相关疑难解决方法(0)

单向和双向关联有什么区别？

由于在db中生成的表都是相同的,因此我发现的唯一区别是双向关联的每一侧都将引用另一侧,而单向不引用.

这是一个单向关联

public class User {
    private int     id;
    private String  name;
    @ManyToOne
    @JoinColumn(
            name = "groupId")
    private Group   group;
}

public class Group {
    private int     id;
    private String  name;
}

双向关联

public class User {
    private int     id;
    private String  name;
    @ManyToOne
    @JoinColumn(
            name = "groupId")
    private Group   group;
}
public class Group {
    private int         id;
    private String      name;
    @OneToMany(mappedBy="group")
    private List<User>  users;
}

区别在于该组是否拥有用户的参考.

所以我想知道这是唯一的区别吗？哪个推荐？

我有两个例子,第一个是@OneToOne单向映射和第二个双向映射.在单向映射中,拥有方表必须包含引用另一个表的id的连接列; 然后在双向中它们都必须包含彼此的外键列.但是在使用自动生成策略生成数据库模式之后,两个示例对数据库模式具有相同的效果.单向映射是正常的,但双向示例只包含一个外键列,但它必须涉及彼此的外键!

单向映射

@Entity
public class Customer43 {

@Id
@GeneratedValue
private Long id;
private String firstName;
private String lastName;
private String email;
private String phoneNumber;
@OneToOne
@JoinColumn(name = "address_fk")
private Address43 address;

// Getters, Setters and Constructors.
}

@Entity
public class Address43 {

@Id
@GeneratedValue
private Long id;
private String street1;
private String street2;
private String city;
private String state;
private String zipcode;
private String country;

// Getters, Setters and Constructors.
}

双向映射

@Entity
public class Customer44 {

@Id
@GeneratedValue …