前言

这个问题在面试中经常被遇到，主要考察对线程的理解，以及多线程如何轮流占有等问题

关于该问题的一些相关知识可看我之前的文章
JUC高并发编程从入门到精通（全）

以及关于Thread的用法解析可看我之前的文章
java之Thread类详细分析（全）
java之Thread类实战模板（全）

线程间定制化通信

所谓定制化通信，需要让线程进行一定的顺序操作

案列：启动三个线程，按照如下要求：
AA打印5此，BB打印10次，CC打印15次，一共进行10轮

具体思路：
每个线程添加一个标志位，是该标志位则执行操作，并且修改为下一个标志位，通知下一个标志位的线程

创建一个可重入锁private Lock lock = new ReentrantLock();
分别创建三个开锁通知private Condition c1 = lock.newCondition();

具体资源类中的A线程代码操作
上锁，（执行具体操作（判断、操作、通知），解锁）放于try、finally

public void print5(int loop) throws InterruptedException {
    //上锁
    lock.lock();
    try {
        //判断
        while(flag != 1) {
            //等待
            c1.await();
        }
        //干活
        for (int i = 1; i <=5; i++) {
            System.out.println(Thread.currentThread().getName()+" :: "+i+" ：轮数："+loop);
        }
        //通知
        flag = 2; //修改标志位 2
        c2.signal(); //通知BB线程
    }finally {
        //释放锁
        lock.unlock();
    }
}

资源类的其他线程操作也同理

具体完整代码

//第一步 创建资源类
class ShareResource {
    //定义标志位
    private int flag = 1;  // 1 AA     2 BB     3 CC

    //创建Lock锁
    private Lock lock = new ReentrantLock();

    //创建三个condition
    private Condition c1 = lock.newCondition();
    private Condition c2 = lock.newCondition();
    private Condition c3 = lock.newCondition();

    //打印5次，参数第几轮
    public void print5(int loop) throws InterruptedException {
        //上锁
        lock.lock();
        try {
            //判断
            while(flag != 1) {
                //等待
                c1.await();
            }
            //干活
            for (int i = 1; i <=5; i++) {
                System.out.println(Thread.currentThread().getName()+" :: "+i+" ：轮数："+loop);
            }
            //通知
            flag = 2; //修改标志位 2
            c2.signal(); //通知BB线程
        }finally {
            //释放锁
            lock.unlock();
        }
    }

    //打印10次，参数第几轮
    public void print10(int loop) throws InterruptedException {
        lock.lock();
        try {
            while(flag != 2) {
                c2.await();
            }
            for (int i = 1; i <=10; i++) {
                System.out.println(Thread.currentThread().getName()+" :: "+i+" ：轮数："+loop);
            }
            //修改标志位
            flag = 3;
            //通知CC线程
            c3.signal();
        }finally {
            lock.unlock();
        }
    }

    //打印15次，参数第几轮
    public void print15(int loop) throws InterruptedException {
        lock.lock();
        try {
            while(flag != 3) {
                c3.await();
            }
            for (int i = 1; i <=15; i++) {
                System.out.println(Thread.currentThread().getName()+" :: "+i+" ：轮数："+loop);
            }
            //修改标志位
            flag = 1;
            //通知AA线程
            c1.signal();
        }finally {
            lock.unlock();
        }
    }
}

public class ThreadDemo3 {
    public static void main(String[] args) {
        ShareResource shareResource = new ShareResource();
        new Thread(()->{
            for (int i = 1; i <=10; i++) {
                try {
                    shareResource.print5(i);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"AA").start();

        new Thread(()->{
            for (int i = 1; i <=10; i++) {
                try {
                    shareResource.print10(i);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"BB").start();

        new Thread(()->{
            for (int i = 1; i <=10; i++) {
                try {
                    shareResource.print15(i);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"CC").start();
    }
}

代码截图

为了更好的铺垫，我们看其他的例子，如果是ABC各打印相关次数

正文

1. 通过synchronized的wait与notifyAll

public class test {
    private static String message = "A";
    private static Object lock = new Object();

    public static void main(String[] args) {

        new Thread(() -> {
            int count = 1;
            synchronized (lock) {
                while (count < 101) {

                    while (!message.equals("A")) {
                        try {
                            lock.wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    System.out.println(message);
                    message = "B";
                    count = count + 1;
                    lock.notifyAll();
                }
            }

        },"A").start();

        new Thread(() -> {
            int count = 1;
            synchronized (lock) {
                while (count < 101) {
                    while (!message.equals("B")) {
                        try {
                            lock.wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    System.out.println(message);
                    message = "C";
                    count = count + 1;
                    lock.notifyAll();
                }
            }

        },"B").start();
        new Thread(() -> {
            int count = 1;
            synchronized (lock) {
                while (count < 101) {
                    while (!message.equals("C")) {
                        try {
                            lock.wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    System.out.println(message);
                    message = "A";
                    count = count + 1;
                    lock.notifyAll();
                }
            }

        },"C").start();


    }
}

2. 通过线程不加锁定义状态变量

public class test {
    private static String message = "A";

    public static void main(String[] args) {

        new Thread(() -> {
            int count = 1;
            while (count < 101) {
                while (!message.equals("A")) {
                    try {
                        Thread.sleep(10);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
                System.out.println(message);
                message = "B";
                count = count + 1;
            }

        },"A").start();

        new Thread(() -> {
            int count = 1;
            while (count < 101) {
                while (!message.equals("B")) {
                    try {
                        Thread.sleep(10);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
                System.out.println(message);
                message = "C";
                count = count + 1;
            }

