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本文主要研究一下logback的AsyncAppender
AsyncAppender
ch/qos/logback/classic/AsyncAppender.java
代码语言：javascript复制public class AsyncAppender extends AsyncAppenderBase<ILoggingEvent> {
boolean includeCallerData = false;
/**
* Events of level TRACE, DEBUG and INFO are deemed to be discardable.
* @param event
* @return true if the event is of level TRACE, DEBUG or INFO false otherwise.
*/
protected boolean isDiscardable(ILoggingEvent event) {
Level level = event.getLevel();
return level.toInt() <= Level.INFO_INT;
}
protected void preprocess(ILoggingEvent eventObject) {
eventObject.prepareForDeferredProcessing();
if (includeCallerData)
eventObject.getCallerData();
}
public boolean isIncludeCallerData() {
return includeCallerData;
}
public void setIncludeCallerData(boolean includeCallerData) {
this.includeCallerData = includeCallerData;
}
}AsyncAppender继承了AsyncAppenderBase，它新增了includeCallerData配置，另外覆盖了isDiscardable、preprocess方法，isDiscardable针对TRACE、DEBUG的级别返回true，INFO返回false；preprocess则判断是否includeCallerData，是的话则执行eventObject.getCallerData()
AsyncAppenderBase
ch/qos/logback/core/AsyncAppenderBase.java
代码语言：javascript复制public class AsyncAppenderBase<E> extends UnsynchronizedAppenderBase<E> implements AppenderAttachable<E> {
AppenderAttachableImpl<E> aai = new AppenderAttachableImpl<E>();
BlockingQueue<E> blockingQueue;
/**
* The default buffer size.
*/
public static final int DEFAULT_QUEUE_SIZE = 256;
int queueSize = DEFAULT_QUEUE_SIZE;
int appenderCount = 0;
static final int UNDEFINED = -1;
int discardingThreshold = UNDEFINED;
boolean neverBlock = false;
Worker worker = new Worker();
/**
* The default maximum queue flush time allowed during appender stop. If the
* worker takes longer than this time it will exit, discarding any remaining
* items in the queue
*/
public static final int DEFAULT_MAX_FLUSH_TIME = 1000;
int maxFlushTime = DEFAULT_MAX_FLUSH_TIME;
/**
* Is the eventObject passed as parameter discardable? The base class's implementation of this method always returns
* 'false' but sub-classes may (and do) override this method.
* <p/>
* <p>Note that only if the buffer is nearly full are events discarded. Otherwise, when the buffer is "not full"
* all events are logged.
*
* @param eventObject
* @return - true if the event can be discarded, false otherwise
*/
protected boolean isDiscardable(E eventObject) {
return false;
}
/**
* Pre-process the event prior to queueing. The base class does no pre-processing but sub-classes can
* override this behavior.
*
* @param eventObject
*/
protected void preprocess(E eventObject) {
}
@Override
public void start() {
if (isStarted())
return;
if (appenderCount == 0) {
addError("No attached appenders found.");
return;
}
if (queueSize < 1) {
addError("Invalid queue size [" queueSize "]");
return;
}
blockingQueue = new ArrayBlockingQueue<E>(queueSize);
if (discardingThreshold == UNDEFINED)
discardingThreshold = queueSize / 5;
addInfo("Setting discardingThreshold to " discardingThreshold);
worker.setDaemon(true);
worker.setName("AsyncAppender-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();
InterruptUtil interruptUtil = new InterruptUtil(context);
try {
interruptUtil.maskInterruptFlag();
worker.join(maxFlushTime);
// check to see if the thread ended and if not add a warning message
if (worker.isAlive()) {
addWarn("Max queue flush timeout (" maxFlushTime " ms) exceeded. Approximately " blockingQueue.size()
" queued events were possibly discarded.");
} else {
addInfo("Queue flush finished successfully within timeout.");
}
} catch (InterruptedException e) {
int remaining = blockingQueue.size();
addError("Failed to join worker thread. " remaining " queued events may be discarded.", e);
} finally {
interruptUtil.unmaskInterruptFlag();
}
}
@Override
protected void append(E eventObject) {
if (isQueueBelowDiscardingThreshold() && isDiscardable(eventObject)) {
return;
}
preprocess(eventObject);
put(eventObject);
}
protected boolean isDiscardable(E eventObject) {
return false;
}
protected void preprocess(E eventObject) {
}
private boolean isQueueBelowDiscardingThreshold() {
return (blockingQueue.remainingCapacity() < discardingThreshold);
}
private void put(E eventObject) {
if (neverBlock) {
blockingQueue.offer(eventObject);
} else {
putUninterruptibly(eventObject);
}
}
private void putUninterruptibly(E eventObject) {
boolean interrupted = false;
try {
while (true) {
try {
blockingQueue.put(eventObject);
break;
} catch (InterruptedException e) {
interrupted = true;
}
}
} finally {
if (interrupted) {
Thread.currentThread().interrupt();
}
}
}
//......
} AsyncAppenderBase继承了UnsynchronizedAppenderBase，实现了AppenderAttachable接口，它定义了queueSize、discardingThreshold、neverBlock等属性，其start方法会根据queueSize创建ArrayBlockingQueue，discardingThreshold默认为
queueSize / 5，之后启动Wroker；stop方法则执行worker.interrupt()，然后等待maxFlushTime让log进行flush；其append方法会先判断isQueueBelowDiscardingThreshold及isDiscardable，都为true则直接返回，否则执行preprocess、put方法
Worker
ch/qos/logback/core/AsyncAppenderBase.java
代码语言：javascript复制 class Worker extends Thread {
public void run() {
AsyncAppenderBase<E> parent = AsyncAppenderBase.this;
AppenderAttachableImpl<E> 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);
parent.blockingQueue.remove(e);
}
aai.detachAndStopAllAppenders();
}
}Worker的run方法会不断循环从blockingQueue阻塞取出原生，然后添加到AppenderAttachableImpl；在started为false的时候跳槽循环，然后遍历blockingQueue，添加到AppenderAttachableImpl，然后将其从blockingQueue；最后执行detachAndStopAllAppenders
AppenderAttachableImpl
ch/qos/logback/core/spi/AppenderAttachableImpl.java
代码语言：javascript复制 public int appendLoopOnAppenders(E e) {
int size = 0;
final Appender<E>[] appenderArray = appenderList.asTypedArray();
final int len = appenderArray.length;
for (int i = 0; i < len; i ) {
appenderArray[i].doAppend(e);
size ;
}
return size;
}
/**
* Remove and processPriorToRemoval all previously attached appenders.
*/
public void detachAndStopAllAppenders() {
for (Appender<E> a : appenderList) {
a.stop();
}
appenderList.clear();
}AppenderAttachableImpl的appendLoopOnAppenders方法会遍历所有的appenderList执行doAppend方法；其detachAndStopAllAppenders则遍历appenderList，挨个执行stop，最后clear掉整个appenderList
小结
logback的AsyncAppender使用ArrayBlockingQueue(默认size为256)来进行缓冲，每次append的时候会先判断isQueueBelowDiscardingThreshold及isDiscardable，为true则直接返回/丢弃，之后执行preprocess，最后执行put，put的时候有个参数neverBlock，为true则使用的是offer方法，队列满的时候会被丢弃，为false则是阻塞的方法，等到put成功才返回；另外它有个worker线程，不断从blockingQueue阻塞take元素出来然后写入到appenderList，在关闭时还会遍历队列写入到appenderList然后从队列移除，最后清空队列。
