指定多个线程按顺序循环执行

import java.util.concurrent.Semaphore;
public class test {

    private static Semaphore s1 = new Semaphore(1);
    private static Semaphore s2 = new Semaphore(0);
    private static Semaphore s3 = new Semaphore(0);
    private static int num = 6;
    static Thread t1 = new Thread(new Runnable() {
        private int sum = 0;
        public void run() {
            while (sum < num) {
                try {
                    s1.acquire();
                    System.out.print("A");
                    sum ++;
                } catch (Exception e) {
                    e.printStackTrace();
                }finally {
                    s2.release();
                }
            }
        }
    });
    static  Thread t2 = new Thread(new Runnable() {
        private int sum = 0;
        public void run() {
            while (sum < num) {
                try {
                    s2.acquire();
                    System.out.print("B");
                    sum ++ ;
                } catch (Exception e) {
                    e.printStackTrace();
                }finally {
                    s3.release();
                }
            }
        }
    });
    static Thread t3 = new Thread(new Runnable() {
        private int sum = 0;
        public void run() {
            while (sum < num) {
                try {
                    s3.acquire();
                    System.out.println("C");
                    sum++;
                } catch (Exception e) {
                    e.printStackTrace();
                }finally {
                    s1.release();
                }
            }
        }
    });
}