在EJB3.0 规范中 多对一与一对多的双向关系，
多对一（就是@ManyToOne注解的这端，是多端哦 不要搞混了）
这端总是双向关联端的主题(owner)端，
而一对多端的关联注解为 @OneToMany（mappedBy=" " ）其值是：多对一端的属性
demo：
被动方：其实也就是一方 或者说（OneToMany方）
@Entity
public class Customer extends AbstractEntity {
private String name;
@OneToMany(mappedBy="customer",cascade=CascadeType.ALL)
private Set<Order> orders;
public void addOrder(Order order){
if(orders == null){
orders = new HashSet<Order>();
}
orders.add(order);
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Set<Order> getOrders() {
return orders;
}
public void setOrders(Set<Order> orders) {
this.orders = orders;
}
}
主动方：1.关系的维护方2.ManyToOne方3.多方
@Entity
@Table(name="orders")
public class Order extends AbstractEntity {
private String name;
@ManyToOne(cascade=CascadeType.ALL)
private Customer customer;
public Customer getCustomer() {
return customer;
}
public void setCustomer(Customer customer) {
this.customer = customer;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
以上是实体
下面是测试用列哦
public void testCRUD() {
// 第一种情况： 调用的被动方的Dao 绑定主动方关系，但主动方没有绑定被动方
Customer entity = new Customer();
entity.setName("customer1");
Order order = new Order();
order.setName("order1");
entity.addOrder(order);
entity = customerDao.create(entity); // 这种情况下 orders.customer_id == null
//控制台的信息
//Hibernate: insert into Customer (name, id) values (?, ?)
//这里的customer_id 为null
//Hibernate: insert into orders (name, customer_id, id) values (?, ?, ?)
System.out.println("entity id = " + entity.getId());
System.out.println("order id = "+ order.getId());
System.out.println("1111********************** over");
// 第二种情况： 调用的被动方的Dao 绑定主动方关系，并且主动方也绑定被动方
entity = new Customer();
entity.setName("customer2");
order = new Order();
order.setName("order2");
entity.addOrder(order);
order.setCustomer(entity); //这里进行双向关联
entity = customerDao.create(entity);
//
//Hibernate: insert into Customer (name, id) values (?, ?)
//这里的customer_id 有值哦
//Hibernate: insert into orders (name, customer_id, id) values (?, ?, ?)
System.out.println("entity id = " + entity.getId());
System.out.println("order id = "+ order.getId());
System.out.println("2222********************** over");
// 第三种情况： 调用的主动方的Dao 绑定被动方关系，但是被东方不绑定主动方
entity = new Customer();
entity.setName("customer3");
order = new Order();
order.setName("order3");
order.setCustomer(entity); //绑定被动方
orderDao.create(order);
//Hibernate: insert into Customer (name, id) values (?, ?)
//Hibernate: insert into orders (name, customer_id, id) values (?, ?, ?)
System.out.println("entity id = " + entity.getId());
System.out.println("order id = "+ order.getId());
System.out.println("3333********************** over");
// 第四种情况： 调用的主动方的Dao 绑定被动方关系，并且被东方也绑定主动方
entity = new Customer();
entity.setName("customer4");
order = new Order();
order.setName("order4");
order.setCustomer(entity); //绑定被动方
orderDao.create(order);
System.out.println("entity id = " + entity.getId());
System.out.println("order id = "+ order.getId());
System.out.println("4444********************** over");
//Hibernate: insert into Customer (name, id) values (?, ?)
//Hibernate: insert into orders (name, customer_id, id) values (?, ?, ?)
//总结： 经测验二三四种方法结果都是一样都能持久化到数据库，并且关系也建立好了
// 也就说只要主动方绑定了被动方关系就维护好了
}
*****************************************做级联删除测试************************************************************
public void testCascade(){
//1. 做个级联删除吧 测试删除主动方 是否删除关联方
// 这里会级联删除主动方的关联对象，以及该关联对象（被动方）所关联的所有主动方都会被级联删除
orderDao.delete("order_id_1");
//Hibernate: delete from orders where id=?
// Hibernate: delete from orders where id=?
// Hibernate: delete from orders where id=?
// Hibernate: delete from orders where id=?
// Hibernate: delete from orders where id=?
// Hibernate: delete from Customer where id=?
assertNull( orderDao.findById("orderDao"));
//2. 做个级联删除吧 测试删除被动方 是否删除关联方
//删除该被动方，以及所关联的所有主动方（维护关系方or多方）与上面的调用主动方的结果一样
//Hibernate: delete from orders where id=?
// Hibernate: delete from orders where id=?
// Hibernate: delete from orders where id=?
// Hibernate: delete from orders where id=?
// Hibernate: delete from orders where id=?
// Hibernate: delete from Customer where id=?
customerDao.delete("2");
assertNull( customerDao.findById("2"));
}
****************************************************l延迟加载************************************************************
@ManyToOne Default: FetchType.EAGER 默认是即时抓取 做连接 如果,fetch=FetchType.LAZY不是左联接 在需要One的时候直接select
@OneToMany Default: FetchType.LAZY 默认是延迟抓取 不需要左联接 如果是FetchType.EAGER 即时抓取 就会是左联接查询
如果 order（ManyToOne）设置了 @ManyToOne(cascade=CascadeType.ALL,fetch=FetchType.LAZY)
private Customer customer; 查找Order是直接查找不需要左连接 customer
Hibernate: select order0_.id as id5_0_, order0_.name as name5_0_, order0_.customer_id as customer3_5_0_ from orders order0_ where order0_.id=?
如果 order（ManyToOne）设置了 @ManyToOne(cascade=CascadeType.ALL) //没有设置延迟加载
private Customer customer; 查找Order是直接查找需要左连接 customer
Hibernate: select order0_.id as id5_1_, order0_.name as name5_1_, order0_.customer_id as customer3_5_1_, customer1_.id as id2_0_, customer1_.name as name2_0_ from orders order0_ left outer join Customer customer1_ on order0_.customer_id=customer1_.id where order0_.id=?