新线程和调用线程将被阻塞Java信号量

Question

I have some trouble with my java swing program. I try to stop my main Frame thread when the ExportWithoutEntryPointFrm Frame appears in an own thread. I implemented that with java.util.concurrent.Semaphore . The appearing Frame shows only an empty Frame, the buttons, lables and so on won´t be shown and both threads are blocked. I think there is a deadlock but I don´t find it.

My code for the new warning Frame, which will be called from the main Frame:

public class ExportWithoutEntryPointFrm extends javax.swing.JFrame implements Runnable
{

    private Semaphore sema;
    private boolean decision = false;


    public ExportWithoutEntryPointFrm(Semaphore semaphore)
    {
        initComponents();
        this.setLocationRelativeTo(null);
        this.sema = semaphore;

    }

    @Override
    public void run()
    {
        this.setVisible(true);

        try
        {
            sema.acquire();

        }
        catch (InterruptedException e)
        {
            this.decision = false;
            this.sema.release();

            this.setVisible(false);

        }
    }
}

And the calling code from the main Frame:

Semaphore waitForDecisionSema = new Semaphore(1, true);

ExportWithoutEntryPointFrm warningFrm = new ExportWithoutEntryPointFrm(waitForDecisionSema);

warningFrm.run();
waitForDecisionSema.acquire();

Answer 1

First of all, calling a run() method of a Runnable doesn't start a new thread.

Second, even if it did, Swing components like JFrame MUST be used from the event dispatch thread only.

Third: since everything is done from a single thread, the EDT, as soon as this line is executed:

waitForDecisionSema.acquire();

the EDT is blocked waiting for some other thread to release the semaphore, and that will never happen, so the EDT is blocked forever, making your GUI unresponsive.

You seriously need to reconsider your design. But I don't know what you're trying to achieve, so it's hard to advise. Given the name of your semaphore, I think that what you're looking for is a modal JDialog, that would prevent the user to use the parent frame of the dialog until the dialog is closed.

Answer 2

I try to stop my main Frame thread when the ExportWithoutEntryPointFrm Frame appears in an own thread

Well, that's a massive contradiction in terms, Swing is a single threaded framework, you can operate components/frames/windows in separate threads, it won't work and you'll end up within no end of issues, dead locks been the most obviously.

Start by having a look at Concurrency in Swing for more details.

Now, there a number of mechanisms you can use to off load long running or blocking code to a separate thread and still interact with Swing, a Swing Timer for regular scheduled callbacks, SwingWorker for long running or potentially blocking calls, but which supports callbacks to the EDT, making it easy to use and even SwingUtilities.invokeLater for those times you have no other choice.

Have a look at How to use Swing Timers and Worker Threads and SwingWorker for more details

Based on you description though, I would suggest that what you really want, is a modal dialog, which will block the current frame/code execution at the point the dialog is made visible, but which will allow the UI to continue responding to the user.

See How to Make Dialogs for more details