log4j2用asyncRoot配置異步日誌是如何使用disruptor

用asyncRoot配置對應的對接disruptor類是AsyncLoggerConfigDisruptor,用Log4jContextSelector啓動參數配置全局異步的對應的對接disruptor類是AsyncLoggerDisruptor。下面分析的是AsyncLoggerConfigDisruptor

disruptor的建立與啓動須要的部件實現

AsyncLoggerConfigDisruptor.start方法用來建立並啓動disruptor實例
建立disruptor須要EventFactory，ringBuffer的大小，ThreadFactory，ProducerType，等待策略waitStrategy
建立後須要設置ExceptionHandler，設置EventHandler。
發佈(生產)事件的translator。

EventFactory

分是否可變(mutable字段)場景對應兩個不一樣的EventFactory。
不可變的factory的邏輯是：

@Override
public Log4jEventWrapper newInstance() {
    return new Log4jEventWrapper();
}

可變的factory邏輯是：

public Log4jEventWrapper newInstance() {
    return new Log4jEventWrapper(new MutableLogEvent());
}

會在 Log4jEventWrapper的構造函數中傳入MutableLogEvent實例。

ringBuffer的大小

是根據AsyncLoggerConfig.RingBufferSize配置值算出來的。
這個配置項的值最小不能小於128，默認值分兩種狀況進行設定：若是啓用了ENABLE_THREADLOCALS(優化GC的一個配置項)，那麼默認值是4 * 1024，不然是256 * 1024。
這個配置是經過System properties指定，一樣存在不一樣版本，配置項名稱不一致的狀況，log4j2.asyncLoggerRingBufferSize (AsyncLogger.RingBufferSize)。詳細能夠參見這裏

ThreadFactory

主要是定製線程名：
線程名格式是："AsyncLoggerConfig-" + FACTORY_NUMBER(自增) + "-" + threadFactoryName + "-" + THREAD_NUMBER(自增)
默認的實際示例是： Log4j2-TF-1-AsyncLoggerConfig--1，跟上面的有些差別，上面的分析錯了嗎？？

ProducerType

多生產者

等待策略waitStrategy

默認是10ms的TimeoutBlockingWaitStrategy。 支持可配置SleepingWaitStrategy、YieldingWaitStrategy、BlockingWaitStrategy、BusySpinWaitStrategy、TimeoutBlockingWaitStrategy。
這個配置是經過System properties指定，一樣存在不一樣版本，配置項名稱不一致的狀況，log4j2.asyncLoggerWaitStrategy (AsyncLogger.WaitStrategy)。

ExceptionHandler

能夠配置，配置項名稱是AsyncLoggerConfig.ExceptionHandler，默認是用AsyncLoggerConfigDefaultExceptionHandler，打印: AsyncLogger error handling event seq=..., value=...,並打出異常棧。

EventHandler

此處使用了 Log4jEventWrapperHandler RingBufferLogEventHandler，是disruptor的SequenceReportingEventHandler實現。

/**
 * EventHandler performs the work in a separate thread.
 */
private static class Log4jEventWrapperHandler implements SequenceReportingEventHandler<Log4jEventWrapper> {
    private static final int NOTIFY_PROGRESS_THRESHOLD = 50;
    private Sequence sequenceCallback;
    private int counter;

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

    @Override
    public void onEvent(final Log4jEventWrapper event, final long sequence, final boolean endOfBatch)
            throws Exception {
        event.event.setEndOfBatch(endOfBatch);
        event.loggerConfig.asyncCallAppenders(event.event);
        event.clear();

        notifyIntermediateProgress(sequence);
    }

    /**
     * Notify the BatchEventProcessor that the sequence has progressed. Without this callback the sequence would not
     * be progressed until the batch has completely finished.
     */
    private void notifyIntermediateProgress(final long sequence) {
        if (++counter > NOTIFY_PROGRESS_THRESHOLD) {
            sequenceCallback.set(sequence);
            counter = 0;
        }
    }
}

