Hibernate annotation的关系定义

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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 employees = new HashSet();

@OneToMany(mappedBy="department")
public Collection getEmployees() {
return employees;
}
public void setEmployees(Collection 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 getEmployees() {
return employees;
}

方法返回的是Collection，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();

@ManyToMany
public Set getAuthors(){
return authors;
}
    
Public void setAuthors(Set authors){
   This.authors = authors;
}
}

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


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

Public void setBooks(Set 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。