SwapQueue.java

/*
 * Copyright 2012 LinkedIn Corp.
 * 
 * Licensed 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 azkaban.utils;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;

/**
 * Queue that swaps its lists. Allows for non-blocking writes when reading.
 * Swap should be called before every read.
 */
public class SwapQueue<T> implements Iterable<T> {
	ArrayList<T> primaryQueue;
	ArrayList<T> secondaryQueue;
	
	public SwapQueue() {
		primaryQueue = new ArrayList<T>();
		secondaryQueue = new ArrayList<T>();
	}
	
	/**
	 * Swaps primaryQueue with secondary queue. The previous primary queue will be released.
	 */
	public synchronized void swap() {
		primaryQueue = secondaryQueue;
		secondaryQueue = new ArrayList<T>();
	}
	
	/**
	 * Returns a count of the secondary queue.
	 * @return
	 */
	public synchronized int getSwapQueueSize() {
		return secondaryQueue.size();
	}
	
	public synchronized int getPrimarySize() {
		return primaryQueue.size();
	}
	
	public synchronized void addAll(Collection<T> col) {
		secondaryQueue.addAll(col);
	}
	
	/**
	 * Returns both the secondary and primary size
	 * @return
	 */
	public synchronized int getSize() {
		return secondaryQueue.size() + primaryQueue.size();
	}
	
	public synchronized void add(T element) {
		secondaryQueue.add(element);
	}

	/**
	 * Returns iterator over the primary queue.
	 */
	@Override
	public synchronized Iterator<T> iterator() {
		return primaryQueue.iterator();
	}
}