如何在一些子线程完成之前返回一个Java线程？

public class Main {
    public static void main(String[] args) {
        Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("子线程1执行完成");
            }
        });

        Thread thread2 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("子线程2执行完成");
            }
        });

        thread1.start();
        thread2.start();

        try {
            thread1.join();
            thread2.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("所有子线程执行完成，主线程继续执行");
    }
}