hibernate多对一关联映射

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">     <class name="Dept" table="dept" schema="svse">         <id name="deptno" type="int">             <generator class="native"></generator>         </id>         <property name="dname" length="50" type="string"></property>     </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">     <class name="Employee" table="employee" schema="ADMIN" select-before-update="true">         <id name="empno"  type="int">             <generator class="native"></generator>         </id>         <property name="ename" length="50" type="string"></property>                 <!-- 映射部门（员工与部门是多对一的关系） -->         <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！