java三线程循环有序打印ABC

迅雷笔试题：

编写一个程序，开启3个线程，这3个线程的ID分别为A、B、C，每个线程将自己的ID在屏幕上打印10遍，要求输出结果必须按ABC的顺序显示；如：ABCABC….依次递推。

解决思路：每个线程运行时先检查他依赖的线程是否已完成工作，线程B依赖线程A的完成，线程C依赖线程B和线程A的完成，线程A依赖线程C的完成。如果当前线程依赖的线程没有执行完，则阻塞当前线程直到条件满足再执行。

Condition.await()会使当前线程暂时阻塞，并释放ReentrantLock锁.

Condition.signalAll()会通知激活Condition.await()而阻塞的线程，这时被激活的线程就会继续检查（通过while循环））是否满足条件，如果满足而且再次获得ReentrantLock锁就能继续运行，否则继续等待。

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

public class ThreadDemo implements Runnable {

    private ReentrantLock lock = new ReentrantLock();
    private Condition condition = lock.newCondition();
    //标记线程A的状态，true为刚执行完。为什么要两个变量？因为只使用一个变量，当线程A执行完后，a为true，B线程就会不停执行
    //所以线程B执行要执行必须满足a==true&&a2==true
    private volatile Boolean a = false, a2 = false;
    //标记线程B的状态，true为刚执行完。
    private volatile Boolean b = false;
    //标记线程C的状态，true为刚执行完。
    private volatile Boolean c = true;

    @Override
    public void run() {
        String name = Thread.currentThread().getName();
        lock.lock();
        //进入临界区
        try {
            for (int i = 0; i < 10; i++) {
                if (name.equals("B")) {
                    //只有a和a2同时为true时才打印B，否则阻塞当前线程
                    while (!a || !a2) {
                        condition.await();//条件不满足，暂时阻塞线程，暂时释放lock
                    }
                    b = true;
                    a2 = false;
                } else if (name.equals("C")) {
                    while (!a || !b) {
                        condition.await();
                    }
                    c = true;
                    b = false;
                } else if (name.equals("A")) {
                    while (!c) {
                        condition.await();
                    }
                    a = true;
                    a2 = true;
                    b = false;
                    c = false;
                }
                System.out.print(name);
                condition.signalAll();//通知正在等待的线程，此时有可能已经满足条件
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();// 记得要释放锁
        }
    }

    public static void main(String[] args) throws InterruptedException {
        ThreadDemo task = new ThreadDemo();
        Thread thread1 = new Thread(task);
        Thread thread2 = new Thread(task);
        Thread thread3 = new Thread(task);
        thread1.setName("A");
        thread2.setName("B");
        thread3.setName("C");
        thread1.start();
        thread2.start();
        thread3.start();
    }

}

 下面是更简单的实现方法：

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

public class ThreadDemo implements Runnable {

    private ReentrantLock lock = new ReentrantLock();
    private Condition condition = lock.newCondition();
    private int state = 0;

    @Override
    public void run() {
        String name = Thread.currentThread().getName();
        lock.lock();
        // 进入临界区
        try {
            for (int i = 0; i < 10; i++) {
                if (name.equals("B")) {
                    // 只有a和a2同时为true时才打印B，否则阻塞当前线程
                    while (state % 3 != 1) {
                        condition.await();// 条件不满足，暂时阻塞线程，暂时释放lock
                    }
                } else if (name.equals("C")) {
                    while (state % 3 != 2) {
                        condition.await();
                    }
                } else if (name.equals("A")) {
                    while (state % 3 != 0) {
                        condition.await();
                    }
                }
                state++;
                System.out.print(name);
                condition.signalAll();// 通知正在等待的线程，此时有可能已经满足条件
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();// 记得要释放锁
        }
    }

    public static void main(String[] args) throws InterruptedException {
        ThreadDemo task = new ThreadDemo();
        Thread thread1 = new Thread(task);
        Thread thread2 = new Thread(task);
        Thread thread3 = new Thread(task);
        thread1.setName("A");
        thread2.setName("B");
        thread3.setName("C");
        thread1.start();
        thread2.start();
        thread3.start();
    }

}