面试题：JAVA多线程交替打印ABC

JAVA实现，3个线程交替A,B,C，一共完成10次“ABC”打印，结束后打印“END”。

打印示例：

abc
abc
abc
abc
abc
abc
abc
abc
abc
abc
END

分析：

打印10次ABC，3个线程分别打印A,B,C字符，打印完唤醒下一个线程，需要使用锁在子线程间精确唤醒控制，synchronized + 3个对象锁，或ReentrantLock+3个Condition，效果一样。

打印结束时打印END，如果由主线程打印，则可以用CountDownLatch进行主、子线程同步控制。

示例代码：

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

public class PrintABC extends Thread {
    private static ReentrantLock lock = new ReentrantLock();
    private static int count = 10;
    private static CountDownLatch cdl = new CountDownLatch(3);

    private Condition cur;
    private Condition next;
    private char ch;

    public PrintABC(Condition cur, Condition next, char ch) {
        this.cur = cur;
        this.next = next;
        this.ch = ch;
    }

    @Override
    public void run() {
        lock.lock();
        try {
            for (int i = 1; i <= count; ++i) {
                System.out.print(ch);
                if (ch == 'c') {
                    System.out.println();
                }
                next.signal();
                if (i < count) {
                    try {
                        cur.await();
                    } catch (Exception e) {
                        //ignore
                    }
                }
            }
        } finally {
            lock.unlock();
        }
        cdl.countDown();
    }

    public static void main(String[] args) throws Exception {
        Condition a = lock.newCondition();
        Condition b = lock.newCondition();
        Condition c = lock.newCondition();

        new PrintABC(a, b, 'a').start();
        new PrintABC(b, c, 'b').start();
        new PrintABC(c, a, 'c').start();

        cdl.await();
        System.out.println("END");
    }
}