wait()/notify() 无法正常工作

Question

我有一个 ConsumerProducer 对象，我想从两个不同的线程获取它的锁。 class如下：

public class ConsumerProducer {

    public String stringPool = null;

    public void put(String s){
        stringPool = s;
    }

    public String get(){
        String ret = stringPool;
        stringPool = null;
        return ret;
    }

}

线程实现class如下：

public class WaitNotifyTest implements Runnable {

    private String threadType;
    public ConsumerProducer cp;
    public static volatile int i = 1;

    public WaitNotifyTest(String threadType, ConsumerProducer cp) {
        this.threadType = threadType;
        this.cp = cp;
    }

    public static void main(String[] args) throws InterruptedException {

        ConsumerProducer cp = new ConsumerProducer();
        WaitNotifyTest test1 = new WaitNotifyTest("Consumer", cp);
        WaitNotifyTest test2 = new WaitNotifyTest("Producer", cp);

        Thread t1 = new Thread(test1);
        Thread t2 = new Thread(test2);

        t1.start();
        t2.start();
        t1.join();
        t2.join();
    }

    @Override
    public void run() {
        while (true) {

            if (threadType.equalsIgnoreCase("Consumer")) {
                synchronized (cp) {
                    try {
                        if (null != cp.get()) {
                            cp.wait();
                        }
                        consume();
                        System.out.println("notify from Consumer");
                        cp.notify();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }

            } else {
                synchronized (cp) {
                    try {
                        if (null == cp.get()) {
                            cp.wait();
                        }
                        produce();
                        System.out.println("notify from Producer");
                        cp.notify();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
            if (i == 5) {
                break;
            }
            i++;
        }
    }

    public void consume() {
        System.out.println("Putting: Counter" + i);
        cp.put("Counter" + i);
    }

    public void produce() {
        System.out.println("getting: " + cp.get());
    }

}

但是当我运行代码时它正面临某种死锁并且它像

一样被卡住打印

Putting: Counter3
notify from Consumer

出现了严重错误，但我无法确定。请帮忙。

Answer 1

您的消费者正在做生产者的工作，而生产者正在做消费者的工作。交换他们的责任并修改条件等待。请参考下面的代码。

消费者会等到什么都拿不到的时候，他就会释放cp的锁。这样生产者就有机会进入同步块。
生产者只有在什么都没有的时候才生产，否则他会等待。之后，他就会释放cp的锁。这样消费者就有机会进入同步块。
消费者是拿东西的人。
Producer 是谁把东西放到 table。
根据您的评论。你想把计数器从 1 到 5，所以你应该只在生产者线程中添加 i++。如何控制它在两个线程中的增加？

从cp对象中调用get()不判断是消费者还是生产者，而是将null赋给stringPool。这显然是错误的，并且会使消费者从 public space 中获取 null。我添加了一个新方法 clearString()，只有当消费者消费了产品时，它才会将 public space 设置为 null。

public class WaitNotifyTest implements Runnable {

private String threadType;
public ConsumerProducer cp;
public static volatile int i = 0;

public WaitNotifyTest(String threadType, ConsumerProducer cp) {
    this.threadType = threadType;
    this.cp = cp;
}

public static void main(String[] args) throws InterruptedException {

    ConsumerProducer cp = new ConsumerProducer();
    WaitNotifyTest test1 = new WaitNotifyTest("Consumer", cp);
    WaitNotifyTest test2 = new WaitNotifyTest("Producer", cp);

    Thread t1 = new Thread(test1);
    Thread t2 = new Thread(test2);

    t1.start();
    t2.start();
    t1.join();
    t2.join();
}

@Override
public void run() {
    while (true) {
        if (threadType.equalsIgnoreCase("Consumer")) {
            synchronized (cp) {
                try {
                    /*
                     * Consumer will wait when there is nothing to get and he will release the lock of cp.
                     * So that producer has change to go into the synchronized block.
                     */

                    if (null == cp.get()) {
                        cp.wait();
                    }
                    consume();
                    System.out.println("notify from Consumer");
                    cp.notify();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }

        } else {
            synchronized (cp) {
                try {
                    /*
                     * Producer only produce when there is nothing or he will wait. At the same time, he will release the lock of cp.
                     * So that consumer has chance to go into the synchronized block.
                     */
                    if (null != cp.get()) {
                        cp.wait();
                    }
                    i++;
                    produce();
                    System.out.println("notify from Producer");
                    cp.notify();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
        if (i == 5) {
            break;
        }

    }
}

public void consume() {
    System.out.println("getting: " + cp.get());
    cp.clearString();
}

public void produce() {
    System.out.println("Putting: Counter" + i);
    cp.put("Counter" + i);
}}

另见 ConsumerProducer class。

public class ConsumerProducer {
        public String stringPool = null;

        public void put(String s){
            stringPool = s;
        }

        public String get(){
            return stringPool;
        }

        public void clearString(){
            stringPool = null;
        }
}

Answer 2

更新代码在这里： ConsumerProducer.java:
public class 消费者生产者 {

    public volatile String stringPool = null;

    public void put(String s){
        this.stringPool = s;
    }

    public String get(){
        String ret = this.stringPool;
        //this.stringPool = null;
        return ret;
    }
    //added
    public void clearString(){
        this.stringPool = null;
    }

}

WaitNotifyTest.java public class WaitNotifyTest 实现 Runnable {

    private String threadType;
    public ConsumerProducer cp;
    public static volatile int i = 0;

    public WaitNotifyTest(String threadType, ConsumerProducer cp) {
        this.threadType = threadType;
        this.cp = cp;
    }

    public static void main(String[] args) throws InterruptedException {

        ConsumerProducer cp = new ConsumerProducer();
        WaitNotifyTest test1 = new WaitNotifyTest("Consumer", cp);
        WaitNotifyTest test2 = new WaitNotifyTest("Producer", cp);

        Thread t1 = new Thread(test1);
        Thread t2 = new Thread(test2);

        t1.start();
        t2.start();
        t1.join();
        t2.join();
    }

    @Override
    public void run() {
        while (true) {

            if (threadType.equalsIgnoreCase("Consumer")) {
                synchronized (cp) {
                    try {
                        if (null == cp.get()) {
                            cp.wait();
                        }
                        consume();
                        System.out.println("notify from Consumer");
                        cp.notify();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }

            } else {
                synchronized (cp) {
                    try {
                        if (null != cp.get()) {
                            cp.wait();
                        }
                        i++;
                        produce();
                        System.out.println("notify from Producer");
                        cp.notify();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
            if (i == 5) {
                break;
            }

        }
    }

    public void produce() {
        System.out.println("Putting: Counter" + i);
        cp.put("Counter" + i);
    }

    public void consume() {
        System.out.println("getting: " + cp.get());
        cp.clearString();
    }

}

wait()/notify() 无法正常工作

wait()/notify() not working properly

java

multithreading

deadlock

notify

wait