public String findCommentByNewsId(String id) {
		Integer i = Integer.parseInt(id);
		System.out.println("service = " + new CommentDao().findCommentByNewsId(i).toString());
		System.out.println("serviceGson = " + new Gson().toJson(new CommentDao().findCommentByNewsId(i)));
		return new Gson().toJson(new CommentDao().findCommentByNewsId(i));
	}

public List<Comment> findCommentByNewsId(int id){
		System.out.println("dao = " + Comment.dao.find("select * from comment_info where comment_belone = ?",id).toString());
		return Comment.dao.find("select * from comment_info where comment_belone = " + id);
	}

dao = [Comment{comment_id=null, comment_author=null, comment_class=null, comment_content='null', comment_likeNum=null, comment_time=null, comment_belone=null}, Comment{comment_id=null, comment_author=null, comment_class=null, comment_content='null', comment_likeNum=null, comment_time=null, comment_belone=null}]
service = [Comment{comment_id=null, comment_author=null, comment_class=null, comment_content='null', comment_likeNum=null, comment_time=null, comment_belone=null}, Comment{comment_id=null, comment_author=null, comment_class=null, comment_content='null', comment_likeNum=null, comment_time=null, comment_belone=null}]
serviceGson = [{"attrs":{"comment_likenum":12,"comment_time":"十一月 27, 2019","comment_belone":1,"comment_author":1,"comment_class":3,"commentId":1,"comment_content":"涌起的情绪一言两语说不清楚，惊喜中带着不置信，随之而来的感动。"}},{"attrs":{"comment_likenum":23,"comment_time":"十一月 28, 2019","comment_belone":1,"comment_author":1,"comment_class":3,"commentId":2,"comment_content":"涌起的情绪一言两语说不清楚，惊喜中带着不置信，随之而来的感动。"}}]

转JSON的时候他居然能在控制台输出出来数据，但是两个调用toString方法打印出来的却是null

public class Comment extends Model<Comment>{
    private Integer commentId;
    private Integer comment_author;
    private Integer comment_class;
    private String comment_content;
    private Integer comment_likeNum;
    private Date comment_time;
    private Integer comment_belone;
    
    public static final Comment dao = new Comment().dao();
    
    public Comment(){}

    public Integer getCommentId() {
        return commentId;
    }

    public void setCommentId(Integer comment_id) {
        this.commentId = comment_id;
    }