@JoinColumn

@ManyToOne // 하나의 Board의 여러개의 Reply이 달린다
@JoinColumn(name = "boardId") ->board 의 id 참조
private Board board;

@ManyToOne // 하나의 user가 여러개의 Reply를 단다
@JoinColumn(name="userId") -> user의 id 참조
private User user;

Model _Reply.java

@Data
@NoArgsConstructor
@AllArgsConstructor
@Builder
@Entity
public class Reply {
	
	@Id 
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	private int id;
	
	@Column(nullable = false, length = 200)
	private String content;
	
	//어디 게시글
	@ManyToOne 
	@JoinColumn(name = "boardId")
	private Board board;
	
	@ManyToOne 
	@JoinColumn(name="userId")
	private User user;
		

	@CreationTimestamp
	private Timestamp createDate;
}