并发编程 - JUC中常用的同步工具

ReentrantLock - 独占锁

特性：①独占锁 ②可重入 ③公平/非公平 ④可超时中断

// ReentrantLock
public class ReentrantLockTest {
    private static Lock lock = new ReentrantLock();
    private static int count = 0;

    public static void main(String[] args) throws InterruptedException {
        for(int i = 0;i < 1000; i++){
            new Thread(ReentrantLockTest::incre).start();
        }

        Thread.sleep(1000);
        System.out.println(count);
    }

    public static void incre() {
        lock.lock();   // 抢占锁
        try {
            count++;
        }catch (Exception e){
            e.printStackTrace();
        }finally {
            lock.unlock();  // 释放锁
        }
    }
}

ReentrantReadWriteLock - 读写锁

ReentrantReadWriteLock使用同一个Sync队列，重写了共享资源/互斥资源的获取与释放逻辑，通过state的高16位存储共享状态，state的低16位存储互斥状态。
API：①readLock() - 获取读锁 ②writeLock() - 获取写锁 =③lock.lock()/unlock()

// 读写锁，允许读读共享，读写互斥，写写互斥
public class ReentrantReadWriteLockTest {
    private static ReadWriteLock readWriteLock = new ReentrantReadWriteLock(true);
    private static Lock readLock = readWriteLock.readLock();
    private static Lock writeLock = readWriteLock.writeLock();

    static String message = null;

    public static void main(String[] args) {
        new Thread(ReentrantReadWriteLockTest::producer).start();
        new Thread(ReentrantReadWriteLockTest::consumer).start();
    }

    public static void producer(){
        Random random = new Random(47);
        while (true) {
            writeLock.lock();
            try {
                message = "" + random.nextInt();
                System.out.println("write " + message);
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                writeLock.unlock();
            }
        }
    }

    public static void consumer(){
        while (true) {
            readLock.lock();
            try {
                System.out.println("read " + message);
            } finally {
                readLock.unlock();
            }
        }
    }
}

Condition - 条件等待

Condition是JUC提供的，功能等同于wait/notify功能的接口，具体实现为AbstractQueuedSynchronizer的内部类ConditionObject(只能在AQS对象实例内部创建)，基于一个单向的Fifo队列实现等待与唤醒
API：①await() ②signal() ③signalAll()

// 阻塞队列的put()/take()方法即为通过两个contidion实现等待
public class ConditionTest{
    public static void main(String[] args) {
        ReentrantLock lock = new ReentrantLock();
        Condition empty = lock.newCondition();
        Condition full = lock.newCondition();
        Queue<Integer> queue = new LinkedList<Integer>();  // 线程不安全的队列

        new Thread(new Producer(queue,4,lock,empty,full)).start();
        new Thread(new Consumer(queue,lock,empty,full)).start();
    }
}

// Producer 
public class Producer implements Runnable{
    private Queue<Integer> msg;
    private int maxSize;
    private Lock lock;
    private Condition empty;
    private Condition full;

    public Producer(Queue<Integer> msg, int maxSize, Lock lock, Condition empty, Condition full) {
        this.msg = msg;
        this.maxSize = maxSize;
        this.lock = lock;
        this.empty = empty;
        this.full = full;
    }

    @Override
    public void run() {
        for(int i = 0;;i++){
            lock.lock();
            try {
                while (msg.size() == maxSize){
                    System.out.println("消息队列已满 - waiting");
                    full.await();                                    // 队列已满，加入full等待队列
                }
                System.out.print("生产消息："+ msg.add(i));
                empty.signal();                                      // 生成了消息，唤醒empty等待队列
                Thread.sleep(500);
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                lock.unlock();
            }
            System.out.println();
        }
    }
}

// Consumer 
public class Consumer implements Runnable{
    private Queue<Integer> msg;
    private Lock lock;
    private Condition empty;
    private Condition full;

    public Consumer(Queue<Integer> msg, Lock lock, Condition empty, Condition full) {
        this.msg = msg;
        this.lock = lock;
        this.empty = empty;
        this.full = full;
    }

    @Override
    public void run() {
        for(int i = 0;;i++){
            lock.lock();
            try {
                while (msg.isEmpty()){
                    System.out.println("消息队列已空 - waiting");
                    empty.await();
                }
                System.out.print("消费消息：" + msg.remove());
                full.signal();
                Thread.sleep(500);
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                lock.unlock();
            }
            System.out.println();
        }
    }
}

CountDownLatch 计数器

线程调用countDownLatch.await()方法时将会被挂起，直到计数器计数值减为0时，唤醒所有await()线程 - 底层为共享模式的实现
API：①await() ②countDown() - 计数值减一

