hystrix在执行命令的同事，会对执行延时进行检测，如果超过设置的延时时间，那么将停止执行，并返回fallback执行结果。

if (properties.executionTimeoutEnabled().get()) {
execution = executeCommandWithSpecifiedIsolation(_cmd)
.lift(new HystrixObservableTimeoutOperator(_cmd));
} else {
execution = executeCommandWithSpecifiedIsolation(_cmd);
}

　　本质就是开启一个定时器，当到达指定时间时，去检测执行命令是否完成，如果没有完成，则返回异常，外部方法监听异常，执行fallback。

private static class HystrixObservableTimeoutOperator implements Operator {
final AbstractCommand originalCommand;
public HystrixObservableTimeoutOperator(final AbstractCommand originalCommand) {
this.originalCommand = originalCommand;
}
@Override
public Subscriber call(final Subscriber child) {
…
final HystrixContextRunnable timeoutRunnable = new HystrixContextRunnable(originalCommand.concurrencyStrategy, new Runnable() {
@Override
public void run() {
child.onError(new HystrixTimeoutException());
}
});
TimerListener listener = new TimerListener() {
…
timeoutRunnable.run();
…
}
}
@Override
public int getIntervalTimeInMilliseconds() {
return originalCommand.properties.executionTimeoutInMilliseconds().get();
}
};
final Reference tl = HystrixTimer.getInstance().addTimerListener(listener);
…
}

}