如果之前的代碼沒有解鎖，為什么我的代碼進入了鎖定區域？

Question

我的項目有一個按鈕。 按下按鈕時，它會執行 function, SimulateDataUpload 。 這個 SimulateDataUpload 被lock.lock()鎖定。 有一個 for 循環，封裝在一個 runnable 中，用於更新進度條。 可運行對象的 onRun 也使用lock.lock()鎖定。 當我快速單擊該按鈕兩次時，很明顯 for 循環尚未完成迭代，因此不應釋放鎖。 為什么代碼再次進入SimulateDataUpload ？

MainActivity 中的代碼：

// defined before onCreate();
private Lock lock = new ReentrantLock();

// defined in onCreate();
simulateDataUploadButton.setOnClickListener(new View.OnClickListener()
{
    @Override
    public void onClick(View v)
    {
       SimulateDataUpload();
    }
});


public void SimulateDataUpload()
{
    lock.lock();
    try 
    {
        Log.i("myTag", "Thread name: " + Thread.currentThread().getName()
           + ". Inside SimulateDataUpload. lock acquired.");
        Handler uiOperationsHandler = new Handler();
        setUiElements(true);

        Runnable simulateDataUploadRunnable = new Runnable() 
        {
            @Override
            public void run() 
            { 
                lock.lock();
                    try 
                    {
                         Log.i("myTag", "Thread name: " 
                            + Thread.currentThread().getName() 
                            + ". Inside runnable threadLockObject. lock 
                            acquired.");
                        for (int i = 0; i < maxProgressBar + 1; i++) 
                        {
                            try 
                            {
                                Thread.sleep(500);
                            } 
                            catch (InterruptedException e) 
                            {
                                e.printStackTrace();
                            }

                            int finalI = i;
                            uiOperationsHandler.post(new Runnable() 
                            {
                                @Override
                                public void run() 
                                {
                                    Log.i("myTag", "Thread name: " 
                                        + Thread.currentThread().getName() 
                                        + ". i: " + finalI 
                                        + ". Inside 
                                        uiOperationsHandler.post.");
                                    taskCompletedEventListener.
                                             getCallBackFunction().
                                             onTaskCompletedEvent(finalI == 
                                             maxProgressBar);
                                }
                            });
                        }
                    }
                    finally
                    {
                        lock.unlock();
                        Log.i("myTag", "Thread name: " 
                            +Thread.currentThread().getName() 
                            +". lock released.");
                    }
            }
        };

        Thread simulateDataUploadThread = new 
            Thread(simulateDataUploadRunnable);
        simulateDataUploadThread.start();
    }
    finally
    {
        lock.unlock();
        Log.i("myTag", "Thread name: " +Thread.currentThread().getName() 
           +". lock released.");
    }
}

這是僅單擊一次按鈕時的正常 output ：

I/myTag: Thread name: main. Inside SimulateDataUpload. lock acquired.
I/myTag: Thread name: Thread-8. Inside runnable threadLockObject. lock acquired.
I/myTag: Thread name: main. lock released.
I/myTag: Thread name: main. i: 0. Inside uiOperationsHandler.post.
I/myTag: Thread name: main. i: 1. Inside uiOperationsHandler.post.
I/myTag: Thread name: main. i: 2. Inside uiOperationsHandler.post.
I/myTag: Thread name: main. i: 3. Inside uiOperationsHandler.post.
I/myTag: Thread name: Thread-8. lock released.

這是單擊兩次按鈕時的 output。 我不明白為什么我要第二次輸入“Inside SimulateDataUpload ...”，因為鎖定的 object 的可運行對象尚未釋放（尚未完成其 for 循環迭代）：

I/myTag: Thread name: main. Inside SimulateDataUpload. lock acquired.
I/myTag: Thread name: main. lock released.
I/myTag: Thread name: Thread-11. Inside runnable threadLockObject. lock acquired.
I/myTag: Thread name: main. i: 0. Inside uiOperationsHandler.post.
I/myTag: Thread name: Thread-11. lock released.
I/myTag: Thread name: main. Inside SimulateDataUpload. lock acquired.
I/myTag: Thread name: main. lock released.
I/myTag: Thread name: Thread-12. Inside runnable threadLockObject. lock acquired.
I/myTag: Thread name: main. i: 1. Inside uiOperationsHandler.post.
I/myTag: Thread name: main. i: 2. Inside uiOperationsHandler.post.
I/myTag: Thread name: main. i: 3. Inside uiOperationsHandler.post.
I/myTag: Thread name: main. i: 0. Inside uiOperationsHandler.post.
I/myTag: Thread name: main. i: 1. Inside uiOperationsHandler.post.
I/myTag: Thread name: main. i: 2. Inside uiOperationsHandler.post.
I/myTag: Thread name: Thread-12. lock released.
I/myTag: Thread name: main. i: 3. Inside uiOperationsHandler.post.

