fifoqueueimpl.java

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//
// This file is part of the OpenNMS(R) Application.
//
// OpenNMS(R) is Copyright (C) 2002-2003 The OpenNMS Group, Inc.  All rights reserved.
// OpenNMS(R) is a derivative work, containing both original code, included code and modified
// code that was published under the GNU General Public License. Copyrights for modified 
// and included code are below.
//
// OpenNMS(R) is a registered trademark of The OpenNMS Group, Inc.
// 
// Copyright (C) 1999-2001 Oculan Corp.  All rights reserved.
//
// This program is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation; either version 2 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
//
// For more information contact:
//      OpenNMS Licensing       <[e-mail]>
//      http://www.opennms.org/
//      http://www.opennms.com/
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package org.opennms.core.queue;

import java.util.LinkedList;

/**
 * </p>
 * This interface defines a queue that uses <em>F</em> irst <em>I</em>n,
 * <em>F</em> irst <em>O</em> ut semantics when adding and removing objects.
 * Each object that is added to the queue is effectively placed at the end of
 * the list of previous elements. Each call to <code>remove</code> will result
 * in the removal of the next element, or the oldest element in the queue.
 * </p>
 * 
 * @author <a href="mailto:[e-mail]">Brian Weaver </a>
 * @author <a href="http://www.opennms.org/">OpenNMS </a>
 * 
 */
public class FifoQueueImpl implements FifoQueue {
    /**
     * The delegate list where queue elements are stored. The elements are
     * removed from the front of the list and added to the end of the list,
     * always!
     */
    private LinkedList m_delegate;

    /**
     * Constructs a new First In, First Out queue that can be used to exchange
     * data. The implementation is thread safe and can be used to exchange data
     * between to concurrent processes.
     */
    public FifoQueueImpl() {
        m_delegate = new LinkedList();
    }

    /**
     * Inserts a new element into the queue.
     * 
     * @param element
     *            The object to append to the queue.
     * 
     * @exception org.opennms.core.queue.FifoQueueException
     *                Thrown if a queue error occurs.
     * @exception java.lang.InterruptedException
     *                Thrown if the thread is interrupted.
     */
    public void add(Object element) throws FifoQueueException, InterruptedException {
        synchronized (m_delegate) {
            m_delegate.addLast(element);
            m_delegate.notifyAll();
        }
    }

    /**
     * Inserts a new element into the queue. If the queue has reached an
     * implementation limit and the <code>
     * timeout</code> expires, then a false
     * value is returned to the caller.
     * 
     * @param element
     *            The object to append to the queue.
     * @param timeout
     *            The time to wait on the insertion to succeed.
     * 
     * @exception org.opennms.core.queue.FifoQueueException
     *                Thrown if a queue error occurs.
     * @exception java.lang.InterruptedException
     *                Thrown if the thread is interrupted.
     * 
     * @return True if the element was successfully added to the queue before
     *         the timeout expired, false otherwise.
     */
    public boolean add(Object element, long timeout) throws FifoQueueException, InterruptedException {
        synchronized (m_delegate) {
            m_delegate.addLast(element);
            m_delegate.notifyAll();
        }
        return true;
    }

    /**
     * Removes the oldest element from the queue.
     * 
     * @exception org.opennms.core.queue.FifoQueueException
     *                Thrown if a queue error occurs.
     * @exception java.lang.InterruptedException
     *                Thrown if the thread is interrupted.
     * 
     * @return The oldest object in the queue.
     */
    public Object remove() throws FifoQueueException, InterruptedException {
        synchronized (m_delegate) {
            while (m_delegate.isEmpty()) {
                m_delegate.wait();
            }
            return m_delegate.removeFirst();
        }
    }

    /**
     * Removes the next element from the queue if one becomes available before
     * the timeout expires. If the timeout expires before an element is
     * available then a <code>null</code> reference is returned to the caller.
     * 
     * @param timeout
     *            The time to wait on an object to be available.
     * 
     * @exception org.opennms.core.queue.FifoQueueException
     *                Thrown if a queue error occurs.
     * @exception java.lang.InterruptedException
     *                Thrown if the thread is interrupted.
     * 
     * @return The oldest object in the queue, or <code>null</code> if one is
     *         not available.
     */
    public Object remove(long timeout) throws FifoQueueException, InterruptedException {
        Object rval = null;
        synchronized (m_delegate) {
            if (m_delegate.isEmpty()) {
                long start = System.currentTimeMillis();
                long diff = 0;
                do {
                    m_delegate.wait(timeout - diff);
                    diff = System.currentTimeMillis() - start;
                } while (diff < timeout && m_delegate.isEmpty());
            }

            if (!m_delegate.isEmpty())
                rval = m_delegate.removeFirst();
        }

        return rval;
    }

    /**
     * Returns the current number of elements that are in the queue.
     * 
     * @return The number of elements in the queue.
     */
    public int size() {
        synchronized (m_delegate) {
            return m_delegate.size();
        }
    }

    /**
     * Used to test if the current queue has no stored elements.
     * 
     * @return True if the queue is empty.
     */
    public boolean isEmpty() {
        synchronized (m_delegate) {
            return m_delegate.isEmpty();
        }
    }
}