hibernate多对一关联映射

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hibernate多对一关联映射

关联映射，就是将关联关系映射到数据库中，所谓的关联关系在对象模型中就是一个或多个引用

多对一关联映射原理：在多的一端加入一个外键，指向一的一端

在多的一端采用如下标签映射：

<many-to-onename="group" column="groupid"/>

掌握级联的含义？

*级联是对象之间的连锁操作，它只影响添加、删除和修改

下面就以员工和部门的关系来进行说明

Dept.java

package org.xiaolong.entity;

public class Dept {

   private Integer deptno;
   private String dname;
   //省略getter和setter方法
}

<?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="org.xiaolong.entity">

</id>

</class>

</hibernate-mapping>

Dept.hbm.xml

Employee.java

package org.xiaolong.entity;

public class Employee {

private Integer empno;

private String ename;

private Dept dept;

//省略getter和setter方法

}

Employee.hbm.xml

<?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="org.xiaolong.entity">

</id>

<many-to-one name="dept" column="deptno" lazy="false"></many-to-one>

</class>

</hibernate-mapping>

IEmployeeDAO.java

package org.xiaolong.dao;

import java.util.List;

import org.xiaolong.entity.Dept;

import org.xiaolong.entity.Employee;

public interface IEmployeeDAO {

public void addDept(Dept dept);

public void addEmployee(Employee employee);

public void updateEmployee(Employee employee);

public void deleteEmployee(Integer empno);

public Employee getEmployeeById(Integer empno);

public List<Employee> getAllUserInfo();

public List<Employee> getUserInfo(String ename);

public Employee getUserInfoByName(String ename);

}

BaseDAO.java

package org.xiaolong.dao.impl;

import org.hibernate.Session;

import org.xiaolong.util.HibernateUtil;

public class BaseDAO {

public Session getSession(){

return HibernateUtil.getSession();

}

public void closeSession(){

HibernateUtil.closeSession();

}

EmployeeDAOImpl.java

package org.xiaolong.dao.impl;

import java.util.List;

import org.apache.log4j.Logger;

import org.hibernate.HibernateException;

import org.hibernate.Session;

import org.hibernate.Transaction;

import org.xiaolong.dao.IEmployeeDAO;

import org.xiaolong.entity.Dept;

import org.xiaolong.entity.Employee;

public class EmployeeDAOImpl extends BaseDAO implements IEmployeeDAO {

private Logger logger = Logger.getLogger(EmployeeDAOImpl.class);

@Override

public void addDept(Dept dept) {

logger.info("add dept start........");

Transaction tx = null;

try {

Session session = this.getSession();

tx = session.beginTransaction();

session.save(dept);

//提交事务

tx.commit();

logger.info("add dept success.......");

} catch (HibernateException e) {

if (tx != null) {

tx.rollback();

}

e.printStackTrace();

logger.info("add dept failure.......");

throw e;

} finally {

this.closeSession();

}

logger.info("add dept end........");

}

//添加员工

public void addEmployee(Employee employee) {

logger.info("add employee start........");

Transaction tx = null;

try {

Session session = this.getSession();

tx = session.beginTransaction();

session.save(employee);

//提交事务

tx.commit();

logger.info("add employee success.......");

} catch (HibernateException e) {

if (tx != null) {

tx.rollback();

}

e.printStackTrace();

logger.info("add employee failure.......");

throw e;

} finally {

this.closeSession();

}

logger.info("add employee end........");

}

public void deleteEmployee(Integer empno) {

}

public List<Employee> getAllUserInfo() {

return null;

}

public Employee getEmployeeById(Integer empno) {

logger.info("get employee by id start........");

Employee employee = null;

try {

Session session = this.getSession();

employee = (Employee)session.get(Employee.class, empno);

//employee = (Employee)session.load(Employee.class, empno);

} catch (HibernateException e) {

e.printStackTrace();

logger.info("get employee by id failure.......");

throw e;

} finally {

this.closeSession();

}

return employee;

}

public List<Employee> getUserInfo(String ename) {

return null;

}

public Employee getUserInfoByName(String ename) {

return null;

}

public void updateEmployee(Employee employee) {

}

编写单元测试类EmployeeDAOTest.java

public class EmployeeDAOTest {

private static IEmployeeDAO dao =null;

@BeforeClass

public static void setUpBeforeClass() throws Exception {

dao = new EmployeeDAOImpl();

}

@Test

public void addEmployee(){

Dept dept = new Dept();

dept.setDname("销售部");

Employee emp = new Employee();

emp.setEname("张三");

emp.setDept(dept);

Employee emp2 = new Employee();

emp2.setEname("张三");

emp2.setDept(dept);

//在清理缓存是发生错误TransientObjectException

//由于部门实体处理临时状态，没有被session管理，数据库中没有相应的记录

//而员工实体处于持久状态，在清理缓存的时候找不到Dept对象

//结论：Persistent状态的对象不能引用Transient状态的对象

dao.addEmployee(emp);

dao.addEmployee(emp2);

}

@Test

public void addEmployee1(){

Dept dept = new Dept();

dept.setDname("销售部");

dao.addDept(dept);

Employee emp = new Employee();

emp.setEname("张三");

emp.setDept(dept);

Employee emp2 = new Employee();

emp2.setEname("张三");

emp2.setDept(dept);

//可以正确保存

//因为Dept和Employee都是持久状态的对象

//Hibernate在清理缓存的时候能找到关联的对象

dao.addEmployee(emp);

dao.addEmployee(emp2);

}

@Test

public void addEmployee2(){

Dept dept = new Dept();

dept.setDname("策划部");

dao.addDept(dept);

Employee emp = new Employee();

emp.setEname("三毛");

emp.setDept(dept);

Employee emp2 = new Employee();

emp2.setEname("小龙");

emp2.setDept(dept);

//可以正确保存,因为此处使用了级联特性

//Hibernate会首先保存与Employee关联的对象Dept

dao.addEmployee(emp);

dao.addEmployee(emp2);

}

@Test

public void loadEmployee(){

Employee employee = dao.getEmployeeById(2);

System.out.println(employee.getEname());

System.out.println(employee.getDept().getDname());

}

注意：在多对一关联映射中，采用级联删除操作是有问题的，如果删除员工，同时把员工所在部门删掉啦，而其他在此部门的员工怎么办。。。。。。

而在一对多种还是符合逻辑的！