JUC学习笔记（三）

JUC学习笔记（一）https://www.cnblogs.com/lm66/p/15118407.html

JUC学习笔记（二）https://www.cnblogs.com/lm66/p/15118813.html

1、线程间通信

线程间通信的模型有两种：共享内存和消息传递，以下方式都是基本这两种模型来实现的。我们来基本一道面试常见的题目来分析

场景---两个线程，一个线程对当前数值加 1，另一个线程对当前数值减 1,要求用线程间通信

1.1、synchronized 方案

public class ThreadDemo1 {

    public static void main(String[] args) {
        Share share = new Share();

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

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

}

class Share {

    private int num = 0;

    public synchronized void increment() throws Exception {
        while (num != 0) {
            this.wait();
        }
        num++;
        System.out.println(Thread.currentThread().getName() + "::" + num);
        this.notifyAll();

    }

    public synchronized void decrement() throws Exception {
        while (num != 1) {
            this.wait();
        }
        num--;
        System.out.println(Thread.currentThread().getName() + "::" + num);
        this.notifyAll();
    }
}

1.2、Lock方案

public class ThreadDemo2 {

    public static void main(String[] args) {
        Share2 share = new Share2();

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

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

}

class Share2 {

    private Lock lock = new ReentrantLock();
    private Condition condition = lock.newCondition();

    private int num = 0;

    public void increment() throws Exception {
        lock.lock();
        try {
            while (num != 0) {
                condition.await();
            }
            num++;
            System.out.println(Thread.currentThread().getName() + "::" + num);
            condition.signalAll();
        } finally {
            lock.unlock();
        }

    }

    public void decrement() throws Exception {
        lock.lock();
        try {
            while (num != 1) {
                condition.await();
            }
            num--;
            System.out.println(Thread.currentThread().getName() + "::" + num);
            condition.signalAll();
        } finally {
            lock.unlock();
        }
    }
}

1.3、线程间定制化通信

1.3.1、案例介绍

问题: A 线程打印 5 次 A，B 线程打印 10 次 B，C 线程打印 15 次 C,按照此顺序循环 10 轮

1.3.2、实现流程

public class ThreadDemo3 {

    public static void main(String[] args) {
        ShareResource shareResource = new ShareResource();
        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    shareResource.print5(i);
                } catch (Exception e) {
                }
            }
        },"AA").start();
        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    shareResource.print10(i);
                } catch (Exception e) {
                }
            }
        },"BB").start();
        new Thread(()->{
            for (int i = 0; i < 10; i++) {
                try {
                    shareResource.print15(i);
                } catch (Exception e) {
                }
            }
        },"CC").start();
    }
}

class ShareResource {
    // 定义标志位
    private int flag = 1;
    // 创建Lock锁
    private Lock lock = new ReentrantLock();
    private Condition c1 = lock.newCondition();
    private Condition c2 = lock.newCondition();
    private Condition c3 = lock.newCondition();

    public void print5(int loop) throws Exception {
        lock.lock();
        try {
            // 判断。防止虚假唤醒
            while (flag != 1) {
                // 等待
                c1.await();
            }
            for (int i = 0; i < 5; i++) {
                System.out.println(Thread.currentThread().getName() + "::" + loop);
            }
            // 修改标志位
            flag = 2;
            // 通知
            c2.signal();
        } finally {
            lock.unlock();
        }
    }

    public void print10(int loop) throws Exception {
        lock.lock();
        try {
            while (flag != 2) {
                c2.await();
            }
            for (int i = 0; i < 10; i++) {
                System.out.println(Thread.currentThread().getName() + "::" + loop);
            }
            flag = 3;
            c3.signal();
        } finally {
            lock.unlock();
        }
    }

    public void print15(int loop) throws Exception {
        lock.lock();
        try {
            while (flag != 3) {
                c3.await();
            }
            for (int i = 0; i < 15; i++) {
                System.out.println(Thread.currentThread().getName() + "::" + loop);
            }
            flag = 1;
            c1.signal();
        } finally {
            lock.unlock();
        }
    }
}