qpid/java/client/src/main/java/org/apache/qpid/client/util/FlowControllingBlockingQueue.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.qpid.client.util;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.Iterator;
import java.util.Queue;
import java.util.concurrent.ConcurrentLinkedQueue;

/**
 * A blocking queue that emits events above a user specified threshold allowing the caller to take action (e.g. flow
 * control) to try to prevent the queue growing (much) further. The underlying queue itself is not bounded therefore the
 * caller is not obliged to react to the events. <p/> This implementation is <b>only</b> safe where we have a single
 * thread adding items and a single (different) thread removing items.
 *
 * @todo Make this implement java.util.Queue and hide the implementation. Then different queue types can be substituted.
 */
public class FlowControllingBlockingQueue
{
	private static final Logger _logger = LoggerFactory.getLogger(FlowControllingBlockingQueue.class);
	
    /** This queue is bounded and is used to store messages before being dispatched to the consumer */
    private final Queue _queue = new ConcurrentLinkedQueue();

    private final int _flowControlHighThreshold;
    private final int _flowControlLowThreshold;

    private final ThresholdListener _listener;

    /** We require a separate count so we can track whether we have reached the threshold */
    private int _count;
    
    private boolean disableFlowControl; 

    public boolean isEmpty()
    {
        return _queue.isEmpty();
    }

    public interface ThresholdListener
    {
        void aboveThreshold(int currentValue);

        void underThreshold(int currentValue);
    }

    public FlowControllingBlockingQueue(int threshold, ThresholdListener listener)
    {
        this(threshold, threshold, listener);
    }

    public FlowControllingBlockingQueue(int highThreshold, int lowThreshold, ThresholdListener listener)
    {
        _flowControlHighThreshold = highThreshold;
        _flowControlLowThreshold = lowThreshold;
        _listener = listener;
        if (highThreshold == 0)
        {
        	disableFlowControl = true;
        }
    }

    public Object take() throws InterruptedException
    {
        Object o = _queue.poll();
        if(o == null)
        {
            synchronized(this)
            {
                while((o = _queue.poll())==null)
                {
                    wait();
                }
            }
        }
        if (!disableFlowControl && _listener != null)
        {
            synchronized (_listener)
            {
                if (_count-- == _flowControlLowThreshold)
                {
                    _listener.underThreshold(_count);
                }
            }
            
        }

        return o;
    }

    public void add(Object o)
    {
        synchronized(this)
        {
            _queue.add(o);

            notifyAll();
        }
        if (!disableFlowControl && _listener != null)
        {
            synchronized (_listener)
            {
                if (++_count == _flowControlHighThreshold)
                {
                    _listener.aboveThreshold(_count);
                }
            }
        }
    }

    public Iterator iterator()
    {
        return _queue.iterator();
    }
}