public class Comment {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "commentID")
    private long id;
    //другие поля...

    @OneToMany(fetch = FetchType.LAZY)
    @JoinTable(name = "ReplyToComment",
            joinColumns = @JoinColumn(name = "replyCommentID"),
            inverseJoinColumns = @JoinColumn(name = "commentID")
    )
    private List<Comment> replies;
}