hibernate 多对一(Many-to-one)单向关联

多对一实例（Employee-Department）

1. E-R图：

2. 实体类：

Department类：

package com.reiyen.hibernate.domainpublic class Department {private int id;private String name; //setter和getter方法....}

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?Employee类:

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package com.reiyen.hibernate.domainpublic class Employee {private int id;private String name;private Department department;//setter和getter方法}

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? 3. 实体映射文件：

Department.hbm.xml如下：

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?Employee.hbm.xml如下：

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?<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）

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<mapping resource="com/reiyen/hibernate/domain/Department.hbm.xml" /><mapping resource="com/reiyen/hibernate/domain/Employee.hbm.xml" />

?4. 测试：

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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：

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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：

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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

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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：

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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()); // 3tx.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 = ?

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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

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Hibernate.initialize(emp.getDepartment())

否则，就不初始化代理对象）。

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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。