logback异步输出日志的配置方法和源码分析

1，异步输出日志的配置

logback中的异步输出日志使用了AsyncAppender这个appender

配置方式如下：

    
        logs/context-log.%d{yyyy-MM-dd}.log
        30
    
    
        [%-5level] %date --%thread-- [%logger] %msg %n
    



    0
    1234
    

AsyncAppender的父类是AsyncAppenderBase，用到的代码基本都在这个父类里面。

2，异步输出日志时会把信息放到BlockingQueue中

当执行logger.info()方法时，Logger里的源码是这样的：

Logger类：ch.qos.logback.classic.Logger

info方法：

public void info(String msg) {
  filterAndLog_0_Or3Plus(FQCN, null, Level.INFO, msg, null, null);
}

filterAndLog_0_Or3Plus方法：

  private void filterAndLog_0_Or3Plus(final String localFQCN,
      final Marker marker, final Level level, final String msg,
      final Object[] params, final Throwable t) {

    final FilterReply decision = loggerContext
        .getTurboFilterChainDecision_0_3OrMore(marker, this, level, msg,
            params, t);

    if (decision == FilterReply.NEUTRAL) {
      if (effectiveLevelInt > level.levelInt) {
        return;
      }
    } else if (decision == FilterReply.DENY) {
      return;
    }

    buildLoggingEventAndAppend(localFQCN, marker, level, msg, params, t);
  }

中间判断了一下日志级别，如果本日志级别比配置的级别低，就不打日志了。比如配置的日志级别是ERROR，但是这段代码的级别是INFO，这段日志就不打印了。
最后的buildLoggingEventAndAppend方法：

   private void buildLoggingEventAndAppend(final String localFQCN,
      final Marker marker, final Level level, final String msg,
      final Object[] params, final Throwable t) {
    LoggingEvent le = new LoggingEvent(localFQCN, this, level, msg, t, params);
    le.setMarker(marker);
    callAppenders(le);
  }

在这一步封装了LoggingEvent对象，Logback后面对日志的处理基本都是以LoggingEvent对象为单位了。

最后的callAppenders方法的代码：

  /**
   * Invoke all the appenders of this logger.
   * 
   * @param event
   *          The event to log
   */
  public void callAppenders(ILoggingEvent event) {
    int writes = 0;
    for (Logger l = this; l != null; l = l.parent) {
      writes += l.appendLoopOnAppenders(event);
      if (!l.additive) {
        break;
      }
    }
    // No appenders in hierarchy
    if (writes == 0) {
      loggerContext.noAppenderDefinedWarning(this);
    }
  }

这个方法调用了Logger的appendLoopOnAppenders方法

appendLoopOnAppenders方法：

  private int appendLoopOnAppenders(ILoggingEvent event) {
    if (aai != null) {
      return aai.appendLoopOnAppenders(event);
    } else {
      return 0;
    }
  }

aai是AppenderAttachableImpl类的对象，这个类在ch.qos.logback.core.spi包下，是专门用来处理相关appender的，维护了appender的列表，并且提供appender的添加、删除等方法。
Logger相关的appender就是配置文件中的ch.qos.logback.classic.AsyncAppender类，另外配置文件中的ch.qos.logback.core.rolling.RollingFileAppender不算，他属于AsyncAppender，不属于Logger。

AppenderAttachableImpl类的appendLoopOnAppenders方法：

  /**
   * Call the doAppend method on all attached appenders.
   */
  public int appendLoopOnAppenders(E e) {
    int size = 0;
      for (Appender appender : appenderList) {
        appender.doAppend(e);
        size++;
      }
    return size;
  }

执行了所有相关appender的doAppend方法，异步输出日志的appender是AsyncAppender

AsyncAppender的父类是AsyncAppenderBase，在ch.qos.logback.core包下

AsyncAppenderBase的父类是UnsynchronizedAppenderBase，也在ch.qos.logback.core包下，doAppend方法在这个类中：

  public void doAppend(E eventObject) {
    // WARNING: The guard check MUST be the first statement in the
    // doAppend() method.
      
