启动三个ID号为ABC的线程，循环输出线程ID号3次

题目：有3个线程，ID号分别为ABC，启动这3个线程，使其输出它的ID号3次，ABCABCABC。

思路一  要按照顺序输出ABC，循环3次，就要控制三个线程的同步，也就是说要让三个线程轮流输出，直到3个ABC全部输出则结束线程。这里可以使用Lock对象来控制三个线程的同步，用一个int型变量state标识由哪个线程输出。

public class ThreadTest {
    private static int state = 0;
    private static final Lock lock = new ReentrantLock();

    public static void main(String[] args) throws Exception {
        Thread A = new Thread(new Runnable() {
            @Override
            public void run() {
                while (state <= 8) {
                    lock.lock();
                    //再进行一次 state <= 8的判断
                    if (state % 3 == 0 && state <= 8) {
                        System.out.println(Thread.currentThread().getName() + ":" + state);
                        state++;
                    }
                    lock.unlock();
                }
            }
        }, "A");

        Thread B = new Thread(new Runnable() {
            @Override
            public void run() {
                while (state <= 8) {
                    lock.lock();
                    if (state % 3 == 1 && state <= 8) {
                        System.out.println(Thread.currentThread().getName() + ":" + state);
                        state++;
                    }
                    lock.unlock();
                }
            }
        }, "B");

        Thread C = new Thread(new Runnable() {
            @Override
            public void run() {
                while (state <= 8) {
                    lock.lock();
                    if (state % 3 == 2 && state <= 8) {
                        System.out.println(Thread.currentThread().getName() + ":" + state);
                        state++;
                    }
                    lock.unlock();
                }
            }
        }, "C");
        A.start();
        B.start();
        C.start();
    }
}

思路二  上述思路中引入了一个state来控制轮到谁来执行，同时也要保证在输出打印线程名之前再次判断state <= 8，防止打印次数超出。我们也可以给每个线程指定信号量Semaphore来控制输出的顺序，实现代码如下：

public class ThreadTest {
    private static Semaphore semaphoreA = new Semaphore(1);
    private static Semaphore semaphoreB = new Semaphore(1);
    private static Semaphore semaphoreC = new Semaphore(1);

    public static void main(String[] args) throws Exception {
        Thread A = new Thread(new Runnable() {
            @Override
            public void run() {
                for (int state = 0; state < 3; state++) {
                    try {
                        semaphoreA.acquire();
                        System.out.println(Thread.currentThread().getName() + ":" + state);
                        semaphoreB.release();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }, "A");

        Thread B = new Thread(new Runnable() {
            @Override
            public void run() {
                for (int state = 0; state < 3; state++) {
                    try {
                        semaphoreB.acquire();
                        System.out.println(Thread.currentThread().getName() + ":" + state);
                        semaphoreC.release();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }, "B");

        Thread C = new Thread(new Runnable() {
            @Override
            public void run() {
                for (int state = 0; state < 3; state++) {
                    try {
                        semaphoreC.acquire();
                        System.out.println(Thread.currentThread().getName() + ":" + state);
                        semaphoreA.release();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }, "C");

        semaphoreB.acquire();
        semaphoreC.acquire();
        A.start();
        B.start();
        C.start();
    }
}