多对一实例（Employee-Department）
1. E-R图：
2. 实体类：
Department类：
- package com.reiyen.hibernate.domain
- public class Department {
- private int id;
- private String name;
- //setter和getter方法....
- }
package com.reiyen.hibernate.domain
public class Department {
private int id;
private String name;
//setter和getter方法....
}
Employee类:
- package com.reiyen.hibernate.domain
- public class Employee {
- private int id;
- private String name;
- private Department department;
- //setter和getter方法
- }
package com.reiyen.hibernate.domain
public class Employee {
private int id;
private String name;
private Department department;
//setter和getter方法
}
3. 实体映射文件：
Department.hbm.xml如下：
- <?xml version="1.0"?>
- <!DOCTYPE hibernate-mapping PUBLIC
- "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
- "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
- <hibernate-mapping package="com.reiyen.hibernate.domain">
- <class name="Department" >
- <id name="id" >
- <generator class="native" />
- </id>
- <property name="name" />
- </class>
- </hibernate-mapping>
<?xml version="1.0"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping package="com.reiyen.hibernate.domain"> <class name="Department" > <id name="id" > <generator class="native" /> </id> <property name="name" /> </class> </hibernate-mapping>
Employee.hbm.xml如下：
- <?xml version="1.0"?>
- <!DOCTYPE hibernate-mapping PUBLIC
- "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
- "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
- <hibernate-mapping package="com.reiyen.hibernate.domain">
- <class name="Employee">
- <id name="id">
- <generator class="native" />
- </id>
- <property name="name" />
- <!-- name="department" 这个名称必须与Employee中的属性名一致. -->
- <many-to-one name="department" column="depart_id" class="Department" />
- </class>
- </hibernate-mapping>
<?xml version="1.0"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping package="com.reiyen.hibernate.domain"> <class name="Employee"> <id name="id"> <generator class="native" /> </id> <property name="name" /> <!-- name="department" 这个名称必须与Employee中的属性名一致. --> <many-to-one name="department" column="depart_id" class="Department" /> </class> </hibernate-mapping>
<many-to-one name=”department” column=”depart_id” />这种缺省情况,
hibernate会默认去department对象中查找主键值,因为hibernate默认的是外键对应另一个表中的主键的,
如果想对应department中的其它属性,如”name”,则可以使用<many-to-one name=”department” column=”depart_id” property-ref=”name”/>也可以使用<many-to-one name=”department” />,这与 <many-to-one name=”depart” column=”depart_id” />的唯一区别就是它的column名也为department了,而不是depart_id.
<many-to-one >元素建立了department属性和employee表的外键depart_id之间的映射.
name: 设定待映射的持久化类的属性名,此外为employee类的department属性.
column: 设定和持久化类的属性对应的表的外键,此外为employee表的外键depart_id.
class(可选):设定持久化类的属性的类型,此处设定department的类型是Department类.
not-null(可选):如果为true,表示department属性不能为null,该属性的默认值为false.当为true时,生成的employee表中depart_id外键会设置not-null约束,所以当Hibernate保存Employee对象时,会先检查它的department属性是否为null,如果为null,则会抛出异常.
3. 将两个实体映射文件在hibernate.cfg.xml配置文件中注册：（hibernate.cfg.xml在我一篇文章：第一个Hibernate实例 中有）
- <mapping resource="com/reiyen/hibernate/domain/Department.hbm.xml" />
- <mapping resource="com/reiyen/hibernate/domain/Employee.hbm.xml" />
<mapping resource="com/reiyen/hibernate/domain/Department.hbm.xml" /> <mapping resource="com/reiyen/hibernate/domain/Employee.hbm.xml" />
4. 测试：
- public class Many2One {
- public static void main(String[] args) {
- Department depart = add();
- System.out.println("depart name: " + depart.getName());
- }
- static Department add() {
- Session s = null;
- Transaction tx = null;
- try {
- Department depart = new Department();
- depart.setName("department name");
- Employee employee = new Employee();
- employee.setDepartment(depart); // 对象模型：建立两个对象的关联
- employee.setName("employee name");
- s = HibernateUtil.getSession();
- tx = s.beginTransaction();
- s.save(depart);
- s.save(employee);
- tx.commit();
- return depart;
- } finally {
- if (s != null)
- s.close();
- }
- }
- }
public class Many2One {
public static void main(String[] args) {
Department depart = add();
System.out.println("depart name: " + depart.getName());
}
static Department add() {
Session s = null;
Transaction tx = null;
try {
Department depart = new Department();
depart.setName("department name");
Employee employee = new Employee();
employee.setDepartment(depart); // 对象模型：建立两个对象的关联
employee.setName("employee name");
s = HibernateUtil.getSession();
tx = s.beginTransaction();
s.save(depart);
s.save(employee);
tx.commit();
return depart;
} finally {
if (s != null)
s.close();
}
}
}
运行后，控制台打印信息如下：
Hibernate: insert into Department (name) values (?)
