memento) {
        return queue instanceof TransferQueue
            ? ((TransferQueue<LogEvent>) queue).tryTransfer(memento)
            : queue.offer(memento);
    }

如流程图所示，首先会判断用户是否设置了blocking选项，默认是true，如果设置为false，则Appender直接会ToErrorAppender，如果用户没有配置或者配置为true，则会按照一定的策略来处理这些消息。策略可以分为2种，他们分别为：

1、DefaultAsyncQueueFullPolicy---等待队列，转为同步操作策略

public class DefaultAsyncQueueFullPolicy implements AsyncQueueFullPolicy {
    @Override
    public EventRoute getRoute(final long backgroundThreadId, final Level level) {

        // LOG4J2-471: prevent deadlock when RingBuffer is full and object
        // being logged calls Logger.log() from its toString() method
        if (Thread.currentThread().getId() == backgroundThreadId) {
            return EventRoute.SYNCHRONOUS;
        }
        return EventRoute.ENQUEUE;
    }

2、DiscardingAsyncQueueFullPolicy---按照日志等级抛弃日志策略

//DiscardingAsyncQueueFullPolicy.java
    @Override
    public EventRoute getRoute(final long backgroundThreadId, final Level level) {
        if (level.isLessSpecificThan(thresholdLevel)) {
            if (discardCount.getAndIncrement() == 0) {
                LOGGER.warn("Async queue is full, discarding event with level {}. " +
                        "This message will only appear once; future events from {} " +
                        "are silently discarded until queue capacity becomes available.",
                        level, thresholdLevel);
            }
            return EventRoute.DISCARD;
        }
        return super.getRoute(backgroundThreadId, level);
    }

二、后台线程执行后续操作。

主要就是后台线程从队列中取出数据然后进行后续的操作。

//AsyncAppender.java
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);
        }

        @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);
        }

    ...
}

该线程会一直尝试从阻塞队列中获取LogEvent，如果获取成功，调用AppenderRef所引用Appender的append方法。我们也可以看到，AsyncAppender实际上主要是类似于中转，日志异步化，当消息放入阻塞队列，返回成功，这样能够大幅提高日志记录的吞吐。用户可以在权衡性能与日志收集质量上进行权衡配置策略(设置blocking选项)，当然也可以设置不同类型的阻塞队列已到达更好的日志记录吞吐。

AsyncAppender配置参数  

https://logging.apache.org/log4j/2.x/manual/appenders.html#AsyncAppender