KeyedRoundRobinQueue.java

package backtype.storm.utils;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Queue;
import java.util.concurrent.Semaphore;

public class KeyedRoundRobinQueue<V> {
    private final Object _lock = new Object();
    private Semaphore _size = new Semaphore(0);
    private Map<Object, Queue<V>> _queues = new HashMap<Object, Queue<V>>();
    private List<Object> _keyOrder = new ArrayList<Object>();
    private int _currIndex = 0;

    public void add(Object key, V val) {
        synchronized(_lock) {
            Queue<V> queue = _queues.get(key);
            if(queue==null) {
                queue = new LinkedList<V>();
                _queues.put(key, queue);
                _keyOrder.add(key);
            }
            queue.add(val);
        }
        _size.release();
    }

    public V take() throws InterruptedException {
        _size.acquire();
        synchronized(_lock) {
            Object key = _keyOrder.get(_currIndex);
            Queue<V> queue = _queues.get(key);
            V ret = queue.remove();
            if(queue.isEmpty()) {
                _keyOrder.remove(_currIndex);
                _queues.remove(key);
                if(_keyOrder.size()==0) {
                    _currIndex = 0;
                } else {
                    _currIndex = _currIndex % _keyOrder.size();
                }
            } else {
                _currIndex = (_currIndex + 1) % _keyOrder.size();
            }
            return ret;
        }
    }
}