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This artifact provides a single jar that contains all classes required to use remote EJB and JMS, including all dependencies. It is intended for use by those not using maven, maven users should just import the EJB and JMS BOM's instead (shaded JAR's cause lots of problems with maven, as it is very easy to inadvertently end up with different versions on classes on the class path).

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package org.jgroups.util;

import java.util.Collection;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.locks.ReentrantLock;

/**
 * A queue with many producers and consumers. However, only one consumer gets to remove and process elements
 * at any given time. This is done through the use of locks.
 * @author Bela Ban
 * @since  3.5
 */
public class ProcessingQueue {
    protected final java.util.Queue  queue=new ConcurrentLinkedQueue<>();
    protected final ReentrantLock       producer_lock=new ReentrantLock(), consumer_lock=new ReentrantLock();
    protected int                       count=0;
    protected Handler                handler;


    public java.util.Queue getQueue()   {return queue;}
    public int                size()       {return queue.size();}

    public ProcessingQueue setHandler(Handler handler) {this.handler=handler; return this;}

    public void add(T element) {
        producer_lock.lock();
        try {
            queue.add(element);
            count++;
        }
        finally {
            producer_lock.unlock();
        }

        process();
    }

    public boolean retainAll(Collection elements) {
        return queue.retainAll(elements);
    }

    public String toString() {
        return queue.toString();
    }


    protected void process() {
        if(consumer_lock.tryLock()) {
            try {
                while(true) {
                    T element=queue.poll();
                    if(element != null) {
                        if(handler != null) {
                            try {
                                handler.handle(element);
                            }
                            catch(Throwable t) {
                                t.printStackTrace(System.err);
                            }
                        }
                    }

                    producer_lock.lock();
                    try {
                        if(count == 0 || count-1 == 0) {
                            count=0;
                            consumer_lock.unlock();
                            return;
                        }
                        count--;
                    }
                    finally {
                        producer_lock.unlock();
                    }
                }
            }
            finally {
                if(consumer_lock.isHeldByCurrentThread())
                    consumer_lock.unlock();
            }
        }
    }


    public interface Handler {
        void handle(T element);
    }
}