小牛大牛

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;   
}   
}  

@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。
原帖出处：http://tapestry.javaeye.com/blog/31928