import javax.swing.SwingUtilities;/** * This is the 3rd version of SwingWorker (also known as * SwingWorker 3), an abstract class that you subclass to * perform GUI-related work in a dedicated thread.  For * instructions on and examples of using this class, see: *  * http://java.sun.com/docs/books/tutorial/uiswing/misc/threads.html * * Note that the API changed slightly in the 3rd version: * You must now invoke start() on the SwingWorker after * creating it. */public abstract class SwingWorker {    private Object value;  // see getValue(), setValue()    /**      * Class to maintain reference to current worker thread     * under separate synchronization control.     */    private static class ThreadVar {        private Thread thread;        ThreadVar(Thread t) { thread = t; }        synchronized Thread get() { return thread; }        synchronized void clear() { thread = null; }    }    private ThreadVar threadVar;    /**      * Get the value produced by the worker thread, or null if it      * hasn't been constructed yet.     */    protected synchronized Object getValue() {         return value;     }    /**      * Set the value produced by worker thread      */    private synchronized void setValue(Object x) {         value = x;     }    /**      * Compute the value to be returned by the <code>get</code> method.      */    public abstract Object construct();    /**     * Called on the event dispatching thread (not on the worker thread)     * after the <code>construct</code> method has returned.     */    public void finished() {    }    /**     * A new method that interrupts the worker thread.  Call this method     * to force the worker to stop what it's doing.     */    public void interrupt() {        Thread t = threadVar.get();        if (t != null) {            t.interrupt();        }        threadVar.clear();    }    /**     * Return the value created by the <code>construct</code> method.       * Returns null if either the constructing thread or the current     * thread was interrupted before a value was produced.     *      * @return the value created by the <code>construct</code> method     */    public Object get() {        while (true) {              Thread t = threadVar.get();            if (t == null) {                return getValue();            }            try {                t.join();            }            catch (InterruptedException e) {                Thread.currentThread().interrupt(); // propagate                return null;            }        }    }    /**     * Start a thread that will call the <code>construct</code> method     * and then exit.     */    public SwingWorker() {        final Runnable doFinished = new Runnable() {           public void run() { finished(); }        };        Runnable doConstruct = new Runnable() {             public void run() {                try {                    setValue(construct());                }                finally {                    threadVar.clear();                }                SwingUtilities.invokeLater(doFinished);            }        };        Thread t = new Thread(doConstruct);        threadVar = new ThreadVar(t);    }    /**     * Start the worker thread.     */    public void start() {        Thread t = threadVar.get();        if (t != null) {            t.start();        }    }}