在Spring Boot框架下的web评论系统，实体有评论、子评论、用户和回复，实现浏览、发布、更新和删除功能。

好的，首先你需要定义四个实体类，分别为Comment、SubComment、User和Reply。其中Comment和SubComment是一对多的关系（即一个Comment下可以有多个SubComment），SubComment和Reply也是一对多的关系（即一个SubComment下可以有多个Reply），User则是一对多的关系（即一个User可以对应多个Comment和SubComment）。

下面是针对这四个实体类的建表语句：

CREATE TABLE `user` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `username` varchar(255) NOT NULL,
  `password` varchar(255) NOT NULL,
  PRIMARY KEY (`id`)
);

CREATE TABLE `comment` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `content` varchar(255) NOT NULL,
  `user_id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  FOREIGN KEY (`user_id`) REFERENCES `user`(`id`)
);

CREATE TABLE `sub_comment` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `content` varchar(255) NOT NULL,
  `comment_id` int(11) NOT NULL,
  `user_id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  FOREIGN KEY (`comment_id`) REFERENCES `comment`(`id`),
  FOREIGN KEY (`user_id`) REFERENCES `user`(`id`)
);

CREATE TABLE `reply` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `content` varchar(255) NOT NULL,
  `sub_comment_id` int(11) NOT NULL,
  `user_id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  FOREIGN KEY (`sub_comment_id`) REFERENCES `sub_comment`(`id`),
  FOREIGN KEY (`user_id`) REFERENCES `user`(`id`)
);

接下来是Spring Boot框架下的web评论系统的实现：

1. 在pom.xml文件中添加依赖：

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>

<dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
</dependency>

2. 在application.properties文件中添加数据库连接信息：

spring.datasource.url=jdbc:mysql://localhost:3306/comment_system
spring.datasource.username=root
spring.datasource.password=123456
spring.jpa.hibernate.ddl-auto=update

3. 编写实体类：

User.java

@Entity
@Table(name = "user")
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String username;
    private String password;

    @OneToMany(mappedBy = "user", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    private List<Comment> comments;

    @OneToMany(mappedBy = "user", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    private List<SubComment> subComments;

    @OneToMany(mappedBy = "user", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    private List<Reply> replies;

    // 省略getter和setter
}

Comment.java

@Entity
@Table(name = "comment")
public class Comment {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(nullable = false)
    private String content;

    @ManyToOne
    @JoinColumn(name = "user_id")
    private User user;

    @OneToMany(mappedBy = "comment", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    private List<SubComment> subComments;

    // 省略getter和setter
}

SubComment.java

@Entity
@Table(name = "sub_comment")
public class SubComment {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(nullable = false)
    private String content;

    @ManyToOne
    @JoinColumn(name = "comment_id")
    private Comment comment;

    @ManyToOne
    @JoinColumn(name = "user_id")
    private User user;

    @OneToMany(mappedBy = "subComment", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    private List<Reply> replies;

    // 省略getter和setter
}

Reply.java

@Entity
@Table(name = "reply")
public class Reply {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(nullable = false)
    private String content;

    @ManyToOne
    @JoinColumn(name = "sub_comment_id")
    private SubComment subComment;

    @ManyToOne
    @JoinColumn(name = "user_id")
    private User user;

    // 省略getter和setter
}

4. 编写Repository：

UserRepository.java

@Repository
public interface UserRepository extends JpaRepository<User, Long> {
}

CommentRepository.java

@Repository
public interface CommentRepository extends JpaRepository<Comment, Long> {
}

SubCommentRepository.java

@Repository
public interface SubCommentRepository extends JpaRepository<SubComment, Long> {
}

ReplyRepository.java

@Repository
public interface ReplyRepository extends JpaRepository<Reply, Long> {
}

5. 编写Service：

UserService.java

@Service
public class UserService {
    @Autowired
    private UserRepository userRepository;

    public User save(User user) {
        return userRepository.save(user);
    }

    public User findById(Long id) {
        return userRepository.findById(id).orElse(null);
    }

    public List<User> findAll() {
        return userRepository.findAll();
    }

    public void delete(Long id) {
        userRepository.deleteById(id);
    }
}

CommentService.java

@Service
public class CommentService {
    @Autowired
    private CommentRepository commentRepository;

    public Comment save(Comment comment) {
        return commentRepository.save(comment);
    }

    public Comment findById(Long id) {
        return commentRepository.findById(id).orElse(null);
    }

    public List<Comment> findAll() {
        return commentRepository.findAll();
    }

    public void delete(Long id) {
        commentRepository.deleteById(id);
    }
}

SubCommentService.java

@Service
public class SubCommentService {
    @Autowired
    private SubCommentRepository subCommentRepository;

    public SubComment save(SubComment subComment) {
        return subCommentRepository.save(subComment);
    }

    public SubComment findById(Long id) {
        return subCommentRepository.findById(id).orElse(null);
    }

    public List<SubComment> findAll() {
        return subCommentRepository.findAll();
    }

    public void delete(Long id) {
        subCommentRepository.deleteById(id);
    }
}

