Hibernate annotation的关系定义

Hibernate annotation的关系定义

一、@OneToOne

 

代码

@Entity
public class User{
    private Address address;

    @OneToOne
    public Address getAddress() {
        return address;
     }

    public void setAddress(Address address) {
        this.address = address;
     }
}

@Entity
public class Address{
    private User user;

    @OneToOne(mappedBy = "address")
    public User getUser() {
        return user;
     }

    public void setUser(User user) {
        this.user = user;
     }

}

1、两边都定义了@OneToOne，但都没有定义mappedBy,则user和address表都会生成到对方的外键，双方都是这个关系的拥有者。 
2、两边都定义了@OneToOne，如果user定义了mappedBy，则在address表生成到user的外键，address是这个关系的拥有者；如果address定义了mappedBy，则在user表生成到address的外键，user是这个关系的拥有者。 
二、@ManyToOne和@OneToMany

 

 

代码

@Entity
public class Employee {
private Department department;

@ManyToOne
public Department getDepartment() {
return department;
}

public void setDepartment(Department department) {
this.department = department;
}
}

@Entity
public class Department {
private Set<Employee> employees = new HashSet<Employee>();

@OneToMany(mappedBy="department")
public Collection<Employee> getEmployees() {
return employees;
}
public void setEmployees(Collection<Employee> employees) {
this.employees = employees;
}
}

@ManyToOne中Many指的是本类（也就是声明@ManeyToOne的类），One是指关联类，也就是To前边的对应本类，后边的对应关联类。如果方法返回的是单数关联类则定义@ManyToOne，例如： 

代码

@ManyToOne
public Department getDepartment() {
return department;
}

方法返回的是Department，为单数关联类，对应@ManyToOne中的One； 
如果返回的是复数关联类则定义@OneToMany，例如： 

代码

@OneToMany(mappedBy="department")
public Collection<Employee> getEmployees() {
return employees;
}

方法返回的是Collection<Employee>，复数关联类，对应@OneToMany中的Many。其中定义mappedBy的是@OneToMany,也就是说One这一方是关系的拥有者。Many一方的表中生成到关联类的外键。

 

三、@ManyToMany

 

代码

@Entity
public class Book {
private Set authors = new HashSet<Author>();

@ManyToMany
public Set<Author> getAuthors(){
return authors;
}

Public void setAuthors(Set<Author> authors){
    This.authors = authors;
}
}

@Entity
public class Author {
private Set books = new HashSet<Book>();

@ManyToMany(mappedBy="authors")
public Set<Book> getBooks(){
return books;
}

Public void setBooks(Set<Book> books){
    This.books = books;
}
}

@ManyToMany会生成中间表，具体表名和字段可以通过@AssociationTable来定义，默认的就可以了，同样关系的非拥有者，需要定义mappedBy属性。

评论

1 楼 yu_xian81 2009-03-05  

@OneToMany(mappedBy="department")  one方是关系拥有者,相当于设置了inverse="false"?
另从一文档上看来的:
4)OneToMany 与 ManyToOne 双向关联
在ManyToOne的配置上再加以下：

@OneToMany(mappedBy = "articleType")
private List<Article> articles;

补充：对OneToMany的双向映射，如果要OneToMany这一端维护关系，你需要删除mappedBy元素

看起来好矛盾.