生产者和消费者

wait / notify

wait 会释放锁，但 notify 不会释放锁。

public class MyContainer1<T> {
    private final LinkedList<T> list = new LinkedList<>();
    // 最多存 10 个元素
    private final int MAX = 10;
    private int count = 0;

    public synchronized void put(T t) {
        /**
         * 为什么用 while 而不用 if?
         * 用 while 的目的是当线程被唤醒的时候再进行一次判断
         */
        while (list.size() == MAX) {
            try {
                this.wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        list.add(t);
        count++;
        // 通知消费者线程消费
        this.notifyAll();
    }

    public synchronized T get() {
        T t = null;
        while (list.size() == 0) {
            try {
                this.wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        t = list.removeFirst();
        count--;
        // 通知生产者线程生产
        this.notifyAll();
        return t;
    }

    public static void main(String[] args) {
        MyContainer1<String> my = new MyContainer1<>();
        // 启动消费者线程
        for (int i = 0; i < 10; i++) {
            new Thread(() -> {
                for (int j = 0; j < 5; j++) {
                    System.out.println(my.get());
                }
            }, "c" + i).start();
        }
        try {
            TimeUnit.SECONDS.sleep(1);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        // 启动生产者线程
        for (int i = 0; i < 2; i++) {
            new Thread(() -> {
                for (int j = 0; j < 25; j++) {
                    my.put(Thread.currentThread().getName() + " " + j);
                }
            }, "p" + i).start();
        }
    }
}

这种写法有一种缺陷：唤醒线程时无法明确只唤醒生产者线程或消费者线程。

await / signal

await 会释放锁，但 signal 不会释放锁。

public class MyContainer2<T> {
    private final LinkedList<T> list = new LinkedList<>();
    // 最多存 10 个元素
    private final int MAX = 10;
    private int count = 0;

    private Lock lock = new ReentrantLock();
    // Condition 的本质是不同的等待队列
    private Condition p = lock.newCondition();
    private Condition c = lock.newCondition();

    public void put(T t) {
        try {
            lock.lock();
            while (list.size() == MAX) {
                p.await();
            }
            list.add(t);
            count++;
            // 通知消费者线程消费
            c.signalAll();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public T get() {
        T t = null;
        try {
            lock.lock();
            while (list.size() == 0) {
                c.await();
            }
            t = list.removeFirst();
            count--;
            // 通知生产者线程生产
            p.signalAll();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
        return t;
    }

    public static void main(String[] args) {
        MyContainer2<String> my = new MyContainer2<>();
        // 启动消费者线程
        for (int i = 0; i < 10; i++) {
            new Thread(() -> {
                for (int j = 0; j < 5; j++) {
                    System.out.println(my.get());
                }
            }, "c" + i).start();
        }
        try {
            TimeUnit.SECONDS.sleep(1);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        // 启动生产者线程
        for (int i = 0; i < 2; i++) {
            new Thread(() -> {
                for (int j = 0; j < 25; j++) {
                    my.put(Thread.currentThread().getName() + " " + j);
                }
            }, "p" + i).start();
        }
    }
}