    // prevent re-entry.
    if (Boolean.TRUE.equals(guard.get())) {
      return;
    }

    try {
      guard.set(Boolean.TRUE);

      if (!this.started) {
        if (statusRepeatCount++ < ALLOWED_REPEATS) {
          addStatus(new WarnStatus(
              "Attempted to append to non started appender [" + name + "].",
              this));
        }
        return;
      }

      if (getFilterChainDecision(eventObject) == FilterReply.DENY) {
        return;
      }

      // ok, we now invoke derived class' implementation of append
      this.append(eventObject);

    } catch (Exception e) {
      if (exceptionCount++ < ALLOWED_REPEATS) {
        addError("Appender [" + name + "] failed to append.", e);
      }
    } finally {
      guard.set(Boolean.FALSE);
    }
  }

最后的this.append方法在该类中是没有实现的抽象方法，具体实现在他的子类AsyncAppenderBase中：

@Override
protected void append(E eventObject) {
  if (isQueueBelowDiscardingThreshold() && isDiscardable(eventObject)) {
    return;
  }
  preprocess(eventObject);
  put(eventObject);
}

然后append方法里的put方法：

private void put(E eventObject) {
  try {
    blockingQueue.put(eventObject);
  } catch (InterruptedException e) {
  }
}

日志的内容会被放到AsyncAppenderBase里定义的一个BlockingQueue中，至此Logger.info的任务完成了。

所谓的异步输出日志就是Logger.info负责往Queue中放日志，再起个线程把Queue中的日志写到磁盘上。

3，从BlockingQueue中获取信息并写入到文件

在AsyncAppender的父类AsyncAppenderBase里面定义了一个叫Worker的内部类，这个类负责从BlockingQueue中取出信息并处理，Worker的定义如下：

class Worker extends Thread {

  public void run() {
    AsyncAppenderBase parent = AsyncAppenderBase.this;
    AppenderAttachableImpl aai = parent.aai;

    // loop while the parent is started
    while (parent.isStarted()) {
      try {
        E e = parent.blockingQueue.take();
        aai.appendLoopOnAppenders(e);
      } catch (InterruptedException ie) {
        break;
      }
    }

    addInfo("Worker thread will flush remaining events before exiting. ");
    for (E e : parent.blockingQueue) {
      aai.appendLoopOnAppenders(e);
    }

    aai.detachAndStopAllAppenders();
  }
}

另外AsyncAppenderBase还定义了Worker线程的start和stop方法，是重写的父类UnsynchronizedAppenderBase中的方法：

@Override
public void start() {
  if (appenderCount == 0) {
    addError("No attached appenders found.");
    return;
  }
  if (queueSize < 1) {
    addError("Invalid queue size [" + queueSize + "]");
    return;
  }
  blockingQueue = new ArrayBlockingQueue(queueSize);

  if (discardingThreshold == UNDEFINED)
    discardingThreshold = queueSize / 5;
  addInfo("Setting discardingThreshold to " + discardingThreshold);
  worker.setDaemon(true);
  worker.setName("AsyncAppender-Worker-" + worker.getName());
  // make sure this instance is marked as "started" before staring the worker Thread
  super.start();
  worker.start();
}

@Override
public void stop() {
  if (!isStarted())
    return;

  // mark this appender as stopped so that Worker can also processPriorToRemoval if it is invoking aii.appendLoopOnAppenders
  // and sub-appenders consume the interruption
  super.stop();

  // interrupt the worker thread so that it can terminate. Note that the interruption can be consumed
  // by sub-appenders
  worker.interrupt();
  try {
    worker.join(1000);
  } catch (InterruptedException e) {
    addError("Failed to join worker thread", e);
  }
}

可以看到，在while循环期间，Worker从blockingQueue里面拿出一个元素并进行处理，还是调用了AppenderAttachableImpl的appendLoopOnAppenders方法（和上面一样）：

/**
 * Call the doAppend method on all attached appenders.
 */
public int appendLoopOnAppenders(E e) {
  int size = 0;
    for (Appender appender : appenderList) {
      appender.doAppend(e);
      size++;
    }
  return size;
}

调用了所有Appender的doAppend方法，在上面的配置中，AsyncAppenderBase相关的appender是RollingFileAppender，在ch.qos.logback.core.rolling包中

RollingFileAppender的父类是FileAppender，在ch.qos.logback.core包中

FileAppender的父类是OutputStreamAppender，也在ch.qos.logback.core包中

OutputStreamAppender的父类是UnsynchronizedAppenderBase，doAppend方法在这个类中（还是和上面的一样）：

public void doAppend(E eventObject) {
  // WARNING: The guard check MUST be the first statement in the
  // doAppend() method.
    
