SwingWorker.java

/*
 * Copyright 2004-2010 Information & Software Engineering Group (188/1)
 *                     Institute of Software Technology and Interactive Systems
 *                     Vienna University of Technology, Austria
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.ifs.tuwien.ac.at/dm/somtoolbox/license.html
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package at.tuwien.ifs.somtoolbox.util;

import javax.swing.SwingUtilities;

/**
 * @author Michael Dittenbach
 * @version $Id: SwingWorker.java 3883 2010-11-02 17:13:23Z frank $
 */
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() {
            @Override
            public void run() {
                finished();
            }
        };

        Runnable doConstruct = new Runnable() {
            @Override
            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();
        }
    }
}