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orders
  public 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;
  }

  @Override
  public 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;
  }

  @Override
  public 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。

@Test
  public 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 
    into
        CUSTOMERS
        (CUSTOMER_NAME) 
    values
        (?)
Hibernate: 
    insert 
    into
        ORDERS
        (ORDER_NAME) 
    values
        (?)
Hibernate: 
    insert 
    into
        ORDERS
        (ORDER_NAME) 
    values
        (?)
Hibernate: 
    update
        ORDERS 
    set
        customer_id=? 
    where
        ORDER_ID=?
Hibernate: 
    update
        ORDERS 
    set
        customer_id=? 
    where
        ORDER_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：

@Test
  public 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: 
    select
        customer0_.CUSTOMER_ID as CUSTOMER1_0_0_,
        customer0_.CUSTOMER_NAME as CUSTOMER2_0_0_ 
    from
        CUSTOMERS customer0_ 
    where
        customer0_.CUSTOMER_ID=?
Hibernate: 
    select
        orders0_.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_ 
    from
        ORDERS orders0_ 
    where
        orders0_.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：

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

测试结果如下：

Hibernate: 
    select
        customer0_.CUSTOMER_ID as CUSTOMER1_0_0_,
        customer0_.CUSTOMER_NAME as CUSTOMER2_0_0_ 
    from
        CUSTOMERS customer0_ 
    where
        customer0_.CUSTOMER_ID=?
Hibernate: 
    select
        orders0_.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_ 
    from
        ORDERS orders0_ 
    where
        orders0_.customer_id=?
Hibernate: 
    update
        ORDERS 
    set
        ORDER_NAME=? 
    where
        ORDER_ID=?

---

单独更新CustomerName如下：

@Test
  public 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: 
    select
        customer0_.CUSTOMER_ID as CUSTOMER1_0_0_,
        customer0_.CUSTOMER_NAME as CUSTOMER2_0_0_ 
    from
        CUSTOMERS customer0_ 
    where
        customer0_.CUSTOMER_ID=?
CC
Hibernate: 
    select
        orders0_.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_ 
    from
        ORDERS orders0_ 
    where
        orders0_.customer_id=?
Customer [customerId=1, customerName=CC, orders=[Order [orderId=1, orderName=O-XXX-10], Order [orderId=2, orderName=O-JJ-2]]]
Hibernate: 
    update
        CUSTOMERS 
    set
        CUSTOMER_NAME=? 
    where
        CUSTOMER_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：

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

测试结果如下：

//先查询
Hibernate: 
    select
        customer0_.CUSTOMER_ID as CUSTOMER1_0_0_,
        customer0_.CUSTOMER_NAME as CUSTOMER2_0_0_ 
    from
        CUSTOMERS customer0_ 
    where
        customer0_.CUSTOMER_ID=?
//更新Order表的外键Customer_ID 为null
Hibernate: 
    update
        ORDERS 
    set
        customer_id=null 
    where
        customer_id=?
 //删除Customers
Hibernate: 
    delete 
    from
        CUSTOMERS 
    where
        CUSTOMER_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