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：
Java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
@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。
