生产者消费者问题--lock

# 代码：

public class App {

    public static void main(String[] args) {

        Depot depot = new Depot(100);
        Producer producer = new Producer(depot);
        Consumer consumer = new Consumer(depot);

        producer.produce(60);
        consumer.consume(100);
        producer.produce(90);
        consumer.consume(40);
    }
}

class Producer {
    private Depot depot;

    public Producer(Depot depot) {
        this.depot = depot;
    }

    public void produce(final int val) {
        new Thread() {
            public void run() {
                depot.produce(val);
            }
        }.start();
    }
}

class Consumer {
    private Depot depot;

    public Consumer(Depot depot) {
        this.depot = depot;
    }

    public void consume(final int val) {
        new Thread() {
            public void run() {
                depot.consume(val);
            }
        }.start();
    }
}

class Depot {
    private int capacity;
    private int size;
    private Lock lock;
    private Condition fullCondition;
    private Condition emptyCondition;

    public Depot(int capacity) {
        this.size = 0;
        this.capacity = capacity;
        this.lock = new ReentrantLock();
        fullCondition = lock.newCondition();
        emptyCondition = lock.newCondition();
    }

    public void produce(int val) {
        lock.lock();
        try {
            int surplus = val;
            while (surplus > 0) {
                while (size >= capacity) {
                    fullCondition.await();
                }
                int incre = (size + surplus) > capacity ? (capacity - size) : surplus;
                size += incre;
                surplus -= incre;
                System.out.printf("%s plan to produce (%d), actually produce (%d), depot size (%d) \n",
                        Thread.currentThread().getName(), val, incre, size);
                emptyCondition.signal();
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public void consume(int val) {
        lock.lock();
        try {
            int surplus = val;
            while (surplus > 0) {
                while (size <= 0) {
                    emptyCondition.await();
                }
                int desc = (size < surplus) ? size : surplus;
                size -= desc;
                surplus -= desc;
                System.out.printf("%s plan to consume (%d), actutally consume (%d), depot size (%d) \n",
                        Thread.currentThread().getName(), val, desc, size);
                fullCondition.signalAll();
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
}

# 输出：

Thread-0 plan to produce (60), actually produce (60), depot size (60)
Thread-1 plan to consume (100), actutally consume (60), depot size (0)
Thread-2 plan to produce (90), actually produce (90), depot size (90)
Thread-3 plan to consume (40), actutally consume (40), depot size (50)
Thread-1 plan to consume (100), actutally consume (40), depot size (10)

# 有四个线程参与了这个过程，两个生产者，两个消费者

---

# 这是几个月之后的补充。。。。。。。。手动笑哭。

# 几个月之后，我重新回顾了这些内容。。。发现了上面的代码有坑！！！

# 首先我们来重现此坑

    public static void main(String[] args) {
        Depot depot = new Depot(100);
        Producer producer = new Producer(depot);
        Consumer consumer = new Consumer(depot);

        for (int i = 0; i < 10; i++) {
            producer.produce(50);
        }

        for (int i = 0; i < 50; i++) {
            consumer.consume(10);
        }

    }

　　- main 方法是这个样子，在这种情况下，有时候会出现程序卡住的情况

　　- 用jstack看了一下堆栈信息，发现有几个consumer线程一直处于locked状态，但是已经没有人去唤醒他们了

　　- 分析了一下，其实原因很明显，我们在produce时，唤醒consumer使用的是signal()，最后一个生产者在唤醒某一个消费者之后，该消费者消费完了就再也没有生产者了，这时剩下的消费者线程就没有人唤醒他们了。。。换成signalAll()就ok了。

# 还有一个需要注意的是进入 await()的判断条件一定要使用while, 尽量不要使用if, 这个问题我会在 synchronized 实现的生产者-消费者模块中给出原因