问题描述

生产者(Productor)将产品交给店员(Clerk)，而消费者(Consumer)从店员处取走产品，店员一次只能持有固定数量的产品(比如:20），如果生产者试图生产更多的产品，店员会叫生产者停一下，如果店中有空位放产品了再通知生产者继续生产；如果店中没有产品了，店员会告诉消费者等一下，如果店中有产品了再通知消费者来取走产品。

wait()/notify()/notifyall()方法

package com.java.ht;

public class Demo1 {
    public static void main(String[] args) {
        Clerk clerk = new Clerk();
        Producer p = new Producer(clerk);
        Consumer c = new Consumer(clerk);

        new Thread(p, "生产者A").start();
        new Thread(p, "生产者B").start();
        new Thread(c, "消费者A").start();
        new Thread(c, "消费者B").start();
    }
}

class Clerk{
    private int product = 0;

    public synchronized void getProduct(){
        while(product >= 20){
            System.out.println("货架已满");
            try {
                //如果货架满了，则需要让该线程等待，生产者停止生产
                wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println(Thread.currentThread().getName() + "：" + ++product);
        //卖出货物后，需要唤醒全部线程，其他消费者可以继续消费，生产者继续生产
        notifyAll();
    }

    public synchronized void sellProduct(){
        while (product <= 0){
            System.out.println("货架上缺货");
            try {
                //如果没有货物了，消费者需要等待
                wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println(Thread.currentThread().getName() + "：" + --product);
        //消费完需要唤醒其他线程，生产需要生产，其他消费者消费
        notifyAll();
    }
}

class Producer implements Runnable{
    private Clerk clerk;

    public Producer(Clerk clerk) {
        this.clerk = clerk;
    }

    @Override
    public void run() {
        for (int i = 0; i < 20; i++) {
            try {
                Thread.sleep(200);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            clerk.getProduct();
        }

    }
}

class Consumer implements Runnable{
    private Clerk clerk;

    public Consumer(Clerk clerk) {
        this.clerk = clerk;
    }

    @Override
    public void run() {
        for (int i = 0; i < 20; i++) {
            clerk.sellProduct();
        }
    }
}

• 方法最后一定要记得唤醒线程
• wait()一定要放在循环体里面，不然可能会出现虚假唤醒

Lcok和Condition

package com.java.ht;

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class Demo2 {
    public static void main(String[] args) {
        Clerk clerk = new Clerk();
        Producer p = new Producer(clerk);
        Consumer c = new Consumer(clerk);

        new Thread(p, "生产者A").start();
        new Thread(p, "生产者B").start();
        new Thread(c, "消费者A").start();
        new Thread(c, "消费者B").start();
    }
}

class Clerk{
    private int product = 0;
    private Lock lock = new ReentrantLock();
    private Condition condition = lock.newCondition();

    public void getProduct(){
        lock.lock();
        try {
            if(product >= 20){
                //货架已满，停止生产
                System.out.println("货架已满");
                condition.await();
            } else {
                System.out.println(Thread.currentThread().getName() + "：" + ++product);
                //生产了货物，唤醒其他线程
                condition.signalAll();
            }

        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public void sellProduct(){
        lock.lock();
        try {
            if(product <= 0){
                //货架上没有货物，需要让消费者等待
                condition.await();
            } else {
                System.out.println(Thread.currentThread().getName() + "：" + --product);
                //消费者消费完成，唤醒其他线程，消费者可以继续消费，生产者可以继续生产
                condition.signalAll();
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
}

class Producer implements Runnable{
    private Clerk clerk;

    public Producer1(Clerk clerk) {
        this.clerk = clerk;
    }

    @Override
    public void run() {
        for (int i = 0; i < 20; i++) {
            try {
                Thread.sleep(200);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            clerk.getProduct();
        }
    }
}

class Consumer implements Runnable{
    private Clerk clerk;

    public Consumer1(Clerk clerk) {
        this.clerk = clerk;
    }

    @Override
    public void run() {
        for (int i = 0; i < 20; i++) {
            clerk.sellProduct();
        }
    }
}

• 记得最后使用unlock()解锁
• Condition是Lock里面的newCondition()产生的
• await(),signal(),signalAll()方法和Object类中的wait(),notify(),notifyAll()一样
• 与上一种方法比，这种的效率更高