Synchronization between two threads?

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7.6

Synchronization between two threads?

Asked by DJ_AM_Juicebox in Java Programming Language

Hi,

I'm trying to write a callback for use with a thread. A pseudocode example is below. An object from the main thread gives the background thread an instance of a callback object. The screen object then exits without deregistering itself. I'm not sure how the background thread can still access the object even though the original owner has been terminated. How does this not cause an exception?

Thanks

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             public class Screen
{
    private String mTest = "";
 
    public void onDisplay()
    { 
        // Here we register a callback.
        GlobalThread.instance().setCallback(
            new ICallback() {
                public void doit() {
                    println("hello!");   
                    mTest = "test complete!";
                }
            }
        );
        GlobalThread.instance().start();
    }
 
    public void onClose()
    {
        // Here we forget to deregister ourselves.
        //GlobalThread.instance().setCallback(null);
    }
}
 
public class MyThread() 
{
    private static MyThread mInstance;
    Object mGuard = new Object();
 
    public void setCallback(ICallback callback) {
        synchronized (mGuard) {
             mCallback = callback;
        }
    }
 
    public void run() {
 
        // Give the screen object a chance to exit.
        Thread.sleep(15000);
 
        synchronized (mGuard) {
            if (mCallback != null) {
                // How does this not crash with the given scenario?
                mCallback.doit();
            }
        }       
    }
}
 
public interface ICallback
{ 
    public void doit();
}

           
         

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Synchronization between two threads?

Asked by DJ_AM_Juicebox in Java Programming Language

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