1. 指定线程执行顺序

可以使用join()方法（但中间加了Thread.sleep(1000)以后就不按顺序执行，不知道为什么）

public static void main(String[] args) throws CloneNotSupportedException {
    // 创建线程对象
    Thread t1 = new Thread(() -> {
        System.out.println("t1");
    });
    Thread t2 = new Thread(() -> {
        try {
            t1.join();
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
        System.out.println("t2");
    });
    Thread t3 = new Thread(() -> {
        try {
            t2.join();
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
        System.out.println("t3");
    });
    // 启动线程，启动顺序无所谓，因为有join的约束
    t3.start();
    t2.start();
    t1.start();
}

2.使用LOCK.wait()和LOCK.notify()

public static void main(String[] args) throws CloneNotSupportedException, InterruptedException {
    Object LOCK = new Object();
    Thread t1 = new Thread(() -> {
        System.out.println("t1");
        synchronized (LOCK) {
            LOCK.notify();
        }
    });
    Thread t2 = new Thread(() -> {
        try {
            synchronized (LOCK) {
                LOCK.wait();
            }
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
        System.out.println("t2");
    });
    t2.start();
    Thread.sleep(1000);
    t1.start();
}