JUC之各种锁的使用：CountDownLatch，CyclicBarrier，ReadWriteLock，Semaphore

CountDownLatch

CountDownLatch的典型使用环境是：确保某个操作在其需要的一个多线程操作完成后执行。比如说赛跑者必须等发令枪响。await是阻塞，countDown是倒计，构造函数中指定初始值。只有初始值被countDown到0时，阻塞才会被打开。

		 //所有线程阻塞，然后统一开始  
        CountDownLatch begin = new CountDownLatch(1);  
          
        //主线程阻塞，直到所有分线程执行完毕  
        CountDownLatch end = new CountDownLatch(5);  
          
        for(int i = 0; i < 5; i++){  
            Thread thread = new Thread(new Runnable() {  
                  
                @Override  
                public void run() {  
                    try {  
                        begin.await();  
                        System.out.println(Thread.currentThread().getName() + " 起跑");  
                        Thread.sleep(1000);  
                        System.out.println(Thread.currentThread().getName() + " 到达终点");  
                        end.countDown();  
                    } catch (InterruptedException e) {  
                        e.printStackTrace();  
                    }  
                      
                }  
            });  
              
            thread.start();  
        }  
          
        try {  
            System.out.println("1秒后统一开始");  
            Thread.sleep(1000);  
            begin.countDown();  //发令枪响
  
            end.await();  
            System.out.println("停止比赛");  
        } catch (InterruptedException e) {  
            e.printStackTrace();  
        }  
          
    }  

CountDownLatch可以当成一个带数量的join。允许1或者多个线程，等待另外N个线程完成某件事情之后，这1个或者多个线程才能执行。

CyclicBarrier

与CountDownLatch不同的是该barrier在释放等待线程后可以重用，所以称它为循环（Cyclic）的屏障（Barrier）。
设想这个场景，一群工人要在AB两地往返工作，负责接送他们的是一个大巴，这个大巴不等坐满就不发车。

public class JUCMain {

    public static Object object = new Object();

    public static void main(String[] args) {
        //System.out.println("begin");
        JUCMain main = new JUCMain();
        main.testCB();
        //System.out.println("end");

    }



    public static class WorkerThread implements Runnable{

        CyclicBarrier barrier;

        public WorkerThread(CyclicBarrier b){
            this.barrier = b;
        }

        @Override
        public void run() {
            int i = 10;
            // TODO Auto-generated method stub
            try{
                while (i>0){
                    System.out.println("ID:"+Thread.currentThread().getId()+" Waiting");
                    //线程在这里等待，直到所有线程都到达barrier。
                    barrier.await();
                    System.out.println("ID:"+Thread.currentThread().getId()+" Working");
                    i--;
                    barrier.await();
                }
            }catch(Exception e){
                e.printStackTrace();
            }
        }

    }


    public void testCB() {
        // TODO Auto-generated method stub
        CyclicBarrier cb = new CyclicBarrier(5, new Runnable() {
            //当所有线程到达barrier时执行
            @Override
            public void run() {
                // TODO Auto-generated method stub
                System.out.println("Inside Barrier");

            }
        });

        for(int i=0;i<5;i++){
            new Thread(new WorkerThread(cb)).start();
        }
    }

上述代码输出：
ID:10 Waiting
ID:11 Waiting
ID:12 Waiting
ID:13 Waiting
ID:14 Waiting
Inside Barrier
ID:14 Working
ID:10 Working
ID:12 Working
ID:13 Working
ID:11 Working
Inside Barrier

ReadWriteLock

ReadWriteLock是一个接口，常见的实现是ReentrantReadWriteLock。

    public static void main(String[] args) {
        final JUCMain main = new JUCMain();
        // 建N个线程，同时读
        ExecutorService service = Executors.newCachedThreadPool();
        service.execute(new Runnable() {
            @Override
            public void run() {
                main.readFile(Thread.currentThread());
            }
        });
        // 建N个线程，同时写
        ExecutorService service1 = Executors.newCachedThreadPool();
        service1.execute(new Runnable() {
            @Override
            public void run() {
                main.writeFile(Thread.currentThread());
            }
        });
        System.out.println("begin");

    }


    ReentrantReadWriteLock lock = new ReentrantReadWriteLock();

    public void readFile(Thread thread){
        lock.readLock().lock();
        boolean readLock = lock.isWriteLocked();
        if(!readLock){
            System.out.println("当前为读锁！");
        }
        try{
            for(int i=0; i<5; i++){
                try {
                    Thread.sleep(20);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(thread.getName() + ":正在进行读操作……");
            }
            System.out.println(thread.getName() + ":读操作完毕！");
        }finally{
            System.out.println("释放读锁！");
            lock.readLock().unlock();
        }
    }
    // 写操作
    public void writeFile(Thread thread){
        lock.writeLock().lock();
        boolean writeLock = lock.isWriteLocked();
        if(writeLock){
            System.out.println("当前为写锁！");
        }
        try{
            for(int i=0; i<5; i++){
                try {
                    Thread.sleep(20);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(thread.getName() + ":正在进行写操作……");
            }
            System.out.println(thread.getName() + ":写操作完毕！");
        }finally{
            System.out.println("释放写锁！");
            lock.writeLock().unlock();
        }
    }

Semaphore

Semaphore的原意是旗语，技术文档中常翻译成信号量。

    private static Semaphore semaphore = new Semaphore(2);

    public static void main(String[] args) {

        ExecutorService service = Executors.newFixedThreadPool(5);
        for (int i = 0; i < 4; i++) {
            service.execute(new Runnable() {
                @Override
                public void run() {
                    try{
                        System.out.println("require_permit"+Thread.currentThread().getId());
                        semaphore.acquire();
                        System.out.println("begin"+Thread.currentThread().getId());
                        Thread.sleep(1000);
                        System.out.println("end"+Thread.currentThread().getId());
                    }catch (Exception e){
                        e.printStackTrace();
                    }finally {
                        semaphore.release();
                        System.out.println("release_permit"+Thread.currentThread().getId());
                    }

                }
            });
        }
    }

上述程序的输出是
require_permit10
require_permit12
begin10
require_permit11
begin12
require_permit13
end10
end12
release_permit10
begin11
release_permit12
begin13
end11
end13
release_permit11
release_permit13