        },"B").start();
        new Thread(() -> {
            int count = 1;
            while (count < 101) {
                while (!message.equals("C")) {
                    try {
                        Thread.sleep(10);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
                System.out.println(message);
                message = "A";
                count = count + 1;
            }

        },"C").start();



    }
}

3. 通过ReentrantLock的lock以及unlock

对于轮流打印有多种方式进行输出
也参考了网上的一些资料说明
友情链接：三线程按顺序交替打印ABC的四种方法

关于以上方法具体代码示例如下：

• try加锁，finally解锁
• 设置状态变量，通过每个线程的状态变量判断（先加锁，在判断状态变量，在解锁）
• ThreadA extends Thread之后重写run方法
• 主函数 调用的时候通过new ThreadA().start();

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class test {
    static Lock lock = new ReentrantLock();// 通过JDK5中的Lock锁来保证线程的访问的互斥
    private static int state = 1;//通过state的值来确定是否打印

    static class ThreadA extends Thread {
        @Override
        public void run() {
            for (int i = 0; i < 100;) {
                try {
                    lock.lock();
                    while (state % 3 == 1) {// 多线程并发，不能用if，必须用循环测试等待条件，避免虚假唤醒
                        System.out.println("A");
                        state++;
                        i++;
                    }
                } finally {
                    lock.unlock();// unlock()操作必须放在finally块中
                }
            }
        }
    }
    static class ThreadB extends Thread {
        @Override
        public void run() {
            for (int i = 0; i < 100;) {
                try {
                    lock.lock();
                    while (state % 3 == 2) {
                        System.out.println("B");
                        state++;
                        i++;
                    }
                } finally {
                    lock.unlock();// unlock()操作必须放在finally块中
                }
            }
        }
    }
    static class ThreadC extends Thread {
        @Override
        public void run() {
            for (int i = 0; i < 10;) {
                try {
                    lock.lock();
                    while (state % 3 == 0) {
                        System.out.println("C");
                        state++;
                        i++;

                    }
                } finally {
                    lock.unlock();// unlock()操作必须放在finally块中
                }
            }
        }
    }
    public static void main(String[] args) {
        new ThreadA().start();
        new ThreadB().start();
        new ThreadC().start();
    }
}

4. ReentrantLock结合Condition

对于以下代码主要参考了如下资料：
三线程按顺序交替打印ABC的四种方法

Condition是被绑定到Lock上的，必须使用lock.newCondition()才能创建一个Condition

上了锁之后，才可以在锁内部中进行操作
Condition A = lock.newCondition();，在状态量不为0的时候，锁一直等待阻塞A.await()，之后抢到了之后，通过B唤醒A的线程，使用B.signal();

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class test {
    private static Lock lock = new ReentrantLock();
    private static Condition A = lock.newCondition();
    private static Condition B = lock.newCondition();
    private static Condition C = lock.newCondition();
    private static int count = 0;
    static class ThreadA extends Thread {
        @Override
        public void run() {
            try {
                lock.lock();
                for (int i = 0; i < 100; i++) {
                    while (count % 3 != 0)//注意这里是不等于0，也就是说在count % 3为0之前，当前线程一直阻塞状态
                        A.await(); // A释放lock锁
                    System.out.print("A");
                    count++;
                    B.signal(); // A执行完唤醒B线程
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                lock.unlock();
            }
        }
    }
    static class ThreadB extends Thread {
        @Override
        public void run() {
            try {
                lock.lock();
                for (int i = 0; i < 100; i++) {
                    while (count % 3 != 1)
                        B.await();// B释放lock锁，当前面A线程执行后会通过B.signal()唤醒该线程
                    System.out.print("B");
                    count++;
                    C.signal();// B执行完唤醒C线程
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                lock.unlock();
            }
        }
    }
    static class ThreadC extends Thread {
        @Override
        public void run() {
            try {
                lock.lock();
                for (int i = 0; i < 100; i++) {
                    while (count % 3 != 2)
                        C.await();// C释放lock锁，当前面B线程执行后会通过C.signal()唤醒该线程
                    System.out.print("C");
                    count++;
                    A.signal();// C执行完唤醒A线程
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                lock.unlock();
            }
        }
    }
    public static void main(String[] args) throws InterruptedException {
        new ThreadA().start();
        new ThreadB().start();
        new ThreadC().start();
    }
}

5. Semaphore信号量方式

定义的全局变量，使用static
通过static Semaphore A = new Semaphore(1);，代表信号量为1

具体其代码块为：

• semaphore.acquire(); 获取信号量

• semaphore.release();释放信号量

import java.util.concurrent.Semaphore;
public class test {
    // 以A开始的信号量,初始信号量数量为1
    private static Semaphore A = new Semaphore(1);
    // B、C信号量,A完成后开始,初始信号数量为0
    private static Semaphore B = new Semaphore(0);
    private static Semaphore C = new Semaphore(0);
    
    static class ThreadA extends Thread {
        @Override
        public void run() {
            try {
                for (int i = 0; i < 100; i++) {
                    A.acquire();// A获取信号执行,A信号量减1,当A为0时将无法继续获得该信号量
                    System.out.print("A");
                    B.release();// B释放信号，B信号量加1（初始为0），此时可以获取B信号量
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
    static class ThreadB extends Thread {
        @Override
        public void run() {
            try {
                for (int i = 0; i < 100; i++) {
                    B.acquire();
                    System.out.print("B");
                    C.release();
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
    static class ThreadC extends Thread {
        @Override
        public void run() {
            try {
                for (int i = 0; i < 100; i++) {
                    C.acquire();
                    System.out.println("C");
                    A.release();
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) throws InterruptedException {
        new ThreadA().start();
        new ThreadB().start();
        new ThreadC().start();
    }
}