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。