AbstractCommand執行命令前首先會判斷是否開啟了timeout。

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

　　HystrixObservableTimeoutOperator的核心是內部又一個定時器，當達到timeout時間時，將命令狀態設置成timeout，向eventNotifier發送timeout事件，取消監聽，返回一個HystrixTimeoutException的onError。

private static class HystrixObservableTimeoutOperator<R> implements Operator<R, R> {
       ....
        @Override
        public Subscriber<? super
 
 R> call(final Subscriber<? super R> child) {
            final CompositeSubscription s = new CompositeSubscription();
            child.add(s);

            /*
             * Define the action to perform on timeout outside of the TimerListener to it can capture the HystrixRequestContext
             * of the calling thread which doesn‘t exist on the Timer thread.
             
 
*/
            final HystrixContextRunnable timeoutRunnable = new HystrixContextRunnable(originalCommand.concurrencyStrategy, new Runnable() {

                @Override
                public void run() {
                    child.onError(new HystrixTimeoutException());
                }
            });

            TimerListener listener = new TimerListener() {
                @Override
                public void tick() {
                    // if we can go from NOT_EXECUTED to TIMED_OUT then we do the timeout codepath
                    // otherwise it means we lost a race and the run() execution completed or did not start
                    if (originalCommand.isCommandTimedOut.compareAndSet(TimedOutStatus.NOT_EXECUTED, TimedOutStatus.TIMED_OUT)) {
                        // report timeout failure
                        originalCommand.eventNotifier.markEvent(HystrixEventType.TIMEOUT, originalCommand.commandKey);
                        // shut down the original request
                        s.unsubscribe();
                        timeoutRunnable.run();
                        //if it did not start, then we need to mark a command start for concurrency metrics, and then issue the timeout
                    }
                }
                @Override
                public int getIntervalTimeInMilliseconds() {
                    return originalCommand.properties.executionTimeoutInMilliseconds().get();
                }
            };
　　　　　　...

        }

    }

hystrix源碼小貼士之調用timeout實現