Hibernate - 单向一对多关联关系映射

上篇博文描述了Hibernate - 单向多对一关联关系映射，本篇博文继续学习单向一对多关系映射。

这里Customer：Order= 1：N，外键保存在Order表中。

【1】修改Customer和Order

Customer修改如下：

public class Customer {private Integer customerId;private String customerName;private Set<Order> orders;//拥有属性orderspublic Set<Order> getOrders() {return orders;}public void setOrders(Set<Order> orders) {this.orders = orders;}public Integer getCustomerId() {return customerId;}public void setCustomerId(Integer customerId) {this.customerId = customerId;}public String getCustomerName() {return customerName;}public void setCustomerName(String customerName) {this.customerName = customerName;}@Overridepublic String toString() {return "Customer [customerId=" + customerId + ", customerName=" + customerName + ", orders=" + orders + "]";}}

Customer.hbm.xml修改如下：

<hibernate-mapping package="com.jane.model"><class name="Customer" table="CUSTOMERS"><id name="customerId" type="java.lang.Integer"><column name="CUSTOMER_ID" /><generator class="native" /></id><property name="customerName" type="java.lang.String"><column name="CUSTOMER_NAME" default="null" /></property><!--name="orders"表示一的一端拥有多的那端的属性;key节点中属性column="customer_id"表示外键(数据库多的一端拥有的)--><set name="orders" table="ORDERS"><key column="customer_id"></key><one-to-many class="Order"/></set></class>
</hibernate-mapping>

<key>元素设定本持久化类在Set对应表中的外键列名。如上值Customer类在ORDERS表中的外键列名–customer_id。

Order修改如下：

public class Order {private Integer orderId;private String orderName;public Integer getOrderId() {return orderId;}public void setOrderId(Integer orderId) {this.orderId = orderId;}public String getOrderName() {return orderName;}public void setOrderName(String orderName) {this.orderName = orderName;}@Overridepublic String toString() {return "Order [orderId=" + orderId + ", orderName=" + orderName+"]";}}

Order.hbm.xml如下：

<hibernate-mapping package="com.jane.model"><class name="Order" table="ORDERS"><id name="orderId" type="java.lang.Integer"><column name="ORDER_ID" /><generator class="native" /></id><property name="orderName" type="java.lang.String"><column name="ORDER_NAME" default="null" /></property></class>
</hibernate-mapping>

生成的数据库表结构如下所示：

//Customers表
CREATE TABLE `customers` (`CUSTOMER_ID` int(11) NOT NULL AUTO_INCREMENT,`CUSTOMER_NAME` varchar(255) DEFAULT NULL,PRIMARY KEY (`CUSTOMER_ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;//Orders表
CREATE TABLE `orders` (`ORDER_ID` int(11) NOT NULL AUTO_INCREMENT,`ORDER_NAME` varchar(255) DEFAULT NULL,`customer_id` int(11) DEFAULT NULL,PRIMARY KEY (`ORDER_ID`),KEY `FK1nbewmmir6psft27yfvvmwpfg` (`customer_id`),CONSTRAINT `FK1nbewmmir6psft27yfvvmwpfg` FOREIGN KEY (`customer_id`) REFERENCES `customers` (`CUSTOMER_ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

可以发现，无论是多对一还是一对多，外键都是多的一端数据表所持有。

单向多对一中，以多的一端为主，在多的一端xml中使用<many-to-one/>进行配置。
单向一对多中，以一的一端为主，在一的一端xml中使用<one-to-many/>进行配置。

【2】代码测试

① 单向一对多持久化

和单向多对一持久化不同的是，单向 1-n 关联关系执行保存时, 一定会多出 UPDATE 语句。因为 n 的一端在插入时不会同时插入外键列。

 @Testpublic void testOneToManySave(){Customer customer = new Customer();customer.setCustomerName("AAA");customer.setOrders(new HashSet<>());Order order1 = new Order();order1.setOrderName("O-JJ-1");Order order2 = new Order();order2.setOrderName("O-JJ-2");System.out.println(customer);//建立关联关系customer.getOrders().add(order1);customer.getOrders().add(order2);session.save(customer);session.save(order1);session.save(order2);}

测试结果如下：

Hibernate: insert intoCUSTOMERS(CUSTOMER_NAME) values(?)
Hibernate: insert intoORDERS(ORDER_NAME) values(?)
Hibernate: insert intoORDERS(ORDER_NAME) values(?)
Hibernate: updateORDERS setcustomer_id=? whereORDER_ID=?
Hibernate: updateORDERS setcustomer_id=? whereORDER_ID=?

