hibernate 多对一关系映射
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实体是Employee和Department,它们之间是多对一的关系。
Department类:
Employee类:
Department.hbm.xml:
Employee.hbm.xml:
many-to-one没有inverse属性,因为关系的维护是many的一方,不可能放弃对关系的维护。
many-to-one的lazy属性有三个取值:false, proxy, no-proxy。
1. 测试cascade属性:
结果是报org.hibernate.TransientObjectException异常,因为没有保存Department实例。
可以加cascade属性,解决问题:
2. 测试fetch
查询语句如下:
Hibernate: select employee0_.id as id1_0_, employee0_.name as name1_0_, employee0_.department as department1_0_, employee0_.skill as skill1_0_, employee0_.sell as sell1_0_, employee0_.type as type1_0_ from Employee employee0_ where employee0_.id=?
Hibernate: select department0_.id as id0_0_, department0_.name as name0_0_ from Department department0_ where department0_.id=?
因为fetch设置为select,所以对每个实体,都分别用一个SELECT语句
如果把fetch设置为join,也就是连表查询,只使用一个SELECT语句。如下:
Hibernate: select employee0_.id as id1_1_, employee0_.name as name1_1_, employee0_.department as department1_1_, employee0_.skill as skill1_1_, employee0_.sell as sell1_1_, employee0_.type as type1_1_, department1_.id as id0_0_, department1_.name as name0_0_ from Employee employee0_ left outer join Department department1_ on employee0_.department=department1_.id where employee0_.id=?
3. 测试lazy
当fetch为select时,设置lazy为proxy或者no-proxy。
结果是报org.hibernate.LazyInitializationException异常。
因为fetch为select,而且lazy为proxy或者no-proxy,所以开始仅仅查询Employee,当需要用SELECT语句查询Department时,Session已经关闭。
解决办法:
1. 设置lazy为false,hibernate会第一时间把Employee和Department查询出来。
如果fetch为select,使用两个SELECT查询语句。
如果fetch为join,使用一个SELECT连表查询语句。
2. 设置fetch为join,这时不管lazy的取值,hibernate会进行连表查询,把两个实体都查询出来。
Department类:
- public class Department {
- private int id;
- private String name;
- public Department() {
- }
- public Department(String name) {
- this.name = name;
- }
- // getters and setters are omitted
- }
Employee类:
- public class Employee {
- private int id;
- private String name;
- private Department department;
- public Employee() {
- }
- public Employee(String name) {
- this.name = name;
- }
- // getters and setters are omitted
Department.hbm.xml:
- <hibernate-mapping
- package="com.john.myhibernate.domain">
- <class name="Department">
- <id name="id">
- <generator class="native"/>
- </id>
- <property name="name" length="20" not-null="true"/>
- </class>
- </hibernate-mapping>
Employee.hbm.xml:
- <hibernate-mapping package="com.john.myhibernate.domain">
- <class name="Employee">
- <id name="id">
- <generator class="native"/>
- </id>
- <property name="name" length="20" not-null="true"/>
- <many-to-one name="department" column="department_id" class="Department" fetch="select"/>
- </class>
- </hibernate-mapping>
many-to-one没有inverse属性,因为关系的维护是many的一方,不可能放弃对关系的维护。
many-to-one的lazy属性有三个取值:false, proxy, no-proxy。
1. 测试cascade属性:
- public void testSaveCascade() {
- Session s = null;
- Transaction tx = null;
- Department depart = new Department();
- depart.setName("FCI");
- Employee em1 = new Employee("John");
- em1.setDepartment(depart);
- Employee em2 = new Employee("Lucy");
- em2.setDepartment(depart);
- try {
- s = HibernateUtil.getSession();
- tx = s.beginTransaction();
- s.save(em1);
- s.save(em2);
- tx.commit();
- } catch (HibernateException e) {
- tx.rollback();
- e.printStackTrace();
- } finally {
- if (s != null)
- s.close();
- }
- }
结果是报org.hibernate.TransientObjectException异常,因为没有保存Department实例。
可以加cascade属性,解决问题:
- <many-to-one name="department" column="department_id" class="Department" fetch="select" cascade="save-update"/>
2. 测试fetch
- Session s = null;
- s = HibernateUtil.getSession();
- Employee em = (Employee) s.get(Employee.class, 2);
- System.out.println(em.getName());
- System.out.println(em.getDepartment());
查询语句如下:
Hibernate: select employee0_.id as id1_0_, employee0_.name as name1_0_, employee0_.department as department1_0_, employee0_.skill as skill1_0_, employee0_.sell as sell1_0_, employee0_.type as type1_0_ from Employee employee0_ where employee0_.id=?
Hibernate: select department0_.id as id0_0_, department0_.name as name0_0_ from Department department0_ where department0_.id=?
因为fetch设置为select,所以对每个实体,都分别用一个SELECT语句
如果把fetch设置为join,也就是连表查询,只使用一个SELECT语句。如下:
Hibernate: select employee0_.id as id1_1_, employee0_.name as name1_1_, employee0_.department as department1_1_, employee0_.skill as skill1_1_, employee0_.sell as sell1_1_, employee0_.type as type1_1_, department1_.id as id0_0_, department1_.name as name0_0_ from Employee employee0_ left outer join Department department1_ on employee0_.department=department1_.id where employee0_.id=?
3. 测试lazy
当fetch为select时,设置lazy为proxy或者no-proxy。
- <many-to-one name="department" column="department_id" class="Department" fetch="select" cascade="save-update" lazy="no-proxy"/>
- Session s = null;
- s = HibernateUtil.getSession();
- Employee em = (Employee) s.get(Employee.class, 2);
- s.close();
- System.out.println(em.getName());
- System.out.println(em.getDepartment());
结果是报org.hibernate.LazyInitializationException异常。
因为fetch为select,而且lazy为proxy或者no-proxy,所以开始仅仅查询Employee,当需要用SELECT语句查询Department时,Session已经关闭。
解决办法:
1. 设置lazy为false,hibernate会第一时间把Employee和Department查询出来。
如果fetch为select,使用两个SELECT查询语句。
如果fetch为join,使用一个SELECT连表查询语句。
2. 设置fetch为join,这时不管lazy的取值,hibernate会进行连表查询,把两个实体都查询出来。
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