mappedBy属性：one2many
    附一个one2many的例子 帮助理解mappedBy属性。
    例子中：
@Entity
public class Department {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private int deptid;
private String dname;
private String location;
@OneToMany(mappedBy = "department")
private Set<User> users = new HashSet<User>();

所以department 表中只有deptid、dname、location三个字段，而users 则交给User表中的department字段维护。