HSQLDB example source code file (HsqlTaskQueue.java)

This example HSQLDB source code file (HsqlTaskQueue.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Java - HSQLDB tags/keywords

hsqldeque, hsqltaskqueue, runnable, runnable, runtimeexception, shutdowntask, shutdowntask, taskrunner, taskrunner, thread, thread

The HSQLDB HsqlTaskQueue.java source code

/* Copyright (c) 2001-2008, The HSQL Development Group
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 * Redistributions of source code must retain the above copyright notice, this
 * list of conditions and the following disclaimer.
 *
 * 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.
 *
 * Neither the name of the HSQL Development Group 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 HSQL DEVELOPMENT GROUP, HSQLDB.ORG,
 * 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.hsqldb.lib;

/**
 * Provides very simple queued execution of Runnable objects in a background
 * thread. The underlying queue is an HsqlDeque instance, an array-based
 * circular queue implementation with automatic capacity expansion.
 *
 * @author boucherb@users
 * @version 1.7.2
 * @since 1.7.2
 */
public class HsqlTaskQueue {

    /** The thread used to process commands */
    protected Thread taskRunnerThread;

    /** Special queue element to signal termination */
    protected static final Runnable SHUTDOWNTASK = new Runnable() {
        public void run() {}
    };

    /**
     * true if thread should shut down after processing current task.
     *
     * Once set true, stays true forever
     */
    protected volatile boolean isShutdown;

    public synchronized Thread getTaskRunnerThread() {
        return taskRunnerThread;
    }

    protected synchronized void clearThread() {
        taskRunnerThread = null;
    }

    protected final HsqlDeque queue = new HsqlDeque();

    protected class TaskRunner implements Runnable {

        public void run() {

            Runnable task;

            try {
                while (!isShutdown) {
                    synchronized (queue) {
                        task = (Runnable) queue.getFirst();
                    }

                    if (task == SHUTDOWNTASK) {
                        isShutdown = true;

                        synchronized (queue) {
                            queue.clear();
                        }

                        break;
                    } else if (task != null) {
                        task.run();

                        task = null;
                    } else {
                        break;
                    }
                }
            } finally {
                clearThread();
            }
        }
    }

    protected final TaskRunner taskRunner = new TaskRunner();

    public HsqlTaskQueue() {}

    public boolean isShutdown() {
        return isShutdown;
    }

    public synchronized void restart() {

        if (taskRunnerThread == null &&!isShutdown) {
            taskRunnerThread = new Thread(taskRunner);

            taskRunnerThread.start();
        }
    }

    public void execute(Runnable command) throws RuntimeException {

        if (!isShutdown) {
            synchronized (queue) {
                queue.addLast(command);
            }

            restart();
        }
    }

    public synchronized void shutdownAfterQueued() {

        if (!isShutdown) {
            synchronized (queue) {
                queue.addLast(SHUTDOWNTASK);
            }
        }
    }

    public synchronized void shutdownAfterCurrent() {

        isShutdown = true;

        synchronized (queue) {
            queue.clear();
            queue.addLast(SHUTDOWNTASK);
        }
    }

    public synchronized void shutdownImmediately() {

        isShutdown = true;

        if (taskRunnerThread != null) {
            taskRunnerThread.interrupt();
        }

        synchronized (queue) {
            queue.clear();
            queue.addLast(SHUTDOWNTASK);
        }
    }
}