uimaj-2.2.0-incubating/uimaj-tools/src/main/java/org/apache/uima/tools/util/gui/SwingWorker.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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.apache.org/licenses/LICENSE-2.0
 * 
 * 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 org.apache.uima.tools.util.gui;

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();
    }
  }
}