Using springboot and mongo's repository, I defined a Comparator class and wanted to implement the comparison method of my own objects. code show as below:

package com.story.utils;

import java.util.Comparator;

import javax.annotation.Resource;

import org.springframework.stereotype.Service;

import com.story.model.Phase;
import com.story.model.Story;
import com.story.repository.StoryRepository;

@Service
public class PhaseComparator implements Comparator<Phase>{
    
    private String field;
    
    private Story story;
    
    @Autowired
    private StoryRepository storyRepository;
    
    
    public PhaseComparator() {
        super();
    }

    public PhaseComparator (String field) {
        this.field = field;
    }

    @Override
    public int compare(Phase phase_1, Phase phase_2) {
        if (this.field.equals("createdDate")) {
            return phase_1.getCreatedDate() < phase_2.getCreatedDate() ? -1 : 1;
        } else {
            Story foundStory_1 = this.storyRepository.findOne(phase_1.getStoryId());
            Story foundStory_2 = this.storyRepository.findOne(phase_1.getStoryId());
            return foundStory_1.getLastUpdatedDate() < foundStory_2.getLastUpdatedDate() ? -1 : 1;
        }
    }

}

But, in this case, storyRepository is null.
How should we deal with it?
Thanks