Hibernate: insert into Employee (name, depart_id) values (?, ?)
depart name: department name
查看Employee数据库表的创建语句如下：（注意红色部分）
DROP TABLE IF EXISTS `test`.`employee`;
CREATE TABLE `test`.`employee` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
`depart_id` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `FK4AFD4ACEA3E9AC0F` (`depart_id`),
CONSTRAINT `FK4AFD4ACEA3E9AC0F` FOREIGN KEY (`depart_id`) REFERENCES `department` (`id`)) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;
表中插入记录如下：
- mysql> select * from employee;
- +----+---------------+-----------+
- | id | name | depart_id |
- +----+---------------+-----------+
- | 1 | employee name | 1 |
- +----+---------------+-----------+
- 1 row in set (0.00 sec)
- mysql> select * from department;
- +----+-----------------+
- | id | name |
- +----+-----------------+
- | 1 | department name |
- +----+-----------------+
- 1 row in set (0.00 sec)
mysql> select * from employee; +----+---------------+-----------+ | id | name | depart_id | +----+---------------+-----------+ | 1 | employee name | 1 | +----+---------------+-----------+ 1 row in set (0.00 sec) mysql> select * from department; +----+-----------------+ | id | name | +----+-----------------+ | 1 | department name | +----+-----------------+ 1 row in set (0.00 sec)
调换测试类两句程序的顺序,如下所示：
- s.save(employee);
- s.save(depart);
s.save(employee); s.save(depart);
则控制台的信息如下所示：（多了一条更新语句）
Hibernate: insert into Employee (name, depart_id) values (?, ?)
Hibernate: insert into Department (name) values (?)
Hibernate: update Employee set name=?, depart_id=? where id=?
depart name: department name
- employee.setDepartment(depart); // 对象模型：建立两个对象的关联
employee.setDepartment(depart); // 对象模型：建立两个对象的关联
这句话很重要，如果没有这句话，则Department和Employee之间就没有任何关系了。
如果你在Employee.hbm.xml中配置了
<many-to-one name="department" column="depart_id" not-null="true"/>
则必须先保存department再保存employee，否则会抛出：
org.hibernate.PropertyValueException: not-null property references a null or transient value 异常！
查询测试：
- public class Many2One {
- public static void main(String[] args) {
- Department depart = add();
- Employee employee = query(1);
- // System.out.println("depart name:" + employee.getDepartment().getName()); // 2
- }
- static Employee query(int empId) {
- Session s = null;
- Transaction tx = null;
- try {
- s = HibernateUtil.getSession();
- tx = s.beginTransaction();
- Employee emp = (Employee) s.get(Employee.class, empId);
- System.out.println("depart name:" + emp.getDepartment().getName()); // 1
- // System.out.println("depart class:" + emp.getDepartment().getClass()); // 4
- //System.out.println("depart id:" + emp.getDepartment().getId()); //5
- // Hibernate.initialize(emp.getDepartment()); // 3
- tx.commit();
- return emp;
- } finally {
- if (s != null)
- s.close();
- }
- }
- }
public class Many2One {
public static void main(String[] args) {
Department depart = add();
Employee employee = query(1);
// System.out.println("depart name:" + employee.getDepartment().getName()); // 2
}
static Employee query(int empId) {
Session s = null;
Transaction tx = null;
try {
s = HibernateUtil.getSession();
tx = s.beginTransaction();
Employee emp = (Employee) s.get(Employee.class, empId);
System.out.println("depart name:" + emp.getDepartment().getName()); // 1
// System.out.println("depart class:" + emp.getDepartment().getClass()); // 4
//System.out.println("depart id:" + emp.getDepartment().getId()); //5
// Hibernate.initialize(emp.getDepartment()); // 3
tx.commit();
return emp;
} finally {
if (s != null)
s.close();
}
}
}
测试程序运行后，控制台打印出的sql如下:
Hibernate: select employee0_.id as id2_0_, employee0_.name as name2_0_, employee0_.depart_id as depart3_2_0_ from Employee employee0_ where employee0_.id=?
Hibernate: select department0_.id as id1_0_, department0_.name as name1_0_ from Department department0_ where department0_.id=?
depart name:department name
当相关联的session没有关闭时，访问这些懒加载对象（代理对象）的属性（getId和getClass除外）时，hibernate会初始化这些代理,所以将Employee对象和Department对象都查询出来了。相当于在数据库中执行两次查询:
首先执行select * from employee where employee.id = ?
然后再执行select * from department where department.id = ?
如果将测试程序中标记为2的语句前的注释去掉，同时把标记为1的语句注释掉，再运行刚会出现如下所示的异常：
org.hibernate.LazyInitializationException: could not initialize proxy - no Session
解决办法是把标记为3的语句前的注释去掉，即初始化懒加载的代理对象。（可以给query方法再传递一个参数，如boolean isInit,如果是true的话就初始化代理对象，即调用
- Hibernate.initialize(emp.getDepartment())
Hibernate.initialize(emp.getDepartment())
否则，就不初始化代理对象）。
5.懒加载说明：
many-to-one（元素）懒加载：1).lazy!=false 2).fetch=select
能够懒加载的对象都是被改写过的代理对象，当相关联的session没有关闭时，访问这些懒加载对象（代理对象）的属性（getId和getClass除外）时，hibernate会初始化这些代理，或用Hibernate.initialize(proxy)来初始化代理对象；
当相关联的session关闭后，再访问懒加载的对象将会出现异常。
（1）把标记为1的语句注释掉，同时去掉标记为4的语句前面的注释，进行测试，控制台打印信息如下所示：
Hibernate: select employee0_.id as id1_0_, employee0_.name as name1_0_, employee0_.depart_id as depart3_1_0_ from Employee employee0_ where employee0_.id=?
depart class:class com.itcast.hibernate.domain.Department$$EnhancerByCGLIB$$b412e7fa
说明实现了懒加载，它只查询出Employee对象，而没有去查询相应的Department对象；同时可以看出，返回的是改写过的代理对象，而不是实体类Department.
（2）把标记为1的语句注释掉，同时去掉标记为5的语句前面的注释，进行测试，控制台打印信息如下所示：
Hibernate: select employee0_.id as id1_0_, employee0_.name as name1_0_, employee0_.depart_id as depart3_1_0_ from Employee employee0_ where employee0_.id=?
depart id:1
当相关联的session没有关闭时，访问这些懒加载对象（代理对象）getId和getClass属性，hibernate不会初始化这些代理。