while循环和队列的奇怪java行为

Question

I have strange behavior of while(true) loop. Here is the code:

As a member of class I have:

static Queue<Object> response = new LinkedList<Object>();

... and a function:

private void read() {

    while (true)
    {
        System.out.println("foo");
        if(!(response.isEmpty()))
        {

            if((Boolean)response.peek() == true)
            {
                view.dispose();
                LogInControler controler= new LogInControler();
                disableMasterLogin();
                response.poll();
                return;
            }
            else if((Boolean)response.poll() == false)
            {
                JOptionPane.showMessageDialog(view.getRootPane(), 
                        "Wrong username or password.");
                view.tfUsername.requestFocus();
                return;
            }
        }
    }
}

When object is received from server(via Socket), InputController class pass that object to appropriate controller, in this case MasterLogInController and put it in Queue response. I am waiting for that response in while(true) loop, but the problem is if I remove "System.out.printline("foo");" loop will be entered only once!? With this syso line I "force" while loop to do loops until response is received. What's wrong here?

Answer 1

I assume you have several threads running.

System.out.println creates a memory barrier which probably helps your code see some variable which is otherwise not visible (because of lack of synchronization).

In particular, your queue is not thread safe and does seem to be safely published. So it is very conceivable that:

• your while loop might see response as null ==> NullPointerException
• reponse.isEmpty() might return false but response.peek() might return null, which you then cast to a Boolean and unbox in your condition if((Boolean)xxx == true) ==> NullPointerException
• etc.

Apart from the sound advice given in the comments to help understand the cause, you should make the code thread safe. For example, you could use a thread safe BlockingQueue . But that will probably not be enough (because of the way your various if / if / else if statements are laid out and the fact that the queue might be changed by another thread between each of those statements).

Answer 2

I suspect what is happening is that your loop is being optimised out of existence by the JIT compiler. If response.isEmpty() is true the first time it is called in your loop, and noticing that response is not inside a synchronized block or method, or marked volatile , it is likely the JIT compiler will decide that it's not going to change and just remove what appears to be an empty busy loop from running code.

Adding in the println() statement at least gives the loop a purpose, in the eyes of the JIT compiler, so it will leave it running in that case.

To fix this, in addition to the salient advice given by assylias, you can put all references to response inside a synchronized block like so:

public void read() {
    Boolean result = null;
    synchronized (response) {
        while (true) {
            result = (Boolean) response.poll();
            if (result != null) break;
            try {
                response.wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
                // You could put return; here
            }
        }
    }
    // result should always be non null here
    if (result) {
         view.dispose();
         LogInControler controler = new LogInControler();
         disableMasterLogin();
    } else {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                JOptionPane.showMessageDialog(view.getRootPane(), "Wrong username or password");
                view.tfUsername.requestFocus();
            }
        });
    }
}

Where your other thread is adding the response to the queue, make sure that is also in an synchronized block and calls notifyAll() :

public void addResult(Object result) {
    synchronized (response) {
        response.add(result);
        response.notifyAll();
    }       
}