ReplyService.java

@Service
public class ReplyService {
    @Autowired
    private ReplyRepository replyRepository;

    public Reply save(Reply reply) {
        return replyRepository.save(reply);
    }

    public Reply findById(Long id) {
        return replyRepository.findById(id).orElse(null);
    }

    public List<Reply> findAll() {
        return replyRepository.findAll();
    }

    public void delete(Long id) {
        replyRepository.deleteById(id);
    }
}

6. 编写Controller：

UserController.java

@RestController
@RequestMapping("/user")
public class UserController {
    @Autowired
    private UserService userService;

    @PostMapping("/save")
    public User save(@RequestBody User user) {
        return userService.save(user);
    }

    @GetMapping("/find/{id}")
    public User findById(@PathVariable("id") Long id) {
        return userService.findById(id);
    }

    @GetMapping("/find-all")
    public List<User> findAll() {
        return userService.findAll();
    }

    @DeleteMapping("/delete/{id}")
    public void delete(@PathVariable("id") Long id) {
        userService.delete(id);
    }
}

CommentController.java

@RestController
@RequestMapping("/comment")
public class CommentController {
    @Autowired
    private CommentService commentService;

    @PostMapping("/save")
    public Comment save(@RequestBody Comment comment) {
        return commentService.save(comment);
    }

    @GetMapping("/find/{id}")
    public Comment findById(@PathVariable("id") Long id) {
        return commentService.findById(id);
    }

    @GetMapping("/find-all")
    public List<Comment> findAll() {
        return commentService.findAll();
    }

    @DeleteMapping("/delete/{id}")
    public void delete(@PathVariable("id") Long id) {
        commentService.delete(id);
    }
}

SubCommentController.java

@RestController
@RequestMapping("/sub-comment")
public class SubCommentController {
    @Autowired
    private SubCommentService subCommentService;

    @PostMapping("/save")
    public SubComment save(@RequestBody SubComment subComment) {
        return subCommentService.save(subComment);
    }

    @GetMapping("/find/{id}")
    public SubComment findById(@PathVariable("id") Long id) {
        return subCommentService.findById(id);
    }

    @GetMapping("/find-all")
    public List<SubComment> findAll() {
        return subCommentService.findAll();
    }

    @DeleteMapping("/delete/{id}")
    public void delete(@PathVariable("id") Long id) {
        subCommentService.delete(id);
    }
}

ReplyController.java

@RestController
@RequestMapping("/reply")
public class ReplyController {
    @Autowired
    private ReplyService replyService;

    @PostMapping("/save")
    public Reply save(@RequestBody Reply reply) {
        return replyService.save(reply);
    }

    @GetMapping("/find/{id}")
    public Reply findById(@PathVariable("id") Long id) {
        return replyService.findById(id);
    }

    @GetMapping("/find-all")
    public List<Reply> findAll() {
        return replyService.findAll();
    }

    @DeleteMapping("/delete/{id}")
    public void delete(@PathVariable("id") Long id) {
        replyService.delete(id);
    }
}

7. 编写前端页面：

在src/main/resources/static目录下添加index.html文件，内容如下：

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Web评论系统</title>
</head>
<body>
    <h1>Web评论系统</h1>
    <h2>用户管理</h2>
    <form id="user-form">
        <label for="username">用户名：</label>
        <input type="text" id="username" name="username">
        <br>
        <label for="password">密码：</label>
        <input type="password" id="password" name="password">
        <br>
        <button type="button" onclick="saveUser()">保存</button>
    </form>
    <table id="user-table">
        <thead>
            <tr>
                <th>id</th>
                <th>用户名</th>
                <th>密码</th>
                <th>操作</th>
            </tr>
        </thead>
        <tbody>
        </tbody>
    </table>
    <h2>评论管理</h2>
    <form id="comment-form">
        <label for="content">评论内容：</label>
        <input type="text" id="content" name="content">
        <br>
        <label for="user-id">用户ID：</label>
        <input type="text" id="user-id" name="user-id">
        <br>
        <button type="button" onclick="saveComment()">保存</button>
    </form>
    <table id="comment-table">
        <thead>
            <tr>
                <th>id</th>
                <th>评论内容</th>
                <th>用户ID</th>
                <th>操作</th>
            </tr>
        </thead>
        <tbody>
        </tbody>
    </table>
    <h2>子评论管理</h2>
    <form id="sub-comment-form">
        <label for="content">子评论内容：</label>
        <input type="text" id="content" name="content">
        <br>
        <label for="comment-id">评论ID：</label>
        <input type="text" id="comment-id" name="comment-id">
        <br>
        <label for="user-id">用户ID：</label>
        <input type="text" id="user-id" name="user-id">
        <br>
        <button type="button" onclick="saveSubComment()">保存</button>
    </form>
    <table id="sub-comment-table">
        <thead>
            <tr>
                <th>id</th>
                <th>子评论内容</th>
                <th>评论ID</th>
                <th>用户ID</th>
                <th>操作</th>
            </tr>
        </thead>
        <tbody>
        </tbody>
    </table>
    <h2>回复管理</h2>
    <form id="reply-form">
        <label for="content">回复内容：</label>
        <input type="text" id="content" name="content">
        <br>
        <label for="sub-comment-id">子评论ID：</label>
        <input type="text" id="sub-comment-id" name="sub-comment-id">
        <br>
        <label for="user-id">用户ID：</label>
        <input type="text" id="user-id" name="user-id">
        <br>
        <button type="button" onclick="saveReply()">保存</button>
    </form>
    <table id="reply-table">
        <thead>
            <tr>
                <th>id</th>
                <th>回复内容</th>
                <th>子评论ID</th>
                <th>用户ID</th>
                <th>操作</th>
            </tr>
        </thead>
        <tbody>
        </tbody>
    </table>
    <script src="https://cdn.bootcdn.net/ajax/libs/jquery/3.6.0/jquery.min.js"></script>
    <script>
        function saveUser() {
            $.ajax({
                url: "/user/save",
                type: "POST",
                dataType: "json",
                contentType: "application/json;charset=utf-8",
                data: JSON.stringify({
                    username: $("#username").val(),
                    password: $("#password").val()
                }),
                success: function (result) {
                    alert("保存成功！");
                    $("#user-form")[0].reset();
                    loadUserTable();
                }
            });
        }

