HystrixCommand在执行的过程中如何探测超时，本篇主要对此进行介绍说明。
1.主入口：executeCommandAndObserve
#com.netflix.hystrix.AbstractCommand#executeCommandAndObserve
private Observable<R> executeCommandAndObserve(final AbstractCommand<R> _cmd) {
···省略部分代码···
Observable<R> execution;
//判断是否开启超时监测
if (properties.executionTimeoutEnabled().get()) {
execution = executeCommandWithSpecifiedIsolation(_cmd)
.lift(new HystrixObservableTimeoutOperator<R>(_cmd));
} else {
execution = executeCommandWithSpecifiedIsolation(_cmd);
}
return execution.doOnNext(markEmits)
.doOnCompleted(markOnCompleted)
.onErrorResumeNext(handleFallback)
.doOnEach(setRequestContext);
}
executeCommandWithSpecifiedIsolation(_cmd) .lift(new HystrixObservableTimeoutOperator<R>(_cmd));
可以简单的认为lift 里面的对前面的Observable包含，类似装饰者，后面的parent就是指上层的Observable。其中 HystrixObservableTimeoutOperator 就是关键的部分。
2.关键点: HystrixObservableTimeoutOperator
先看下HystrixObservableTimeoutOperator.call()，TimerListener的实现
TimerListener listener = new TimerListener() {
@Override
public void tick() {
if (originalCommand.isCommandTimedOut.compareAndSet(TimedOutStatus.NOT_EXECUTED, TimedOutStatus.TIMED_OUT)) {
// 标记事件，可以认为是开的hook，这里暂忽略
originalCommand.eventNotifier.markEvent(HystrixEventType.TIMEOUT, originalCommand.commandKey);
//取消原Obserable的订阅
s.unsubscribe();
final HystrixContextRunnable timeoutRunnable = new HystrixContextRunnable(originalCommand.concurrencyStrategy, hystrixRequestContext, new Runnable() {
@Override
public void run() {
child.onError(new HystrixTimeoutException());
}
});
timeoutRunnable.run();
}
}
//获取配置的超时时间配置
@Override
public int getIntervalTimeInMilliseconds() {
return originalCommand.properties.executionTimeoutInMilliseconds().get();
}
};
这段代码的意思就是，给当前command的超时状态置为超时，如果设置成功就抛出HystrixTimeoutException异常，紧接着被command的 doOnErron接收走 fallback逻辑
fallback
private Observable<R> executeCommandAndObserve(final AbstractCommand<R> _cmd) {
final HystrixRequestContext currentRequestContext = HystrixRequestContext.getContextForCurrentThread();
.................................
final Func1<Throwable, Observable<R>> handleFallback = new Func1<Throwable, Observable<R>>() {
@Override
public Observable<R> call(Throwable t) {
circuitBreaker.markNonSuccess();
Exception e = getExceptionFromThrowable(t);
executionResult = executionResult.setExecutionException(e);
if (e instanceof RejectedExecutionException) {
return handleThreadPoolRejectionViaFallback(e);
} else if (t instanceof HystrixTimeoutException) {
//此处catch到超时异常
return handleTimeoutViaFallback();
} else if (t instanceof HystrixBadRequestException) {
return handleBadRequestByEmittingError(e);
} else {
/*
* Treat HystrixBadRequestException from ExecutionHook like a plain HystrixBadRequestException.
*/
if (e instanceof HystrixBadRequestException) {
eventNotifier.markEvent(HystrixEventType.BAD_REQUEST, commandKey);
return Observable.error(e);
}
return handleFailureViaFallback(e);
}
}
};
.................................