  // prevent re-entry.
  if (Boolean.TRUE.equals(guard.get())) {
    return;
  }

  try {
    guard.set(Boolean.TRUE);

    if (!this.started) {
      if (statusRepeatCount++ < ALLOWED_REPEATS) {
        addStatus(new WarnStatus(
            "Attempted to append to non started appender [" + name + "].",
            this));
      }
      return;
    }

    if (getFilterChainDecision(eventObject) == FilterReply.DENY) {
      return;
    }

    // ok, we now invoke derived class' implementation of append
    this.append(eventObject);

  } catch (Exception e) {
    if (exceptionCount++ < ALLOWED_REPEATS) {
      addError("Appender [" + name + "] failed to append.", e);
    }
  } finally {
    guard.set(Boolean.FALSE);
  }
}

这段代码，经过了一堆的判断和设置，调用了append方法，自己类里的append方法是个抽象方法，具体的实现写在了他的子类OutputStreamAppender里，代码如下：

@Override
protected void append(E eventObject) {
  if (!isStarted()) {
    return;
  }

  subAppend(eventObject);
}

然后是subAppend方法：

/**
 * Actual writing occurs here.
 * 

 * Most subclasses of WriterAppender will need to override this
 * method.
 * 
 * @since 0.9.0
 */
protected void subAppend(E event) {
  if (!isStarted()) {
    return;
  }
  try {
    // this step avoids LBCLASSIC-139
    if (event instanceof DeferredProcessingAware) {
      ((DeferredProcessingAware) event).prepareForDeferredProcessing();
    }
    // the synchronization prevents the OutputStream from being closed while we
    // are writing. It also prevents multiple threads from entering the same
    // converter. Converters assume that they are in a synchronized block.
    lock.lock();
    try {
      writeOut(event);
    } finally {
      lock.unlock();
    }
  } catch (IOException ioe) {
    // as soon as an exception occurs, move to non-started state
    // and add a single ErrorStatus to the SM.
    this.started = false;
    addStatus(new ErrorStatus("IO failure in appender", this, ioe));
  }
}

然后是writeOut方法：

protected void writeOut(E event) throws IOException {
  this.encoder.doEncode(event);
}

调用了encoder的doEncode方法，encoder的类是Encoder，Encoder是OutputStreamAppender定义的最终负责写日志的接口，由LayoutWrappingEncoder类实现：

  public void setLayout(Layout layout) {
    addWarn("This appender no longer admits a layout as a sub-component, set an encoder instead.");
    addWarn("To ensure compatibility, wrapping your layout in LayoutWrappingEncoder.");
    addWarn("See also "+CODES_URL+"#layoutInsteadOfEncoder for details");
    LayoutWrappingEncoder lwe = new LayoutWrappingEncoder();
    lwe.setLayout(layout);
    lwe.setContext(context);
    this.encoder = lwe;
  }

LayoutWrappingEncoder的doEncode方法是这么写的：

  public void doEncode(E event) throws IOException {
    String txt = layout.doLayout(event);
    outputStream.write(convertToBytes(txt));
    if (immediateFlush)
      outputStream.flush();
  }

用OutputStream写日志，immediateFlush可以在配置文件里配置，代表是否立即清空信息流，默认为true，如果配置为false，则会让outputStream快满的时候清空信息流。

至此日志写入完毕

 

4，一些可能有用的配置方式

1，blockingQueue长度。

blockingQueue长度决定了队列能放多少信息，在默认的配置下，如果blockingQueue放满了，后续想要输出日志的线程会被阻塞，直到Worker线程处理掉队列中的信息为止。根据实际情况适当调整队列长度，可以防止线程被阻塞。

2，immediateFlush=false。不立即清空输出流。

immediateFlush参数可以配置在里面，默认是true，代表是否立即刷新OutputStream中的信息。如果设置为false，会在OutputStream放满或隔断时间进行flush，具体由OutputStream类决定。据说设置为false之后输出日志的效率能提高为原来的4倍。

官网说：setting thisproperty to 'false' is likely to quadruple (your mileage may vary) loggingthroughput.

3，neverBlock=true。队列满了也不卡线程

neverBlock参数可以配置在里面，默认是false，代表在队列放满的情况下是否卡住线程。也就是说，如果配置neverBlock=true，当队列满了之后，后面阻塞的线程想要输出的消息就直接被丢弃，从而线程不会阻塞。这个配置用于线程很重要，不能卡顿，而且日志又不是很重要的场景，因为很有可能会丢日志。

4，自定义appender

开发者可以自己写一个appender类，需要继承AppenderBase类并重写append(LoggingEventeventObject)方法，然后像别的appender一样配置到logback.xml里面，就可以定义自己的日志输出方式了。