② 单向一对多获取对象

默认对关联的多的一方使用懒加载的加载策略，只有在使用的时候才会进行查询。

测试代码如下：

 @Testpublic void testOneToManyGet(){Customer customer = session.get(Customer.class, 1);System.out.println(customer);System.out.println(customer.getOrders().size());}

测试结果如下：

Hibernate: selectcustomer0_.CUSTOMER_ID as CUSTOMER1_0_0_,customer0_.CUSTOMER_NAME as CUSTOMER2_0_0_ fromCUSTOMERS customer0_ wherecustomer0_.CUSTOMER_ID=?
Hibernate: selectorders0_.customer_id as customer3_2_0_,orders0_.ORDER_ID as ORDER_ID1_2_0_,orders0_.ORDER_ID as ORDER_ID1_2_1_,orders0_.ORDER_NAME as ORDER_NA2_2_1_ fromORDERS orders0_ whereorders0_.customer_id=?
Customer [customerId=1, customerName=AAA, orders=[Order [orderId=1, orderName=O-JJ-1], Order [orderId=2, orderName=O-JJ-2]]]
2

③ 单向一对多更新操作

代码如下：

 @Testpublic void testUpdate(){Customer customer = session.get(Customer.class, 1);customer.getOrders().iterator().next().setOrderName("O-XXX-10");}

测试结果如下：

Hibernate: selectcustomer0_.CUSTOMER_ID as CUSTOMER1_0_0_,customer0_.CUSTOMER_NAME as CUSTOMER2_0_0_ fromCUSTOMERS customer0_ wherecustomer0_.CUSTOMER_ID=?
Hibernate: selectorders0_.customer_id as customer3_2_0_,orders0_.ORDER_ID as ORDER_ID1_2_0_,orders0_.ORDER_ID as ORDER_ID1_2_1_,orders0_.ORDER_NAME as ORDER_NA2_2_1_ fromORDERS orders0_ whereorders0_.customer_id=?
Hibernate: updateORDERS setORDER_NAME=? whereORDER_ID=?

单独更新CustomerName如下：

 @Testpublic void testUpdate(){Customer customer = session.get(Customer.class, 1);customer.setCustomerName("CC");System.out.println(customer.getCustomerName());System.out.println(customer);}

测试结果如下：

Hibernate: selectcustomer0_.CUSTOMER_ID as CUSTOMER1_0_0_,customer0_.CUSTOMER_NAME as CUSTOMER2_0_0_ fromCUSTOMERS customer0_ wherecustomer0_.CUSTOMER_ID=?
CC
Hibernate: selectorders0_.customer_id as customer3_2_0_,orders0_.ORDER_ID as ORDER_ID1_2_0_,orders0_.ORDER_ID as ORDER_ID1_2_1_,orders0_.ORDER_NAME as ORDER_NA2_2_1_ fromORDERS orders0_ whereorders0_.customer_id=?
Customer [customerId=1, customerName=CC, orders=[Order [orderId=1, orderName=O-XXX-10], Order [orderId=2, orderName=O-JJ-2]]]
Hibernate: updateCUSTOMERS setCUSTOMER_NAME=? whereCUSTOMER_ID=?

④ 单向一对多删除操作

默认情况下, 若删除 1 的一端, 则会先把关联的 n 的一端的外键置空, 然后进行删除。

测试代码如下：

 @Testpublic void testOneToManyDel(){Customer customer = session.get(Customer.class,1);session.delete(customer);}

测试结果如下：

//先查询
Hibernate: selectcustomer0_.CUSTOMER_ID as CUSTOMER1_0_0_,customer0_.CUSTOMER_NAME as CUSTOMER2_0_0_ fromCUSTOMERS customer0_ wherecustomer0_.CUSTOMER_ID=?
//更新Order表的外键Customer_ID 为null
Hibernate: updateORDERS setcustomer_id=null wherecustomer_id=?//删除Customers
Hibernate: delete fromCUSTOMERS whereCUSTOMER_ID=?

综上可知，只有在持久化操作和删除操作时，单向一对多和单向多对一有所不同，在查找和更新时没有什么区别。

博文是基于XML进行讲解，那么注解版的如何使用呢？

参考博文：注解版的单向一对多