root/trunk/LogicMail/src/org/logicprobe/LogicMail/util/ThreadQueue.java

/*-
 * Copyright (c) 2008, Derek Konigsberg
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the project nor the names of its
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
 * OF THE POSSIBILITY OF SUCH DAMAGE.
 */
package org.logicprobe.LogicMail.util;

// TODO: Write rigorous tests for this class

/**
 * Provides a work item queue for <tt>Runnable</tt> objects.
 * This is similar to a thread pool, except all work items
 * run in sequence.  Also, the thread is not kept alive
 * when there are no pending work items.
 */
public class ThreadQueue {
        private Queue runnableQueue;
        private ThreadQueueThread threadQueueThread;
        private boolean isShutdown;
        
        /**
         * Instantiates a new thread queue.
         */
        public ThreadQueue() {
                runnableQueue = new Queue();
        }
        
        /**
         * Flushes any pending work items, and optionally
         * waits for the thread to join.
         * 
         * @param wait True to wait for the thread to join.
         */
        public void shutdown(boolean wait) {
                isShutdown = true;
                synchronized(runnableQueue) {
                        runnableQueue.clear();
                }
                if(wait && threadQueueThread != null) {
                        try {
                                threadQueueThread.join();
                        } catch (InterruptedException e) { }
                        threadQueueThread = null;
                }
        }
        
        /**
         * Puts the provided <tt>Runnable</tt> object on the
         * work item queue.  Starts the worker thread if necessary.
         * 
         * @param runnable The <tt>Runnable</tt> object.
         * @throws IllegalStateException Thrown if {@link #shutdown(boolean)} has been called.
         */
        public void invokeLater(Runnable runnable) {
                if(isShutdown) {
                        throw new IllegalStateException("Thread queue has been shutdown");
                }
                boolean queued = false;
                synchronized(runnableQueue) {
                        if(threadQueueThread != null && threadQueueThread.isAlive()) {
                                runnableQueue.add(runnable);
                                queued = true;
                        }
                }
                if(!queued) {
                        if(threadQueueThread != null) {
                                try {
                                        threadQueueThread.join();
                                } catch (InterruptedException e) { }
                                threadQueueThread = null;
                        }
                        threadQueueThread = new ThreadQueueThread();
                        runnableQueue.add(runnable);
                        threadQueueThread.start();
                }
        }
        
        /**
         * Actual thread implementation used for the work item queue.
         */
        private class ThreadQueueThread extends Thread {
                /**
                 * Instantiates a new thread queue thread.
                 */
                public ThreadQueueThread() {
                }

                /* (non-Javadoc)
                 * @see java.lang.Thread#run()
                 */
                public void run() {
                        while(true) {
                                Runnable runnable;
                                synchronized(runnableQueue) {
                                        if(runnableQueue.element() != null) {
                                                runnable = (Runnable)runnableQueue.remove();
                                        }
                                        else {
                                                return;
                                        }
                                }
                                Thread.yield();
                                try {
                                        runnable.run();
                                } catch (RuntimeException exp) {
                                        System.err.println(exp.toString());
                                }
                        }
                }
        }
}