一道面试题：多个线程按顺序输出

　　题目：编写一个程序，开启 3 个线程，这三个线程的 ID 分别为 A、B、C，每个线程将自己的 ID 在屏幕上打印 10 遍，要求输出的结果必须按顺序显示。如：ABCABCABC……
　　
　　解题源码：

package concurrent;import java.util.concurrent.locks.Condition;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantLock;class AlternateDemo {    //标志当前由哪一个线程输出，1代表A，2代表B，3代表C    private int number = 1;    Lock lock = new ReentrantLock();    //Condition的强大之处在于它可以为多个线程间建立不同的Condition    Condition condition1 = lock.newCondition();    Condition condition2 = lock.newCondition();    Condition condition3 = lock.newCondition();    // loopNum:当前循环轮数    public void loopA(int loopNum) {        //上锁        lock.lock();        try {            while (number != 1) {                //等待                condition1.await();            }            System.out.println(Thread.currentThread().getName() + ", currentLoopNum is " + loopNum);            number = 2;            //唤醒            condition2.signal();        } catch (InterruptedException e) {            e.printStackTrace();        } finally {            //确保释放锁            lock.unlock();        }    }    public void loopB(int loopNum) {        lock.lock();        try {            while (number != 2) {                condition2.await();            }            System.out.println(Thread.currentThread().getName() + ", currentLoopNum is " + loopNum);            number = 3;            condition3.signal();        } catch (InterruptedException e) {            e.printStackTrace();        } finally {            lock.unlock();        }    }    public void loopC(int loopNum) {        lock.lock();        try {            while (number != 3) {                condition3.await();            }            System.out.println(Thread.currentThread().getName() + ", currentLoopNum is " + loopNum);            number = 1;            condition1.signal();        } catch (InterruptedException e) {            e.printStackTrace();        } finally {            lock.unlock();        }    }}public class TestABCAlternate {    public static void main(String[] args) {        AlternateDemo ad = new AlternateDemo();        new Thread(new Runnable() {            @Override            public void run() {                for (int i = 0; i < 10; i++)                    ad.loopA(i);            }        }, "A").start();        new Thread(new Runnable() {            @Override            public void run() {                for (int i = 0; i < 10; i++)                    ad.loopB(i);            }        }, "B").start();        new Thread(new Runnable() {            @Override            public void run() {                for (int i = 0; i < 10; i++)                    ad.loopC(i);            }        }, "C").start();    }}

　　这道题充分体现了Condition的强大之处：它可以为多个线程间建立不同的Condition，并指定唤醒哪一个线程。