一段阻塞隊列代碼的糾錯與優化

下面的代碼在某處發現後，立馬發現存在問題。

public class BlockingQ {

	private Object notEmpty = new Object();
	private Object notFull = new Object();
	private Queue<Object> linkedList = new LinkedList<Object>();
	private int maxLength = 10;
	
	public Object take() throws InterruptedException{
		synchronized(notEmpty){
			if(linkedList.size() == 0){
				notEmpty.wait();
			}
			synchronized(notFull){
				if(linkedList.size() == maxLength){
					notFull.notifyAll();
				}
				return linkedList.poll();
			}
		}
	}
	
	public void offer(Object object) throws InterruptedException{
		synchronized(notEmpty){
			if(linkedList.size() == 0){
				notEmpty.notifyAll();
			}
			synchronized(notFull){
				if(linkedList.size() == maxLength){
					notFull.wait();
				}
				linkedList.add(object);
			}
		}
	}
}

最簡單的情況就是一個生產者，兩個消費者，

			if(linkedList.size() == 0){
				notEmpty.wait();
			}

上面的if語句在第二線程重新獲取鎖後，進來的時候不會再次判斷是否有消費元素，會直接返回null。

	public Object take() throws InterruptedException{
		synchronized(notEmpty){
			while(linkedList.size() == 0){
				notEmpty.wait();
			}
			synchronized(notFull){
				if(linkedList.size() == maxLength){
					notFull.notifyAll();
				}
				return linkedList.poll();
			}
		}
	}

修正代碼如上面所示。

整個代碼主要用作示例，所以代碼上存在很冗餘的點，下面是代碼優化後的版本：

public class SimpleBlockingQ {
	private Queue<Object> linkedList = new LinkedList<Object>();
	private int maxLength = 10;
	
	public synchronized Object take() throws InterruptedException{
		while(linkedList.size() == 0){
			wait();
		}
		notifyAll();
		return linkedList.poll();
	}
	
	public synchronized void offer(Object object) throws InterruptedException{
		notifyAll();
		if(linkedList.size() == maxLength){
			wait();
		}
		linkedList.add(object);
	}
}

從性能上優化，可以參考《一種簡單無鎖隊列的實現》和《無鎖同步棧實現》。

下面是測試代碼：

import java.util.concurrent.atomic.AtomicInteger;

/**
 * @author 天水
 * @date 2013-4-12 下午03:44:15
 */
public class BlockingQTest {

	public static AtomicInteger index = new AtomicInteger(0);
	
	public static void main(String[] args){
		int tCount = 10; // thread count
		
		final BlockingQ BQ = new BlockingQ();
		final SimpleBlockingQ SBQ = new SimpleBlockingQ();
		
		// provider
		Runnable pr = new Runnable(){
			@Override
			public void run() {
				while(true){
					try {
						Thread.sleep(100);
						
						int tindex = index.getAndIncrement();
						//BQ.offer(tindex);
						//System.out.println("BQ offer: " + tindex);
						SBQ.offer(tindex);
						System.out.println("SBQ offer: " + tindex);
						
						Thread.sleep(100);
					} catch (InterruptedException e) {
						e.printStackTrace();
					}
					
				}
			}
		};
		// consumer
		Runnable cr = new Runnable(){
			@Override
			public void run() {
				while(true){
					try {
						Thread.sleep(100);
					
						//System.out.println("BQ take: " + BQ.take());
						System.out.println("SBQ take: " + SBQ.take());		

						Thread.sleep(100);
					} catch (InterruptedException e) {
						e.printStackTrace();
					}
				}
			}
		};
		
		for(int i=0; i<tCount; i++){
			new Thread(cr).start();
		}
		
		for(int i=0; i<tCount; i++){
			new Thread(pr).start();
		}
	}
}