如果之前的代码没有解锁，为什么我的代码进入了锁定区域？

Question

我的项目有一个按钮。 按下按钮时，它会执行 function, SimulateDataUpload 。 这个 SimulateDataUpload 被lock.lock()锁定。 有一个 for 循环，封装在一个 runnable 中，用于更新进度条。 可运行对象的 onRun 也使用lock.lock()锁定。 当我快速单击该按钮两次时，很明显 for 循环尚未完成迭代，因此不应释放锁。 为什么代码再次进入SimulateDataUpload ？

MainActivity 中的代码：

// 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.");
    }
}

这是仅单击一次按钮时的正常 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.

这是单击两次按钮时的 output。 我不明白为什么我要第二次输入“Inside SimulateDataUpload ...”，因为锁定的 object 的可运行对象尚未释放（尚未完成其 for 循环迭代）：

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

由于 logs synchronised关键字的作用就像一个魅力，您已经创建了两个Thread并启动它们，从未中断，没有保留参考。 第二个必须等待第一个的结束执行，因为方法/代码与相同的final object 同步。 所以两个Thread都会启动，但第二个实际上会在第一个结束后执行

Answer 2

接受了@Shark 的建议并使用了reentrant lock 。 使用锁的isLocked()功能来加强解决方案。 使用synchronized (lockObject)的原始代码。 单独的线程过早地释放了锁，因此多次单击按钮会产生奇怪的结果。 如果有人能解释为什么它会这样，我肯定很想知道。

无论如何，这是可行的解决方案：

//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.");
    }
}