@OneToMany
@JoinColumn(name = "emp_id")

public class Book {
   @Id
   private Long id;

   @OneToMany()
   @JoinColumn(name = "book_id")
   private List<Comment> comments;
	
public class Comment {
  @Id
  private Long id;
  }

@Entity
public class Book {
   @Id
   private Long id;

   @ElementCollection
   private List<Comment> comments;

@Embedded	
public class Comment {
   private String cmnt;
  }

   @OneToMany(cascade...)
   @JoinColumn(name = "book_id" )
   private List<Comment> comments;

    @ManyToOne(optional=false)
    private Book book;

 @OneToMany(cascade= CascadeType.ALL)
    @JoinColumn(name = "book_id",nullable=false)
    private List<Comment> clist = new ArrayList<>();

@Entity
@Table(name = "COMMENTS")
public class Comments
{
    @ManyToOne
    @JoinColumn(name = "BOOK_ID")
    private Book book;
}


@Entity
@Table(name = "BOOKS")
public class Book
{
    @OneToMany(mappedBy = "book")
    private List<Comment> comments;

    public List<Comment> getComments() {}
    public void setComments(List<Comment>) {}
}

@Entity
@Table(name = "COMMENTS")
public class Comments
{
    //здесь вообще не задана связь с книгой, комент ничего про книгу не знает
    String comment;
    
}


@Entity
@Table(name = "BOOKS")
public class Book
{
    @OneToMany()//вся связь замаплена в этом классе
    @JoinColumn(name = "BOOK_ID",nullable = false)    
    private List<Comment> comments;

    public List<Comment> getComments() {}
    public void setComments(List<Comment>) {}
}

@Embeddable
@Table(name = "COMMENTS")
public class Comments
{
    //здесь вообще не задана связь с книгой, комент ничего про книгу не знает
//если не предпринимать специальные меры
    String comment;
    
}


@Entity
@Table(name = "BOOKS")
public class Book
{
    @ElementCollection //для 4-го хибера, в третьем вроде  @CollectionOfElements
    private List<Comment> comments;

    public List<Comment> getComments() {}
    public void setComments(List<Comment>) {}
}