https://stackoverflow.com/questions/16351156/spring-data-jpa-save-can-not-get-id

public class ManModel {
    @Id
    // 要点1如下：这里只能是GenerationType.IDENTITY
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;
}

// 要点二如下：manService.save()返回更新后的
man = manService.save(man);