借助wait和notify方法控制线程以固定的顺序执行： 

 /**
     * 控制输出字符的顺序，必须是固定顺序2,1
     * 采用wait-notify实现
     * @param args
     */
    public static void main(String[] args) {
        new Thread(() -> {
            synchronized (lock) {
                while (!isPrint2) {
                    try {
                        lock.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                log.info("1");
            }
        }, "t1").start();

        new Thread(() -> {
            synchronized (lock) {
                log.info("2");
                isPrint2 = true;
                lock.notify();
            }

        }, "t2").start();
    }
}

借助park和unpark方法控制线程以固定的顺序执行： 

@Slf4j
public class ParkOrderDemo {

    public static void main(String[] args) {
        Thread t1 = new Thread(() -> {
            LockSupport.park();
            log.info("1");
        }, "t1");
        t1.start();

        new Thread(() -> {
            log.info("2");
            LockSupport.unpark(t1);
        }, "t2").start();
    }
}