@Entity
@Table(name = "Users")
public class Users implements Serializable{
   @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "user_id")
    private Long userId;
   
   @OneToMany(targetEntity = Comments.class, cascade = CascadeType.ALL, fetch = FetchType.LAZY)
   private List<Comments> commentsForUser;

@Entity
@Table(name ="Books")
public class Books implements Serializable {
  @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "book_id")
    private Long bookId;
  
    @OneToMany(targetEntity = Comments.class, cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    private List<Comments> commentsForBook;

@Entity
@Table(name ="Comments")
public class Comments implements Serializable{
   @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "comment_id")
    private Long commentId;
   
   @ManyToOne(targetEntity = Users.class, cascade = {CascadeType.REFRESH}, fetch =FetchType.LAZY)
   @JoinColumn(name = "user_id")
   private Users user;
   
    @ManyToOne(targetEntity = Books.class, cascade = {CascadeType.REFRESH}, fetch =FetchType.LAZY)
    @JoinColumn(name = "book_id")
    private Books book;

public class Main {

    public static void main(String[] args) {
    
        Session session = HibernateUtil.getSessionFactory().openSession();
       
        session.beginTransaction();
        Books book = new Books();
        book.setBook("Книга краба");
        book.setAutor("Краб");
        book.setDescription("Книга краба, классная");
        book.setImage("Ссылка на картинку для книги");
        book.setFile("Ссылка на файл для книги");
        book.setNumberComments(0);
        book.setNumberDownloads(0);
        book.setYearToPublication(2009);
        session.save(book);
        session.getTransaction().commit();
        
        session.beginTransaction();
        Users user = new Users();
        user.setLogin("krab");
        user.setPassword("qwerty");
        user.setName("Андрей");
        user.setSName("Коваленко");
        user.setOnLibrary(true);
        user.setCommentsNumber(1);
        user.setRegisterDate(new Date(System.currentTimeMillis()));
        session.save(user);
        session.getTransaction().commit();
        
        
        session.beginTransaction();
        Comments comment = new Comments();
        comment.setTextComment("Классная книга");
        comment.setDateOfComment(new Date(System.currentTimeMillis()));
        comment.setUser(user);
        comment.setBook(book);
        session.save(comment);
        session.getTransaction().commit();
        
        session.close();
        
       System.out.println(user.getCommentsForUser());
       System.out.println(book.getCommentsForBook());
        
        
    }
}

Hibernate: insert into Books (autor, book, description, file, image, numberComments, number_Downloads, year_to_publication) values (?, ?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Users (comments_Number, login, name, on_Library, password, register_Date, s_Name) values (?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into Comments (book_id, date_of_comment, text_comment, user_id) values (?, ?, ?, ?)
null
null

comment.setUser(user);
user.getComments().add(comment);

    public static void main(String[] args) {
    
        Session session = HibernateUtil.getSessionFactory().openSession();
        session.beginTransaction();

        Books book = new Books();
        book.setBook("Книга краба");
        ...
        session.flush();
        
        Users user = new Users();
        user.setLogin("krab");
        ...
       session.flush();
      
       Comments comment = new Comments();
       comment.setTextComment("Классная книга");
       ...
        session.flush();

        session.save(book);
        session.save(user);
        session.save(comment);

        session.getTransaction().commit();
        
        session.close();
        
       System.out.println(user.getCommentsForUser());
       System.out.println(book.getCommentsForBook());
        
        
    }

        

    @OneToMany(mappedBy="user", cascade = CascadeType.ALL)
       private List<Comments> commentsForUser;

    @OneToMany(mappedBy="book", cascade = CascadeType.ALL)
       private List<Comments> commentsForBook;

    @ManyToOne(cascade = {CascadeType.ALL})
        @JoinColumn(name = "user_id")
        private Users user;
       
        @ManyToOne(cascade = {CascadeType.ALL})
        @JoinColumn(name = "book_id")
        private Books book;