return execution.doOnNext(markEmits)
.doOnCompleted(markOnCompleted)
.onErrorResumeNext(handleFallback)
.doOnEach(setRequestContext);
}
同时s.unsubscribe()通知正在执行的线程，终止任务。如何终止呢？
executeCommandWithSpecifiedIsolation.subscribeOn()
subscribeOne的参数就是HystrixContextScheduler, Rxjava里 scheduler具体干活的是 worker，我们先看下Hystrix自定义scheduler的结构示意图
那么我们直奔主题，直接看
ThreadPoolWorker
//ThreadPoolWorker.schedule
@Override
public Subscription schedule(final Action0 action) {
if (subscription.isUnsubscribed()) {
return Subscriptions.unsubscribed();
}
ScheduledAction sa = new ScheduledAction(action);
subscription.add(sa);
sa.addParent(subscription);
ThreadPoolExecutor executor = (ThreadPoolExecutor) threadPool.getExecutor();
FutureTask<?> f = (FutureTask<?>) executor.submit(sa);
sa.add(new FutureCompleterWithConfigurableInterrupt(f, shouldInterruptThread, executor));
return sa;
}
1.开始的时候判断observable是否被订阅
2.被订阅后，将任务 submit到线程池
3.FutureCompleterWithConfigurableInterrupt scheduler在执行的时候，增加了observable的中断探测
private static class FutureCompleterWithConfigurableInterrupt implements Subscription {
private final FutureTask<?> f;
private final Func0<Boolean> shouldInterruptThread;
private final ThreadPoolExecutor executor;
private FutureCompleterWithConfigurableInterrupt(FutureTask<?> f, Func0<Boolean> shouldInterruptThread, ThreadPoolExecutor executor) {
this.f = f;
this.shouldInterruptThread = shouldInterruptThread;
this.executor = executor;
}
@Override
public void unsubscribe() {
executor.remove(f);
if (shouldInterruptThread.call()) {
f.cancel(true);
} else {
f.cancel(false);
}
}
.....省略代码.......
}
当observable 取消订阅时，就会把当前任务移除，并中断任务
到这里只是讲说了超时后的处理，如何认定执行超时呢？
3.匠心之巧
这里有个很巧妙的设计，再探HystrixObservableTimeoutOperator
final Reference<TimerListener> tl = HystrixTimer.getInstance().addTimerListener(listener);
#com.netflix.hystrix.util.HystrixTimer#addTimerListener
public Reference<TimerListener> addTimerListener(final TimerListener listener) {
startThreadIfNeeded();
// add the listener
Runnable r = new Runnable() {
@Override
public void run() {
try {
listener.tick();
} catch (Exception e) {
logger.error("Failed while ticking TimerListener", e);
}
}
};
ScheduledFuture<?> f = executor.get().getThreadPool().scheduleAtFixedRate(r, listener.getIntervalTimeInMilliseconds(), listener.getIntervalTimeInMilliseconds(), TimeUnit.MILLISECONDS);
return new TimerReference(listener, f);
}
利用了ScheduledThreadPoolExecutor，延迟执行，延迟时间就是我们设定的超时时间，我们再看下
#HystrixObservableTimeoutOperator
Subscriber<R> parent = new Subscriber<R>() {
@Override
public void onCompleted() {
if (isNotTimedOut()) {
// stop timer and pass notification through
tl.clear();
child.onCompleted();
}
}
@Override
public void onError(Throwable e) {
if (isNotTimedOut()) {
// stop timer and pass notification through
tl.clear();
child.onError(e);
}
}
..... ..... ..... ..... ..... ..... ..... ..... .....
private boolean isNotTimedOut() {
// if already marked COMPLETED (by onNext) or succeeds in setting to COMPLETED
return originalCommand.isCommandTimedOut.get() == TimedOutStatus.COMPLETED ||
originalCommand.isCommandTimedOut.compareAndSet(TimedOutStatus.NOT_EXECUTED, TimedOutStatus.COMPLETED);
}
};
这里parent就是指上层的obserable，这里可以抽象的认为是我们的HystrixCommand执行线程, 当command执行线程执行完成的时候或异常的时候，会执行 tl.clear()， 也就是Future.cancel()会中断 TimerListener 的ScheduledFuture 线程，迫使超时机制失效。
// tl.clear()
private static class TimerReference extends SoftReference<TimerListener> {
private final ScheduledFuture<?> f;
.... .... .... .... ....
@Override
public void clear() {
super.clear();
// stop this ScheduledFuture from any further executions
f.cancel(false);
}
}
4.回归文字
HystrixCommand里有个 TimedOutStatus 超时状态
TimedOutStatus
现在可以认为有两个线程，一个是hystrixCommand任务执行线程，一个是等着给hystrixCommand判定超时的线程，现在两个线程看谁能先把hystrixCommand的状态置换，只要任何一个线程对hystrixCommand打上标就意味着超时判定结束。
系列文章推荐
Hystrix熔断框架介绍
Hystrix常用功能介绍
Hystrix执行原理
Hystrix熔断器执行机制
Hystrix超时实现机制
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