/*** The AsyncAppender lets users log events asynchronously.* <p/>* <p/>* The AsyncAppender will collect the events sent to it and then dispatch them* to all the appenders that are attached to it. You can attach multiple* appenders to an AsyncAppender.* </p>* <p/>* <p/>* The AsyncAppender uses a separate thread to serve the events in its buffer.* </p>* <p/>* <b>Important note:</b> The <code>AsyncAppender</code> can only be script* configured using the {@link org.apache.log4j.xml.DOMConfigurator}.* </p>** @author Ceki G&uuml;lc&uuml;* @author Curt Arnold* @since 0.9.1*/

//构造

  /*** Create new instance.*/public AsyncAppender() {appenders = new AppenderAttachableImpl();////   only set for compatibilityaai = appenders;dispatcher =new Thread(new Dispatcher(this, buffer, discardMap, appenders));// It is the user's responsibility to close appenders before// exiting.dispatcher.setDaemon(true);// set the dispatcher priority to lowest possible value//        dispatcher.setPriority(Thread.MIN_PRIORITY);dispatcher.setName("Dispatcher-" + dispatcher.getName());dispatcher.start();}
//构造 构造了一个叫despatcher的最低级别守护进程。

//其实唯一需要关注的方法就是append，我们看它怎么做异步append

  /*** {@inheritDoc}*/public void append(final LoggingEvent event) {////   if dispatcher thread has died then//      append subsequent events synchronously//   See bug 23021// 如果dispatcher线程不在了，直接调用各个appender的同步写方法，确保日志的写入if ((dispatcher == null) || !dispatcher.isAlive() || (bufferSize <= 0)) {synchronized (appenders) {appenders.appendLoopOnAppenders(event);}return;}// Set the NDC and thread name for the calling thread as these// LoggingEvent fields were not set at event creation time.event.getNDC();event.getThreadName();// Get a copy of this thread's MDC.event.getMDCCopy();if (locationInfo) {event.getLocationInformation();}synchronized (buffer) { //这里将buffer资源对象当锁，可以想象到所有操作buffer的方法都必须做同步。while (true) {int previousSize = buffer.size();if (previousSize < bufferSize) {buffer.add(event);////   if buffer had been empty//       signal all threads waiting on buffer//       to check their conditions.//if (previousSize == 0) {buffer.notifyAll();  //notifyAll()方法是Object对象的方法，而不是Thread对象的方法。}break;}////   Following code is only reachable if buffer is full//   如果buffer写满了，才会走到下面的代码流程////   if blocking and thread is not already interrupted//      and not the dispatcher then//      wait for a buffer notificationboolean discard = true;if (blocking  //buffer满就阻塞的标志位&& !Thread.interrupted()&& Thread.currentThread() != dispatcher) {try {buffer.wait(); //满了就等待把discard = false;} catch (InterruptedException e) {////  reset interrupt status so//    calling code can see interrupt on//    their next wait or sleep.Thread.currentThread().interrupt(); //这里处理interrupt的方法值得体会：）当然可以学会套用这个模板}}////   if blocking is false or thread has been interrupted//   add event to discard map.//if (discard) { //如果没能成功进入wait(),放到丢弃消息map中保存String loggerName = event.getLoggerName();DiscardSummary summary = (DiscardSummary) discardMap.get(loggerName);if (summary == null) {summary = new DiscardSummary(event);discardMap.put(loggerName, summary);} else {summary.add(event);}break;}}}}

//这里出现了2个新的东西，dispatcher 和DiscardSummary，下面分析dispatcher线程的实现

  /*** Event dispatcher.*/private static class Dispatcher implements Runnable {/*** Parent AsyncAppender.*/private final AsyncAppender parent;/*** Event buffer.*/private final List buffer;/*** Map of DiscardSummary keyed by logger name.*/private final Map discardMap;/*** Wrapped appenders.*/private final AppenderAttachableImpl appenders;/*** Create new instance of dispatcher.** @param parent     parent AsyncAppender, may not be null.* @param buffer     event buffer, may not be null.* @param discardMap discard map, may not be null.* @param appenders  appenders, may not be null.*/public Dispatcher(final AsyncAppender parent, final List buffer, final Map discardMap,final AppenderAttachableImpl appenders) {this.parent = parent;this.buffer = buffer;this.appenders = appenders;this.discardMap = discardMap;}/*** {@inheritDoc}*/public void run() {boolean isActive = true;////   if interrupted (unlikely), end thread//try {////   loop until the AsyncAppender is closed.//while (isActive) {LoggingEvent[] events = null;////   extract pending events while synchronized//       on buffer//synchronized (buffer) {int bufferSize = buffer.size();isActive = !parent.closed;while ((bufferSize == 0) && isActive) {buffer.wait();bufferSize = buffer.size();isActive = !parent.closed;}if (bufferSize > 0) {events = new LoggingEvent[bufferSize + discardMap.size()];buffer.toArray(events);////   add events due to buffer overflow//int index = bufferSize;for (Iterator iter = discardMap.values().iterator();iter.hasNext();) {events[index++] = ((DiscardSummary) iter.next()).createEvent();}////    clear buffer and discard map//buffer.clear();discardMap.clear();////    allow blocked appends to continuebuffer.notifyAll();}}////   process events after lock on buffer is released.//if (events != null) {for (int i = 0; i < events.length; i++) {synchronized (appenders) {appenders.appendLoopOnAppenders(events[i]);}}}}} catch (InterruptedException ex) {Thread.currentThread().interrupt();}}}