swingworker.java

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package com.bbn.openmap.util;

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 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();
        }
    }

    /**
     * For compatibility with old versions of SwingWorker, calls
     * start().
     */
    public void execute() {
        start();
    }
}