兩個線程進入一個同步塊

Question

我有一個包含在 synchronized(this) 中的塊，並且我在調試模式和日志中都看到有 2 個線程同時進入此部分。

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);
    }
}

日志：

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

如您所見，每次進入同步塊時，我都會增加數據成員索引。 它應該為每個索引打印 3 行日志，但正如您在日志中看到的那樣，索引 1 打印了兩次，索引 3 打印了 4 次。

謝謝

更新：原來它正在發生，因為同一個線程多次進入這個方法。 同步塊僅適用於不同線程。 這如何在同步代碼中發生是新的謎團。

Answer 1

看起來您的線程使用 StateMachine 的不同實例。 當您this同步時，您將使用該類的特定實例作為監視器。 這意味着只有當它們都操作相同的 StateMachine 實例時，trace A 才會被阻塞等待線程 B。