event.loggerConfig.asyncCallAppenders(event.event); 這個會觸發日誌的輸出

translator

跟EventFactory同樣分mutable是否可變的兩種狀況。
不可變：

private static final EventTranslatorTwoArg<Log4jEventWrapper, LogEvent, AsyncLoggerConfig> TRANSLATOR =
        new EventTranslatorTwoArg<Log4jEventWrapper, LogEvent, AsyncLoggerConfig>() {
    @Override
    public void translateTo(final Log4jEventWrapper ringBufferElement, final long sequence,
            final LogEvent logEvent, final AsyncLoggerConfig loggerConfig) {
        ringBufferElement.event = logEvent;
        ringBufferElement.loggerConfig = loggerConfig;
    }
};

可變：

/**
 * Object responsible for passing on data to a RingBuffer event with a MutableLogEvent.
 */
private static final EventTranslatorTwoArg<Log4jEventWrapper, LogEvent, AsyncLoggerConfig> MUTABLE_TRANSLATOR =
        new EventTranslatorTwoArg<Log4jEventWrapper, LogEvent, AsyncLoggerConfig>() {
    @Override
    public void translateTo(final Log4jEventWrapper ringBufferElement, final long sequence,
            final LogEvent logEvent, final AsyncLoggerConfig loggerConfig) {
        ((MutableLogEvent) ringBufferElement.event).initFrom(logEvent);
        ringBufferElement.loggerConfig = loggerConfig;
    }
};

都是完成一個事情給ringBufferElement元素的event賦值或者初始化。

事件進隊列的tryEnqueue方法邏輯分析

• prepareEvent

1. ensureImmutable：建立LogEvent的快照，傳遞進來的原LogEvent在應用線程還會被繼續修改，因此此處要建立快照
2. Log4jLogEvent.makeMessageImmutable：格式化消息。由於是異步日誌，針對須要format的消息在此處進行格式化，不然會由於引用對象值的改變致使日誌不許確

• tryPublishEvent

1. 發佈事件對象到disruptor隊列

事件出隊列的處理

先看按下調用棧:

Daemon Thread [Log4j2-TF-1-AsyncLoggerConfig--1] (Suspended (breakpoint at line 37 in PatternFormatter))    
    PatternFormatter.format(LogEvent, StringBuilder) line: 37   
    PatternLayout$PatternSerializer.toSerializable(LogEvent, StringBuilder) line: 334   
    PatternLayout.toText(AbstractStringLayout$Serializer2, LogEvent, StringBuilder) line: 233   
    PatternLayout.encode(LogEvent, ByteBufferDestination) line: 218 
    PatternLayout.encode(Object, ByteBufferDestination) line: 58    
    ConsoleAppender(AbstractOutputStreamAppender<M>).directEncodeEvent(LogEvent) line: 177  
    ConsoleAppender(AbstractOutputStreamAppender<M>).tryAppend(LogEvent) line: 170  
    ConsoleAppender(AbstractOutputStreamAppender<M>).append(LogEvent) line: 161 
    AppenderControl.tryCallAppender(LogEvent) line: 156 
    AppenderControl.callAppender0(LogEvent) line: 129   
    AppenderControl.callAppenderPreventRecursion(LogEvent) line: 120    
    AppenderControl.callAppender(LogEvent) line: 84 
    AsyncLoggerConfig(LoggerConfig).callAppenders(LogEvent) line: 448   
    AsyncLoggerConfig.asyncCallAppenders(LogEvent) line: 129    
    AsyncLoggerConfigDisruptor$Log4jEventWrapperHandler.onEvent(AsyncLoggerConfigDisruptor$Log4jEventWrapper, long, boolean) line: 111  
    AsyncLoggerConfigDisruptor$Log4jEventWrapperHandler.onEvent(Object, long, boolean) line: 97 
    BatchEventProcessor<T>.run() line: 129  
    Log4jThread(Thread).run() line: 748

Log4jEventWrapperHandler.onEvent 111行是event.loggerConfig.asyncCallAppenders(event.event)，參見上面EventHandler，此處完成日誌真正寫出。