3、JUC：生产者和消费者问题

3.1、Sychronized版

package com.chen.pc;

/**
 * 线程交替的通信问题，生产者和消费者问题! 等待唤醒，通知唤醒
 * 线程交替执行 A B 操作同一个变量 num = 0
 * A num + 1
 * B num - 1
 */
public class A {
    public static void main(String[] args) {
        Data data = new Data();
        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.increment();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"A").start();

        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.decrement();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"B").start();
    }
}
// 判断等待，业务，通知
class Data{
    private int number = 0;

    // +1
    public synchronized void increment() throws InterruptedException {
        if (number != 0){
            // 等待
            this.wait();
        }
        number ++;
        // 通知其他线程，我+
        System.out.println(Thread.currentThread().getName()+"=>"+number);
        this.notifyAll();
    }

    // -1
    public synchronized void decrement() throws InterruptedException {
        if (number == 0){
            // 等待
            this.wait();
        }
        number --;
        // 通知其他线程，我-1完毕了
        System.out.println(Thread.currentThread().getName()+"=>"+number);
        this.notifyAll();
    }

}

问题存在，A B C D 4个线程

同步代码块确不会同时进入到代码块中，但是在if语句块中用了wait()等待，导致释放了锁，当其他线程通知该线程重新恢复时，由于恢复位置在if语句块中，那么此时将直接执行，此时，如果A、C都在等待中被相继唤醒时，他们会执行各自下面的+1，也就出现了问题。

哪里wait()，就会在哪里被唤醒继续执行

if改为while判断，

package com.chen.pc;

/**
 * 线程交替的通信问题，生产者和消费者问题! 等待唤醒，通知唤醒
 * 线程交替执行 A B 操作同一个变量 num = 0
 * A num + 1
 * B num - 1
 */
public class A {
    public static void main(String[] args) {
        Data data = new Data();
        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.increment();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"A").start();

        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.decrement();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"B").start();

        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.increment();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"C").start();

        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.decrement();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"D").start();

    }
}
// 判断等待，业务，通知
class Data{
    private int number = 0;

    // +1
    public synchronized void increment() throws InterruptedException {
        while (number != 0){
            // 等待
            this.wait();
        }
        number ++;
        // 通知其他线程，我+
        System.out.println(Thread.currentThread().getName()+"=>"+number);
        this.notifyAll();
    }

    // -1
    public synchronized void decrement() throws InterruptedException {
        while (number == 0){
            // 等待
            this.wait();
        }
        number --;
        // 通知其他线程，我-1完毕了
        System.out.println(Thread.currentThread().getName()+"=>"+number);
        this.notifyAll();
    }

}

3.2、JUC版的生产者和消费者问题

代码实现

package com.chen.pc;

import javax.lang.model.element.VariableElement;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

/**
 * 线程交替的通信问题，生产者和消费者问题! 等待唤醒，通知唤醒
 * 线程交替执行 A B 操作同一个变量 num = 0
 * A num + 1
 * B num - 1
 */
public class B {
    public static void main(String[] args) {
        Data2 data = new Data2();

        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.increment();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"A").start();

        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.decrement();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"B").start();

        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.increment();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"C").start();

        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    data.decrement();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"D").start();

    }
}

// 判断等待，业务，通知
class Data2{
    private int number = 0;

    Lock lock = new ReentrantLock();
    Condition condition = lock.newCondition();
    // condition.await(); 等待
    // condition.signalAll(); 唤醒全部

    // +1
    public void increment() throws InterruptedException {
        lock.lock();
        try {
            while (number != 0){
                // 等待
                condition.await();
            }
            number ++;
            // 通知其他线程，我+
            System.out.println(Thread.currentThread().getName()+"=>"+number);
            condition.signalAll();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    // -1
    public void decrement() throws InterruptedException {
        lock.lock();
        try {
            while (number == 0){
                // 等待
                condition.await();
            }
            number --;
            // 通知其他线程，我-1完毕了
            System.out.println(Thread.currentThread().getName()+"=>"+number);
            condition.signalAll();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

}

3.3、Condition 精准的通知和唤醒线程

代码测试

// A执行完调用B，B执行完调用C，C执行完调用A
public class C {

    public static void main(String[] args) {

        Data3 data3 = new Data3();

        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                data3.printA();
            }
        },"A").start();
        
        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                data3.printB();
            }
        },"B").start();
        
        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                data3.printC();
            }
        },"C").start();
    }

}

class Data3{ // 资源类 Lock

    private Lock lock = new ReentrantLock();
    // 通过监视器判断唤醒哪一个
    Condition condition1 = lock.newCondition();
    Condition condition2 = lock.newCondition();
    Condition condition3 = lock.newCondition();

    private int number = 1; // 1A 2B 3C

    public void printA(){
        lock.lock();
        try {
            // 业务，判断->执行->通知
            while (number != 1){
                condition1.await();
            }
            System.out.println(Thread.currentThread().getName()+"--->A");
            // 唤醒 唤醒指定的人 B
            number = 2;
            condition2.signal();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
    public void printB(){
        lock.lock();
        try {
            // 业务，判断->执行->通知
            while (number != 2){
                condition2.await();
            }
            // 唤醒，唤醒指定的人 C
            System.out.println(Thread.currentThread().getName()+"--->C");
            number = 3;
            condition3.signal();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
    public void printC(){
        lock.lock();
        try {
            // 业务，判断->执行->通知
            while (number != 3){
                condition3.await();
            }
            // 唤醒，唤醒指定的人 C
            System.out.println(Thread.currentThread().getName()+"--->C");
            number = 1;
            condition1.signal();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
}

学习视频链接：https://www.bilibili.com/video/BV1B7411L7tE?p=10