Java中wait、notify、signal的区别

wait与notify、notifyAll：

        -- 属于Object类的方法，一个对象只能提供一个同步控制

        -- 与synchronize搭配使用

wait与signal、signalAll：

        -- 属于Condition类的方法，一个锁可以提供多个Condition同步控制

        -- 与Lock.lock()、unlock()搭配使用

以生产者消费者为例，说明两者的使用情景

public abstract class AbstractProducer implements Runnable{
    protected int a = 0;
    protected Queue queue;
    protected String name;

    protected abstract void create();

    public AbstractProducer(Queue queue, String name){
        this.queue = queue;
        this.name = name;
    }

    protected boolean isFull(Queue queue){
        return queue.size() >= 10;
    }

    @Override
    public void run() {
        // 默认生产100次
        while(true){
            create();
            try {
                if(a<20)
                    Thread.sleep(10);
                else
                    Thread.sleep(40);
                if(a>100) {
                    System.out.println("Producer break");
                    break;
                }
            } catch (InterruptedException e) {
            }
        }
    }
}

public abstract class AbstractConsumer implements Runnable{
    protected Queue queue;
    protected String name;
    public AbstractConsumer(Queue queue, String name){
        this.queue = queue;
        this.name = name;
    }

    protected abstract void handle();

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

            try {
                Thread.sleep(20);
            } catch (InterruptedException e) {
            }
        }
    }
}

wait--notifyAll 实现方式

 同步过程：创建一个队列，当队列为空，消费者进入等待状态；当队列满了，生产者进入等待状态。

当消息队列为空，消费者进入等待，释放锁；然后生产者获取到锁，队列没满，所以生产者生产一个消息，并唤醒所有等待线程，消费者被唤醒，此时消费者还不能运行，需要等生产者退出代码同步块后，消费者才真的开始运行。

public class ProducerWaitNotify extends AbstractProducer {

    ProducerWaitNotify(Queue queue, String name){
        super(queue, name);
    }

    @Override
    protected void create(){
        a++;
        String str = a+"";
        synchronized (queue){
            while(isFull(queue)){
                System.out.println("queue is full,"+name+" wait");
                try{
                    queue.wait();
                }catch(Exception e){}
            }
            queue.add(str);
            System.out.println(name+" create "+str);
            queue.notifyAll();
        }
    }
}

public class ConsumerWaitNotify extends AbstractConsumer {

    public ConsumerWaitNotify(Queue queue, String name){
        super(queue, name);
    }

    public void handle(){
        String str = null;
        synchronized (queue){
            while(queue.isEmpty()){
                System.out.println("queue is empty,"+name+" wait.");
                try{
                    queue.wait();
                }catch(Exception e){}
            }
            str = queue.poll();
            System.out.println(name+" handle "+str);
            queue.notifyAll();
        }
    }
}

public class Main {
    public static void main(String[] args) {
        //创建两个消费者，两个生产者
        Queue queue = new ArrayDeque<>();
        Thread producer1 = new Thread(new ProducerWaitNotify(queue,"p1"));
        Thread producer2 = new Thread(new ProducerWaitNotify(queue,"p2"));

        Thread consumer1 = new Thread(new ConsumerWaitNotify(queue,"c1"));
        Thread consumer2 = new Thread(new ConsumerWaitNotify(queue,"c2"));

        producer1.start();
        producer2.start();
        consumer1.start();
        consumer2.start();
    }
}

wait--signal方式实现

同步过程：先创建一个锁对象，然后再创建两个条件，条件1是队列不为空，条件2是队列不满。

当队列为空时，消费者在条件1上进行等待；当队列满了时，生产者在条件2上等待。因为条件1和条件2不会同时满足，所以消费者和生产者不会同时进入等待。

消费者消费一个消息后，就唤醒条件2；生产者生产一个消息后，就唤醒条件1

public class LockSignal {
    private static Lock lock = new ReentrantLock();
    private static Condition noFullCondition = lock.newCondition();
    private static Condition noEmptyCondition = lock.newCondition();

    public static Lock getLock(){
        return lock;
    }

    public static Condition getNoFullCondition(){
        return noFullCondition;
    }

    public static Condition getNoEmptyCondition(){
        return noEmptyCondition;
    }
}

public class ConsumerWaitSignal extends AbstractConsumer {
    ConsumerWaitSignal(Queue queue, String name){
        super(queue, name);
    }

    protected void handle(){
        String str;
        LockSignal.getLock().lock();
        try {
            while (queue.isEmpty()) {
                System.out.println("queue is empty," + name + " wait.");
                try {
                    LockSignal.getNoEmptyCondition().await();
                } catch (Exception e) {
                    LockSignal.getNoEmptyCondition().signal();
                }
            }
            str = queue.poll();
            System.out.println(name + " handle " + str);
            LockSignal.getNoFullCondition().signal();   //只唤醒生产者
        }finally {
            LockSignal.getLock().unlock();
        }
    }
}

public class ProducerWaitSignal extends AbstractProducer {
    ProducerWaitSignal(Queue queue, String name){
        super(queue,name);
    }

    @Override
    protected void create(){
        a++;
        String str = a+"";
        LockSignal.getLock().lock();
        try {
            while (isFull(queue)) {
                System.out.println("queue is full," + name + " wait");
                try {
                    LockSignal.getNoFullCondition().await();
                } catch (Exception e) {
                    LockSignal.getNoFullCondition().signal();
                }
            }
            queue.add(str);
            System.out.println(name + " create " + str);
            LockSignal.getNoEmptyCondition().signal();//只唤醒消费者
        }finally {
            LockSignal.getLock().unlock();
        }
    }
}

public class Main {
    public static void main(String[] args) {
        Queue queue = new ArrayDeque<>();
        //创建两个生产者，两个消费者
        Thread producer1 = new Thread(new ProducerWaitSignal(queue,"p1"));
        Thread producer2 = new Thread(new ProducerWaitSignal(queue,"p2"));

        Thread consumer1 = new Thread(new ConsumerWaitSignal(queue,"c1"));
        Thread consumer2 = new Thread(new ConsumerWaitSignal(queue,"c2"));

        consumer1.start();
        consumer2.start();
        producer1.start();
        producer2.start();
    }
}

以上内容如果不足欢迎指出