        function loadUserTable() {
            $.ajax({
                url: "/user/find-all",
                type: "GET",
                dataType: "json",
                success: function (result) {
                    var tbody = $("#user-table tbody");
                    tbody.empty();
                    $.each(result, function (index, item) {
                        var tr = $("<tr></tr>");
                        tr.append($("<td></td>").text(item.id));
                        tr.append($("<td></td>").text(item.username));
                        tr.append($("<td></td>").text(item.password));
                        var td = $("<td></td>");
                        td.append($("<button type='button'>删除</button>").click(function () {
                            deleteUser(item.id);
                        }));
                        tr.append(td);
                        tbody.append(tr);
                    });
                }
            });
        }

        function deleteUser(userId) {
            $.ajax({
                url: "/user/delete/" + userId,
                type: "DELETE",
                success: function () {
                    alert("删除成功！");
                    loadUserTable();
                }
            });
        }

        function saveComment() {
            $.ajax({
                url: "/comment/save",
                type: "POST",
                dataType: "json",
                contentType: "application/json;charset=utf-8",
                data: JSON.stringify({
                    content: $("#content").val(),
                    user: {id: $("#user-id").val()}
                }),
                success: function (result) {
                    alert("保存成功！");
                    $("#comment-form")[0].reset();
                    loadCommentTable();
                }
            });
        }

        function loadCommentTable() {
            $.ajax({
                url: "/comment/find-all",
                type: "GET",
                dataType: "json",
                success: function (result) {
                    var tbody = $("#comment-table tbody");
                    tbody.empty();
                    $.each(result, function (index, item) {
                        var tr = $("<tr></tr>");
                        tr.append($("<td></td>").text(item.id));
                        tr.append($("<td></td>").text(item.content));
                        tr.append($("<td></td>").text(item.user.id));
                        var td = $("<td></td>");
                        td.append($("<button type='button'>删除</button>").click(function () {
                            deleteComment(item.id);
                        }));
                        tr.append(td);
                        tbody.append(tr);
                    });
                }
            });
        }

        function deleteComment(commentId) {
            $.ajax({
                url: "/comment/delete/" + commentId,
                type: "DELETE",
                success: function () {
                    alert("删除成功！");
                    loadCommentTable();
                }
            });
        }

        function saveSubComment() {
            $.ajax({
                url: "/sub-comment/save",
                type: "POST",
                dataType: "json",
                contentType: "application/json;charset=utf-8",
                data: JSON.stringify({
                    content: $("#content").val(),
                    comment: {id: $("#comment-id").val()},
                    user: {id: $("#user-id").val()}
                }),
                success: function (result) {
                    alert("保存成功！");
                    $("#sub-comment-form")[0].reset();
                    loadSubCommentTable();
                }
            });
        }

        function loadSubCommentTable() {
            $.ajax({
                url: "/sub-comment/find-all",
                type: "GET",
                dataType: "json",
                success: function (result) {
                    var tbody = $("#sub-comment-table tbody");
                    tbody.empty();
                    $.each(result, function (index, item) {
                        var tr = $("<tr></tr>");
                        tr.append($("<td></td>").text(item.id));
                        tr.append($("<td></td>").text(item.content));
                        tr.append($("<td></td>").text(item.comment.id));
                        tr.append($("<td></td>").text(item.user.id));
                        var td = $("<td></td>");
                        td.append($("<button type='button'>删除</button>").click(function () {
                            deleteSubComment(item.id);
                        }));
                        tr.append(td);
                        tbody.append(tr);
                    });
                }
            });
        }

        function deleteSubComment(subCommentId) {
            $.