Hibernate annotation 的各种关系

转的：

 

一、@OneToOne

@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 

@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;
}
}

 

如果只写了@manytoone，不写其他配置，那么，会生成3张表，其中一张为关系表(默认为多对多)

所以最好是加上@JoinColumn(name="**id")

@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： 

@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： 

@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 

@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。