I have a very simple springboot application as follow :

import org.springframework.web.bind.annotation.*;

import java.util.LinkedList;
import java.util.Queue;

@RestController
public class QuestionController {

    private Queue<String> queue = new LinkedList<>();

    @PostMapping("/api/foos")
    @ResponseBody
    public String addFoo(@RequestParam(name = "id") String fooId) {
        queue.add(fooId);
        return "ID: " + fooId + " " + queue.size();
    }

    @GetMapping("/api/get")
    @ResponseBody
    public String getFoo() {
        return queue.peek() + " " + queue.size();
    }

}

There is a queue in this web service.
The post method will add something to the queue
The get method will remove something from the queue.
I have assurance the number of get is greater than the number of post.

I would like to prevent the app from shutting down if there is still something in the queue!

What I tried:
From Springboot 2.3, there is a property for graceful shutdown.

However, after tring setting the property server.shutdown=graceful to true, the app could still be shut with something in the queue.

Question:

How to prevent the app from shutting down if there is still something inside the queue, while utilizing the concept of the graceful shutdown.