重新加载我的applet时抛出IllegalMonitorStateException？

Question

我对线程的同步知之甚少......我正在Java applet中捕获和解码条形码。它工作正常一次，但每当我尝试在浏览器中重新加载/刷新我的Java小程序时，我想之前的事情都没有发布，因此我得到以下例外：

Exception in thread atomic-processor-1
Exception in thread webcam-updater-thread-1
Exception in thread webcam-panel-scheduled-executor-1
java.lang.IllegalMonitorStateException
    at java.util.concurrent.locks.ReentrantLock$Sync.tryRelease(Unknown Source)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.release(Unknown Source)
    at java.util.concurrent.locks.ReentrantLock.unlock(Unknown Source)
    at java.util.concurrent.LinkedBlockingQueue.take(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor.getTask(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

java.lang.IllegalMonitorStateException
    at java.util.concurrent.locks.ReentrantLock$Sync.tryRelease(Unknown Source)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.release(Unknown Source)
    at java.util.concurrent.locks.ReentrantLock.unlock(Unknown Source)
    at java.util.concurrent.LinkedBlockingQueue.take(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor.getTask(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

java.lang.IllegalMonitorStateException
    at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.signal(Unknown Source)
    at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(Unknown Source)
    at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor.getTask(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

Exception in thread "thread applet-WebcamQRCodeExample.class-1" java.lang.NullPointerException
    at java.awt.EventQueue.isDispatchThread(Unknown Source)
    at com.sun.deploy.uitoolkit.impl.awt.AWTAppletAdapter.runOnEDT(Unknown Source)
    at com.sun.deploy.uitoolkit.impl.awt.AWTAppletAdapter.doClearAppletArea(Unknown Source)
    at sun.plugin2.applet.Plugin2Manager$AppletExecutionRunnable.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

以下是我的applet代码：

import java.awt.FlowLayout;
import java.awt.image.BufferedImage;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadFactory;

import javax.swing.JApplet;
import javax.swing.JFrame;
import javax.swing.JTextArea;

import com.github.sarxos.webcam.Webcam;
import com.github.sarxos.webcam.WebcamPanel;
import com.github.sarxos.webcam.ds.buildin.WebcamDefaultDevice;
import com.google.zxing.BinaryBitmap;
import com.google.zxing.LuminanceSource;
import com.google.zxing.MultiFormatReader;
import com.google.zxing.NotFoundException;
import com.google.zxing.Result;
import com.google.zxing.client.j2se.BufferedImageLuminanceSource;
import com.google.zxing.common.HybridBinarizer;

public class WebcamQRCodeExample extends JApplet implements Runnable, ThreadFactory {

    private static final long serialVersionUID = 6441489157408381878L;

    private Executor executor = Executors.newSingleThreadExecutor( this);
    private volatile boolean running = true;
    private Webcam webcam = null;
    private WebcamPanel panel = null;
    private JTextArea textarea = null;
    private Thread t = null;

    public WebcamQRCodeExample() {
        super();
        setLayout( new FlowLayout());
        webcam = Webcam.getWebcams().get( 0);
        panel = new WebcamPanel( webcam);
        add( panel);
        setVisible( true);
        executor.execute( this);
    }

    @Override
    public void run() {
        do {
            try {
                Thread.sleep( 20);
            } catch ( InterruptedException e) {
                e.printStackTrace();
            }
            Result result = null;
            BufferedImage image = null;
            if ( webcam.isOpen()) {
                if ( ( image = webcam.getImage()) == null) {
                    continue;
                }
                LuminanceSource source = new BufferedImageLuminanceSource( image);
                BinaryBitmap bitmap = new BinaryBitmap( new HybridBinarizer( source));
                try {
                    result = new MultiFormatReader().decode( bitmap);
                } catch ( NotFoundException e) {
                    // fall thru, it means there is no QR code in image
                }
            }
            if ( result != null) {
                textarea.setText( result.getText());
            }
        } while ( running);
    }

    @Override
    public Thread newThread( Runnable r) {
        t = new Thread( r, "example-runner");
        t.setDaemon( true);
        return t;
    }

    public static void main( String[] args) {
        new WebcamQRCodeExample();
    }

    public void destroy() {
        try {
            running = false;
            if ( webcam != null) {
                while ( webcam.isOpen()) {
                    webcam.close();
                    webcam.resetDriver();
                }
                webcam = null;
            }
        } catch ( Exception e) {
            e.printStackTrace();
        }
    }

    public void stop() {
        try {
            running = false;
            while ( webcam.isOpen()) {
                webcam.close();
                webcam.resetDriver();
            }
            if ( webcam != null)
                webcam = null;
            System.out.println( "mmmmmm");
        } catch ( Exception e) {
            e.printStackTrace();
        }
    }
}

当我重新加载applet时，如何避免上述异常？如何在重新加载applet之前等待几秒钟，以防止IllegalMonitorStateException？谢谢。