Java CompletableFuture 使用问题？大佬帮忙看看

正常 main 方法执行完之后会退出：Process finished with exit code 0

但是我执行下边的代码，主线程阻塞了，为什么不退出呢？执行结果如下：

执行 step 1
主流程 0.5 完了
主流程完了
执行 step 2
step1 result , step2 result
执行 step 3
step3 result

代码在这里：

public static void main(String[] args) {
        ExecutorService executor1 = Executors.newFixedThreadPool(5);
        CompletableFuture<String> cf1 = CompletableFuture.supplyAsync(() -> {
            System.out.println("执行 step 1");
            return "step1 result";
        }, executor1);
        CompletableFuture<String> cf2 = CompletableFuture.supplyAsync(() -> {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
            System.out.println("执行 step 2");
            return "step2 result";
        }, executor1);
        System.out.println("主流程 0.5 完了");

        cf1.thenCombine(cf2, (result1, result2) -> {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
            System.out.println(result1 + " , " + result2);
            System.out.println("执行 step 3");
            return "step3 result";
        }).thenAccept(result3 -> System.out.println(result3));


        System.out.println("主流程完了");

    }