生产者和消费者问题

面试：单例模式，排序算法，生产者和消费者，死锁

• Synchronized版

  Synchronized 加锁 ； wait 等待； notify 唤醒；

/**
     * 线程之间的週信题：生产者和消赏者问题！等待唤，知唤
     * 线程交替执行 A B操作问一个变量num=0
     * A 生产num+1
     * B 消费num-1
     */
public class Test01 {
    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();
    }
}

//等待，业务，通知
class Data{
    private int num = 0;
    //+1
    public synchronized void increment() throws InterruptedException {
        if(num != 0){
            //等待线程
            this.wait();
        }
        num++;
        //通知其他线程，+1完毕
        System.out.println(Thread.currentThread().getName() + "->" + num);
        this.notifyAll();
    }
    //-1
    public synchronized void decrement() throws InterruptedException {
        if(num == 0){
            //等待线程
            this.wait();
        }
        num--;
        //通知其他线程，-1完毕
        System.out.println(Thread.currentThread().getName() + "->" + num);
        this.notifyAll();
    }
}

问题存在，现在是A，B线程，没有发现问题，如果有A,B,C,D四个线程，多一个加减线程，就会出现问题

问题原因：

线程会出现虚假唤醒的情况，线程等待应出现在while循环中，不是if判断中

public class Test01 {
    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.decrement();
                } 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();
    }
}

//等待，业务，通知
class Data{
    private int num = 0;
    //+1
    public synchronized void increment() throws InterruptedException {
        while(num != 0){
            //等待线程
            this.wait();
        }
        num++;
        //通知其他线程，+1完毕
        System.out.println(Thread.currentThread().getName() + "->" + num);
        this.notifyAll();
    }
    //-1
    public synchronized void decrement() throws InterruptedException {
        while(num == 0){
            //等待线程
            this.wait();
        }
        num--;
        //通知其他线程，-1完毕
        System.out.println(Thread.currentThread().getName() + "->" + num);
        this.notifyAll();
    }
}

• JUC版的生产者和消费者问题

通过Lock 找到 Condition

Lock lock = new ReentrantLock();
Condition condition = lock.newCondition();
//加锁
lock.lock();
//释放锁
lock.unlock();
//等待线程
condition.await();
//通知其他线程
condition.signalAll();

public class Test01 {
    /**
     * 线程之间的週信题：生产者和消赏者问题！等待唤，知唤
     * 线程交替执行 A B操作问一个变量num=0
     * A num+1
     * B num-1
     */
    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.decrement();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "C").start();
        new Thread(() -> {
            for (int i = 0; i < 10; i++) {
                try {
                    data.increment();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "D").start();
    }
}

//等待，业务，通知
class Data {
    private int num = 0;
    Lock lock = new ReentrantLock();
    Condition condition = lock.newCondition();

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

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

//随机分布
A->1
B->0
A->1
B->0
A->1
B->0
A->1
B->0
A->1
B->0
A->1
B->0
A->1
B->0
A->1
B->0
A->1
B->0
A->1
B->0
D->1
C->0
D->1

任何一个新的技术，绝对不是仅仅只是覆盖了原来的技术，优势和补充！

Condition 精准的通知和唤醒线程：可以有序执行线程

public class Test01 {
    /**
     * Condition 精准的通知和唤醒线程A B C
     */
    public static void main(String[] args) {
        Data data = new Data();
        new Thread(() -> {
            for (int i = 0; i < 10; i++) {
                try {
                    data.note1();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "A").start();
        new Thread(() -> {
            for (int i = 0; i < 10; i++) {
                try {
                    data.note2();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "B").start();
        new Thread(() -> {
            for (int i = 0; i < 10; i++) {
                try {
                    data.note3();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "C").start();
    }
}

//等待，业务，通知
class Data {
    private int num = 1;
    Lock lock = new ReentrantLock();
    Condition condition1 = lock.newCondition();
    Condition condition2 = lock.newCondition();
    Condition condition3 = lock.newCondition();

    public void note1() throws InterruptedException {
        lock.lock();
        try {
            while (num != 1) {
                //等待线程
                condition1.await();
            }
            System.out.println(Thread.currentThread().getName() + "->" + num);
            num = 2;
            //唤醒指定的人 B
            condition2.signal();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public void note2() throws InterruptedException {
        lock.lock();
        try {
            while (num != 2) {
                //等待线程
                condition2.await();
            }
            System.out.println(Thread.currentThread().getName() + "->" + num);
            //唤醒指定的人 C
            num = 3;
            condition3.signal();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public void note3() throws InterruptedException {
        lock.lock();
        try {
            while (num != 3) {
                //等待线程
                condition3.await();
            }
            System.out.println(Thread.currentThread().getName() + "->" + num);
            //唤醒指定的人 A
            num = 1;
            condition1.signal();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
}
//线程顺序执行，比如生产线 ： 下单 -》 支付 -》交易 -》物流
A->1
B->2
C->3
A->1
B->2
C->3
A->1
B->2
C->3
A->1
B->2
C->3