两个线程进入一个同步块

Question

I have a block wrapped in synchronized(this), and I see both in Debug mode and in the logs that 2 threads enter this section at the same time.

public void dispatch(Event.Builder eventBuilder) {

    synchronized (this) {
        index++;
        getLogger().d(TAG, "race condition line A - The index is " + index);

        try {
            Event event = eventBuilder.build();
            getLogger().d(TAG, "race condition line B - The index is " + index);
            mDispatcher.dispatch(event);

        } catch (InstantiationWithoutBuilderException e) {

            // Dev time Exception. Should be caught by Developer
            throw e;
        } catch (StateMachineException e) {

            if (!e.wasWrittenToErrorHistory()) {
                printError(new ExceptionHistoryElement(mState, eventBuilder.getTemporaryEventWithTypeForException(), e));
            }
        } catch (Exception e) {

            printError(new ExceptionHistoryElement(mState, eventBuilder.getTemporaryEventWithTypeForException(), e));
        }
        getLogger().d(TAG, "race condition line C - The index is " + index);
    }
}

Logs:

race condition line A - The index is 1
race condition line B - The index is 1
race condition line A - The index is 2
race condition line B - The index is 2
race condition line C - The index is 2
race condition line A - The index is 3
race condition line B - The index is 3
race condition line C - The index is 3
race condition line C - The index is 3
race condition line A - The index is 4
race condition line B - The index is 4
race condition line C - The index is 4
race condition line A - The index is 5
race condition line B - The index is 5
race condition line C - The index is 5

As you can see I'm increasing the data member index every time I enter the synchronized block. It should print 3 log lines with each index, but as you can see in the logs, index 1 is printed twice and index 3 is printed 4 times.

Thanks

UPDATE : Turns out it was happening because the same thread was entering this method multiple times. synchronized block only works between different threads. how this is happening in a synchronous code is the new mystery.

Answer 1

It looks like your threads use different instances of your StateMachine. When you synchronize on this you use that particular instance of you class as a monitor. That means that tread A will be blocked waiting thread B only if both of them operate the same instance of StateMachine.