// 用法一：初始计数值设为1，所有await线程将在同一时间被唤醒(多个线程await()，一个线程countDown())
    public static void demo1(){
        countDownLatch = new CountDownLatch(1);
        new Thread(CountDownLatchTest::execute1).start();
        new Thread(CountDownLatchTest::execute1).start();
        new Thread(CountDownLatchTest::execute1).start();
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        countDownLatch.countDown();   // 初始计数值计为1，countDown()方法唤醒所有await()线程，使所有线程同步开始
    }

    public static void execute1(){
        System.out.println(Thread.currentThread().getName() + " -> begin");
        try {
            countDownLatch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + " -> end");
    }


// 用法二：初始计数值设为任意，只有任意个线程调用countDown()方法后，方才唤醒awati线程(一个线程await()，等待多个线程countDown())
    public static void demo2(){
        countDownLatch = new CountDownLatch(3);
        new Thread(CountDownLatchTest::execute2).start();
        new Thread(CountDownLatchTest::execute2).start();
        new Thread(CountDownLatchTest::execute2).start();

        System.out.println(Thread.currentThread().getName() + " -> main.await");
        try {
            countDownLatch.await(); // await()线程等待计数值个线程执行countDown()后才能被唤醒
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + " -> main.signal");
    }

    public static void execute2(){
        System.out.println(Thread.currentThread().getName() + " -> 已执行");
        countDownLatch.countDown();
    }

Semaphore 信号灯(令牌桶)

只有获得令牌的线程可以继续执行，用以控制同时访问的线程个数。
API：①acquire() - 获取令牌 ②release() - 释放令牌

// 只有获得令牌的线程可以继续执行，用以控制同时访问的线程个数。
public class SemaphoreTest {
    private static Semaphore semaphore = new Semaphore(5);

    public static void main(String[] args) {
        new Thread(SemaphoreTest::execute).start();
        new Thread(SemaphoreTest::execute).start();
        new Thread(SemaphoreTest::execute).start();
        new Thread(SemaphoreTest::execute).start();
        new Thread(SemaphoreTest::execute).start();
    }

    public static void execute(){
        while(true) {
            try {
                semaphore.acquire();
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            semaphore.release();
            System.out.println(Thread.currentThread().getName() + " -> 获得了令牌");
        }
    }
}

CyclicBarrier - 循环屏障

让一组线程到达一个屏障（也可以叫同步点）时被阻塞，直到最后一个线程到达屏障时，所有被屏障拦截的线程才会继续工作。
(由最后一个进入线程执行barrierAction，并唤醒对应数量的await状态线程，然后计数值会reset,计入下一次循环)
API: await() - CyclicBarrier是当最后一个线程到达屏障时，自动唤醒。

// 屏障，当进入await状态的线程数达到parties时，会由最后一个进入线程执行barrierAction，然后释放对应数量的await状态线程(然后发送reset)
public class CyclicBarrierTest {
    private static CyclicBarrier cyclicBarrier = new CyclicBarrier(4,new Thread(()-> System.out.println(Thread.currentThread().getName())));

    public static void main(String[] args) {
        new Thread(CyclicBarrierTest::execute).start();
        new Thread(CyclicBarrierTest::execute).start();
        new Thread(CyclicBarrierTest::execute).start();
        new Thread(CyclicBarrierTest::execute).start(); // 以上4个线程将会被唤醒
        
        new Thread(CyclicBarrierTest::execute).start(); // 计数值被reset后，由于后来线程数无法达到计数值，因此该线程将被永久阻塞
    }

    public static void execute() {
        System.out.println(Thread.currentThread().getName() + " -> begin");
        try {
            cyclicBarrier.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (BrokenBarrierException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + " -> end");
    }
}

Exchanger - 值交换器

用以实现两个线程之间消息的交换，先到达exchange()的线程将被阻塞，等待下一个线程exchange()唤醒

// 交换器，用于两个线程之间交换数据(先到达的线程将会被阻塞)
public class ExchangerTesst {
    private static Exchanger<String> exchanger = new Exchanger<>();

    public static void main(String[] args) {
        new Thread(ExchangerTesst::execute).start();
        new Thread(ExchangerTesst::execute).start();
    }

    private static void execute(){
        try {
            String msg = Thread.currentThread().getName();
            Thread.sleep(1000);
            System.out.println(Thread.currentThread().getName() + " -> " + msg);
            msg = exchanger.exchange(Thread.currentThread().getName());
            System.out.println(Thread.currentThread().getName() + " -> " + msg);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

// Thread-0 -> Thread-0
// Thread-1 -> Thread-1
// Thread-1 -> Thread-0 -- 发生了值的交换
// Thread-0 -> Thread-1