评论回复功能的数据库设计可以分开设计成两张表，评论表和回复表，也可以将其设计为一张表，我采用的是一张表
评论回复表的相关字段（我做的是商品goods下的评论回复）

字段解释：

gc_id：评论回复表id
gc_uid：评论用户id（外键）
gc_content：评论内容
gc_time：评论时间
gc_gid：评论的商品id（外键）
gc_status：评论状态（1未删除0已删除）
gc_parentid：父级评论的id，如果不是对评论的回复，那么该值为null

我在评论回复表中添加了几条数据

接下来是代码部分了
采用的是 springboot+mybatis
1.GoodsComment 的pojo类

public class GoodsComment {
    private Integer gcId;
    private Integer gcGid;
    private Integer gcUid;
    private Date gcTime;
    private String gcContent;
    private Integer gcStatus;
    private Integer gcParentid;
    private Goods goods;
    private Users users;
    private List<GoodsComment> child;

	// 略去了getter和setter方法
}

2.GoodsCommentMapper.java

public interface GoodsCommentMapper {
    // 根据商品id查询该商品下的所有评论
    List<GoodsComment> findByGoodId(@Param("gId") Integer gId);
}

3.GoodsCommentService.java

public interface GoodsCommentService {
    // 根据商品id查询该商品下的所有评论
    List<GoodsComment> findByGoodId(Integer gId);
}

4.GoodsCommentServiceImpl.java

@Service
public class GoodsCommentServiceImpl implements GoodsCommentService {
    @Resource
    private GoodsCommentMapper goodsCommentMapper;

    @Override
    public List<GoodsComment> findByGoodId(Integer gId) {
        return goodsCommentMapper.findByGoodId(gId);
    }
}

5.GoodsCommentMapper.xml

<select id="findByGoodId" resultMap="BaseResultMap">
    SELECT a.*,b.*
    FROM goods_comment AS a,users AS b
    WHERE a.gc_gId=#{gId}
    AND a.gc_status=b.u_status=1
    AND a.gc_uid=b.u_id
</select>

6.CoodsCommentController.java

@RestController
@RequestMapping("/goodsComment")
public class GoodsCommentController {

    @Autowired
    private GoodsCommentService commentService;

    /**
     * 根据商品id查询该商品下的所有评论
     * @param gId
     * @return
     */
    @GetMapping("/findByGoodId/{gId}")
    public List<GoodsComment> findByGoodId(@PathVariable("gId") Integer gId) {
        List<GoodsComment> allComments = commentService.findByGoodId(gId);
        if (allComments == null || allComments.size() == 0) {
            return new ArrayList<>();
        }
        List<GoodsComment> comments = new ArrayList<>();
        List<GoodsComment> parents = new ArrayList<>();
        for (GoodsComment comment : allComments) {
            if (comment.getGcParentid()==null) {
                comments.add(comment);
                parents.add(comment);
            } else {
                boolean foundParent=false;
                for (GoodsComment parent : parents) {
                    if (comment.getGcParentid() == parent.getGcId()) {
                        if (parent.getChild() == null) {
                            parent.setChild(new ArrayList<>());
                        }
                        parent.getChild().add(comment);
                        parents.add(comment);
                        foundParent=true;
                        //如果对list迭代过程中同时修改list，会报java.util.ConcurrentModificationException
                        // 的异常，所以我们需要break,当然break也可以提高算法效率
                        break;
                    }
                }
                if (!foundParent) {
                    throw new RuntimeException("can not find the parent comment");
                }
            }
        }
        return comments;
    }
}

7.获取到的数据部分

[
    {
        "gcId": 1,
        "gcGid": 1,
        "gcUid": 11,
        "gcTime": "2020-03-13T14:29:52.000+0000",
        "gcContent": "我是评论商品的",
        "gcStatus": 1,
        "gcParentid": null,
        "goods": null,
        "users": {
            "uId": 11,
            "uName": "小陈",
            "uAvatar": "http://imgs/FjrnXL4DP6Ib8xmZmXQU5vy5GWvq"
        },
        "child": [
            {
                "gcId": 3,
                "gcGid": 1,
                "gcUid": 6,
                "gcTime": "2020-03-15T16:31:49.000+0000",
                "gcContent": "我是回复",
                "gcStatus": 1,
                "gcParentid": 1,
                "goods": null,
                "users": {
                    "uId": 6,
                    "uName": "小贰",
                    "uAvatar": "http://imgs/Fi-WU4zsZ-1OTVfPg5OXyIao13SP"
                },
                "child": [
                    {
                        "gcId": 4,
                        "gcGid": 1,
                        "gcUid": 10,
                        "gcTime": "2020-03-15T16:48:15.000+0000",
                        "gcContent": "我是回复的回复",
                        "gcStatus": 1,
                        "gcParentid": 3,
                        "goods": null,
                        "users": {
                            "uId": 10,
                            "uName": "小贰",
                            "uAvatar": "http://imgs/Fi-WU4zsZ-1OTVfPg5OXyIao13SP"
                        },
                        "child": [
                            {
                                "gcId": 5,
                                "gcGid": 1,
                                "gcUid": 9,
                                "gcTime": "2020-03-15T20:26:09.000+0000",
                                "gcContent": "我看一下新增评论",
                                "gcStatus": 1,
                                "gcParentid": 4,
                                "goods": null,
                                "users": {
                                    "uId": 9,
                                    "uName": "小贰",
                                    "uAvatar": "http://imgs/Fi-WU4zsZ-1OTVfPg5OXyIao13SP"
                                },
                                "child": null
                            }
                        ]
                    }
                ]
            }
        ]
    },
    {
        "gcId": 2,
        "gcGid": 1,
        "gcUid": 8,
        "gcTime": "2020-03-14T14:30:12.000+0000",
        "gcContent": "我也是评论这个商品的",
        "gcStatus": 1,
        "gcParentid": null,
        "goods": null,
        "users": {
            "uId": 8,
            "uName": "小贰",
            "uAvatar": "http://imgs/Fi-WU4zsZ-1OTVfPg5OXyIao13SP"
        },
        "child": null
    }
 ]

评论的样式