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();
}
}【一道面试题（多个线程按顺序输出）】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。
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