闭锁CountDownLatch和栅栏CyclicBarrier之异同举例

CountDownLatch和CyclicBarrier的主要联系和区别如下：
1.闭锁CountDownLatch做减计数，而栅栏CyclicBarrier则是加计数。
2.CountDownLatch是一次性的，CyclicBarrier可以重用。
3.CountDownLatch强调一个线程等多个线程完成某件事情。CyclicBarrier是多个线程互等，等大家都完成。
4.鉴于上面的描述，CyclicBarrier在一些场景中可以替代CountDownLatch实现类似的功能。

另外，值得一提的是，CountDownLatch和CyclicBarrier在创建和启动线程时，都没有明确提到同时启动全部线程，事实上这在技术上是不大可能，不必要，不提倡的。

先看例子一：

class SubRunnable implements Runnable {
	private CountDownLatch begin, end;
	private List<Integer> sublist;
	
	public SubRunnable(List<Integer> sublist, CountDownLatch begin,CountDownLatch end) {
		this.sublist = sublist;
		
		this.begin = begin;
		this.end = end;
	}
	
	@Override
	public void run() {
		try {
			begin.await();			
			
			if (sublist != null) {
				for (int i : sublist) {
					System.out.println("线程" + Thread.currentThread().getName() + ", i = " + i);
				}
			}
		} catch (InterruptedException e) {
			e.printStackTrace();
		} finally{
			System.out.println(System.currentTimeMillis() + ",线程" + Thread.currentThread().getName() + ",开始执行！");
			end.countDown();
		}
	}
}

public class BatchWithCountDownLatch {
	private static final int MAX = 3;
	
	private static void list(List<Integer> list) {
		if(list == null){
			list = new ArrayList<Integer>();
		}
		
		for(int i = 0 ;i < 1000;i++){
			list.add(i);
		}
	}

	public static void main(String[] args) {
		List<Integer> list = new ArrayList<Integer>();
		list(list);
		
		//把list拆分成多个
		int mod = list.size() % MAX;
		int threadCount = mod == 0 ? list.size() / MAX : list.size() / MAX + 1;
		ExecutorService executors = Executors.newFixedThreadPool(threadCount);
		
		CountDownLatch begin = new CountDownLatch(1); 
		CountDownLatch end = new CountDownLatch(threadCount); 
				
		for(int i = 0; i< threadCount;i++){
			int subsize = (i + 1) * MAX;
			executors.execute(new SubRunnable(list.subList(i * MAX, subsize > list.size() ? list.size() : subsize),begin,end));
		}
		
		System.out.println("开始 !");
		begin.countDown();
		long startTime = System.currentTimeMillis();
		
		try {
			end.await();
		} catch (InterruptedException e) {
			e.printStackTrace();
		} finally {
			System.out.println("线程" + Thread.currentThread().getName() + "," + System.currentTimeMillis() + ", 所有线程已完成，开始进入下一步！");
			System.out.println("花费时间 -> " + (System.currentTimeMillis() - startTime) + " ms");
		}
		
		System.out.println("开始进入第二步操作! ");		
		
		System.out.println("end! ");
	}
}

这是根据jdk文档中的伪代码例程，编写的一个例子，我们完全可以将这个例程改为只使用一个CountDownLatch来实现之。经过测试，发现begin的引入对程序基本无用，当list是1000的数量级时，最先启动的线程仍然比最后启动的快几十毫秒左右；而不设置begin开始闭锁的程序，也是完全一样的情况。

例子二：

class SubRunnable implements Runnable {
    private CyclicBarrier cyclicBarrier;
    private List<Integer> sublist;
    
    public SubRunnable(List<Integer> sublist, CyclicBarrier cyclicBarrier) {
        this.sublist = sublist;
        this.cyclicBarrier = cyclicBarrier;
    }
    
    @Override
    public void run() {        
        try {
            System.out.println(System.currentTimeMillis() + ",线程" + Thread.currentThread().getName() + ",开始执行！");
            if(sublist != null){
                for(int i : sublist){
                    System.out.println("线程" + Thread.currentThread().getName() + ", i = " + i);
                }
            }

            cyclicBarrier.await();
        } catch (InterruptedException | BrokenBarrierException e) {
            e.printStackTrace();
        }
    }    
}

public class ReplaceCountDownLatch {
    private static final int MAX = 3;
    
    private static void list(List<Integer> list) {
        if(list == null){
            list = new ArrayList<Integer>();
        }
        
        for(int i = 0 ;i < 10;i++){
            list.add(i);
        }
    }
    
    public static void main(String[] args) throws InterruptedException, BrokenBarrierException {
        List<Integer> list = new ArrayList<Integer>();
        list(list);

        //把list拆分成多个
        int mod = list.size() % MAX;
        int threadCount = mod == 0 ? list.size() / MAX : list.size() / MAX + 1;
        ExecutorService executors = Executors.newFixedThreadPool(threadCount);

        final CyclicBarrier cyclicBarrier = new CyclicBarrier(threadCount,new Runnable() {
            @Override
            public void run() {
                //根据jdkdoc里的描述，哪个线程最后运行完，就执行下面的代码。
                System.out.println("线程" + Thread.currentThread().getName() + "," + System.currentTimeMillis() + ", 所有线程已完成，开始进入下一步！");
            }
        });
        
        for(int i = 0; i< threadCount;i++){
            int subsize = (i + 1) * MAX;
            executors.execute(new SubRunnable(list.subList(i * MAX, subsize > list.size() ? list.size() : subsize),cyclicBarrier));
        }
        
        cyclicBarrier.await();
        executors.shutdown();
        System.out.println("开始进入第二步操作! ");
        
        
        System.out.println("end! ");
    }
}

使用栅栏CyclicBarrier实现和上面闭锁CountDownLatch相同的功能。