Answer 1

由於 logs synchronised關鍵字的作用就像一個魅力，您已經創建了兩個Thread並啟動它們，從未中斷，沒有保留參考。 第二個必須等待第一個的結束執行，因為方法/代碼與相同的final object 同步。 所以兩個Thread都會啟動，但第二個實際上會在第一個結束后執行

Answer 2

接受了@Shark 的建議並使用了reentrant lock 。 使用鎖的isLocked()功能來加強解決方案。 使用synchronized (lockObject)的原始代碼。 單獨的線程過早地釋放了鎖，因此多次單擊按鈕會產生奇怪的結果。 如果有人能解釋為什么它會這樣，我肯定很想知道。

無論如何，這是可行的解決方案：

//synchronize (myObject) doesn't work in this example... so using a ReentrantLock.
private ReentrantLock lock = new ReentrantLock();
private final int maxProgressBar = 6;

@Override
protected void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);

    // ... Other stuff defined here

    simulateDataUploadButton.setOnClickListener(new View.OnClickListener()
    {
        @Override
        public void onClick(View v)
        {
            // Problem: The user can click multiple times and our runnable will be concurrently run in multiple different threads
            // Used a lock to prevent this.

            // 1. SimulateDataUpload. Lock Acquired by Main Thread. A runnable package is created and a new thread with that package is started. Lock released by Main Thread.
            // 2. New thread runs and tries to acquire the lock, but is blocked until SimulateDataUpload releases the lock.
            // 3. The new Thread acquires the lock.
            // 4. The new Thread finishes the for loop and releases the lock.
            SimulateDataUpload();
        }
    });


}

public void SimulateDataUpload()
{
    if(lock.isLocked())
    {
        // If we are in the middle of an operation with the lock, return.
        return;
    }
    else
    {
        lock.lock();
    }
    try
    {
        Log.i("myTag", "Thread name: " + Thread.currentThread().getName() + ". Inside SimulateDataUpload. lock acquired.");
        Handler uiOperationsHandler = new Handler();
        setUiElements(true);

        Runnable simulateDataUploadRunnable = new Runnable()
        {
            @Override
            public void run()
            {
                while (lock.isLocked())
                {
                    // If we call this runnable with simulateDataUploadThread.start(), we need it to complete. So just wait for whatever has a lock to release the lock.
                }

                lock.lock();
                try
                {
                    Log.i("myTag", "Thread name: " + Thread.currentThread().getName() + ". Inside runnable threadLockObject. lock acquired.");
                    for (int i = 0; i < maxProgressBar + 1; i++)
                    {
                        try
                        {
                            Thread.sleep(500);
                        }
                        catch (InterruptedException e)
                        {
                            Log.i("myTag", "Thread name: " + "exception: " +e);
                        }

                        // Fire Event. Use a handler because we are going to be updating UI elements.
                        int finalI = i;
                        uiOperationsHandler.post(new Runnable()
                        {
                            @Override
                            public void run() {
                                Log.i("myTag", "Thread name: " + Thread.currentThread().getName() + ". i: " + finalI + ". Inside uiOperationsHandler.post.");
                                taskCompletedEventListener.
                                        getCallBackFunction().
                                        onTaskCompletedEvent(finalI == maxProgressBar);
                            }
                        });
                    }
                }
                finally
                {
                    lock.unlock();
                    Log.i("myTag", "Thread name: " +Thread.currentThread().getName() +". lock released.");
                }
            }
        };

        Thread simulateDataUploadThread = new Thread(simulateDataUploadRunnable);
        simulateDataUploadThread.start();
    }
    finally
    {
        lock.unlock();
        Log.i("myTag", "Thread name: " +Thread.currentThread().getName() +". lock released.");
    }
}