【问题】

有n个桃子， 猴子A每次固定摘2个，猴子B每次固定摘3个，这2只猴子不断摘桃子直到剩余桃子数量不足以摘（必须满足摘桃个数）；

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【1】 使用AtomicInteger（推荐）

1）利用 原子类的CAS原子函数（乐观锁）实现并发控制访问；

2）推荐； 乐观锁实现，代码简单，性能高； 

public class AtomicIntegerTest {
    public static void main(String[] args) {
        ExecutorService executorService = Executors.newFixedThreadPool(2);
        Object lock = new Object();
        AtomicInteger cap = new AtomicInteger(103);
        executorService.execute(new Task(cap, 2));
        executorService.execute(new Task(cap, 3));
        executorService.shutdown();
    }

    private static class Task implements Runnable {
        private AtomicInteger cap;
        private int consumeSize;
        private int takeCount = 0;

        Task(AtomicInteger cap, int consumeSize) {
            this.cap = cap;
            this.consumeSize = consumeSize;
        }

        @Override
        public void run() {
            while (take()) ;
            PrintUtils.print(Thread.currentThread().getName() + "#" + this);
        }

        boolean take() {
            if (cap.addAndGet(-consumeSize) >= 0) {
                takeCount++;
                return true;
            }
            return false;
        }

        @Override
        public String toString() {
            return "Task{" +
                    "consumeSize=" + consumeSize +
                    ", takeCount=" + takeCount +
                    '}';
        }
    }
}

---

【2】使用synchronized同步代码块

监视器锁内存结构：

1）用户线程访问Synchronized同步代码块获取锁流程

2）代码示例

public class SynchronizedTest {
    private static volatile Integer CAPACITY = 14;

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newFixedThreadPool(2);
        Object lock = new Object();
        executorService.execute(new Task( 2, lock));
        executorService.execute(new Task( 3, lock));
        executorService.shutdown();
    }

    static class Task implements Runnable {
        private int consumeSize;
        private Object lock;
        private int takeCount = 0;

        Task(int consumeSize, Object lock) {
            this.consumeSize = consumeSize;
            this.lock = lock;
        }

        @Override
        public void run() {
            while(take());
            PrintUtils.print(Thread.currentThread().getName() + "#" + this);
        }

        boolean take() {
            synchronized (lock) {
                if (CAPACITY >= consumeSize) {
                    CAPACITY -= consumeSize;
                    takeCount++;
                    return true;
                }
                return false;
            }
        }

        @Override
        public String toString() {
            return "Task{" +
                    "consumeSize=" + consumeSize +
                    ", takeCount=" + takeCount +
                    '}';
        }
    }
}

2024-06-09 10:42:56.989 pool-1-thread-1#Task{consumeSize=2, takeCount=4}
2024-06-09 10:42:56.989 pool-1-thread-2#Task{consumeSize=3, takeCount=2}

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【3】使用可重入锁ReentrantLock

public class ReentrantLockTest {
    private static volatile Integer CAPACITY = 103;

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newFixedThreadPool(2);
        ReentrantLock reentrantLock = new ReentrantLock();
        executorService.execute(new Task(2, reentrantLock));
        executorService.execute(new Task(3, reentrantLock));
        executorService.shutdown();
    }

    static class Task implements Runnable {
        private int consumeSize;
        private ReentrantLock reentrantLock;
        private int takeCount = 0;

        Task(int consumeSize, ReentrantLock reentrantLock) {
            this.consumeSize = consumeSize;
            this.reentrantLock = reentrantLock;
        }

        @Override
        public void run() {
            while (take()) ;
            PrintUtils.print(Thread.currentThread().getName() + "#" + this);
        }

        boolean take() {
            reentrantLock.lock(); // 加锁
            try {
                if (CAPACITY >= consumeSize) {
                    CAPACITY -= consumeSize;
                    takeCount++;
                    return true;
                }
                return false;
            } finally {
                reentrantLock.unlock(); // 解锁
            }
        }

        @Override
        public String toString() {
            return "Task{" +
                    "consumeSize=" + consumeSize +
                    ", takeCount=" + takeCount +
                    '}';
        }
    }
}

---

【4】 使用信号量Semaphore实现

semaphore.acquire()#信号量个数减1 和 release()#信号量个数加1 ；如信号量个数等于0，则 acquire() 阻塞；

public class SemaphoreTest {
    private static volatile Integer CAPACITY = 103;

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newFixedThreadPool(2);
        Semaphore semaphore = new Semaphore(1);// 信号量数量初始为1 
        executorService.execute(new Task(2, semaphore));
        executorService.execute(new Task(3, semaphore));
        executorService.shutdown();
    }

    static class Task implements Runnable {
        private int consumeSize;
        private Semaphore semaphore;
        private int takeCount = 0;

        Task(int consumeSize, Semaphore semaphore) {
            this.consumeSize = consumeSize;
            this.semaphore = semaphore;
        }

        @Override
        public void run() {
            while (take()) ;
            PrintUtils.print(Thread.currentThread().getName() + "#" + this);
        }

        boolean take() {
             // 获取信号量（信号数量减1）
            try {
                semaphore.acquire();
                if (CAPACITY >= consumeSize) {
                    CAPACITY -= consumeSize;
                    takeCount++;
                    return true;
                }
                return false;
            } catch (Exception e) {
                throw new RuntimeException(e);
            } finally {
                semaphore.release();
            }
        }

        @Override
        public String toString() {
            return "Task{" +
                    "consumeSize=" + consumeSize +
                    ", takeCount=" + takeCount +
                    '}';
        }
    }
}

2024-06-09 14:20:18.509 pool-1-thread-2#Task{consumeSize=3, takeCount=5}
2024-06-09 14:20:18.509 pool-1-thread-1#Task{consumeSize=2, takeCount=44}