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Hibernate关联映射---多对一实例分析(单项关联)

2011-12-08 10:09 477 查看

关联映射---多对一实例分析
一关联映射类型

1．多对一(Employee - Department)

    多对一关联是一个部门可以有多个员工

    所用到的映射文件为：

       <many-to-one name="depart" column="depart_id"/>

二代码分析

1．首先创建两个类，Department和Employee，并把他们关联起来

（1）Department类

package com.hbsi.domain;
//部门类
public class Department {
    private int id;
    private String name;
    public Department() {
       super();
       // TODO Auto-generated constructor stub
    }
    public Department(int id, String name) {
       super();
       this.id = id;
       this.name = name;
    }
    public int getId() {
       return id;
    }
    public void setId(int id) {
       this.id = id;
    }
    public String getName() {
       return name;
    }
    public void setName(String name) {
       this.name = name;
    }
    public String toString() {
       return "Department [id=" + id + ", name=" + name + "]";
    }
}
（2）Employee类

    package com.hbsi.domain;

//员工类   一般主鍵是建在多的一方

public class Employee {

    private int id;

    private String name;

    // 得到的是一个对象，可以得到员工对应的部门的详细信息

    private Department depart;



    public Employee() {

       super();

       // TODO Auto-generated constructor stub

    }



    public Employee(int id, String name, Department depart) {

       super();

       this.id = id;

       this.name = name;

       this.depart = depart;

    }



    public int getId() {

       return id;

    }



    public void setId(int id) {

       this.id = id;

    }



    public String getName() {

       return name;

    }



    public void setName(String name) {

       this.name = name;

    }



    public Department getDepart() {

       return depart;

    }



    public void setDepart(Department depart) {

       this.depart = depart;

    }

    public String toString() {

       return "Employee [id=" + id + ", name=" + name + ", depart=" + depart

              + "]";

    }

}

注：之所以在员工表中设主键是为了减少资源消耗，而如果设置

private int depart_id;

通过员工查找部门的话，只能找到部门的id,得不到部门的其他信息，

设置的Deparment的类对象作为外键，可以得到部门的所有信息



（3）配置文件

    <!DOCTYPE hibernate-configuration PUBLIC
    "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
    "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
    <session-factory>
       <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
       <property name="hibernate.connection.url">jdbc:mysql:///demo</property>
       <property name="hibernate.connection.username">root</property>
       <property name="hibernate.connection.password">1234</property>

       
       <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
       
       <property name="hibernate.hbm2ddl.auto">update</property>
       
       <property name="hibernate.show_sql">true</property>
       
       <mapping resource="com/hbsi/domain/Department.hbm.xml" />
       <mapping resource="com/hbsi/domain/Employee.hbm.xml" />
    </session-factory>
</hibernate-configuration>

（4）Department映射文件

    <?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.hbsi.domain">

    
    <class name="Department" table="department">
       <id name="id" column="id">
           <generator class="native" />
       </id>
       <property name="name" column="name" />
    </class>
</hibernate-mapping>
（5）Employee映射文件

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.hbsi.domain">
    <class name="Employee" table="employee">
       <id name="id" column="id">
           <generator class="native" />
       </id>
       <property name="name" column="name" />
       
       <many-to-one name="depart" column="depart_id"not-null="true"/>
    </class>
</hibernate-mapping>
注：一般情况下，主键是不为空的，所以应该在<many-to-one>标签中加一条语句 not-null="true"
(6)测试---保存的操作

package com.hbsi.test;

import org.hibernate.Session;
import org.hibernate.Transaction;

import com.hbsi.domain.Department;
import com.hbsi.domain.Employee;
import com.hbsi.hibernate.utils.HibernateUtil;

public class Many2One {

    public static void main(String[] args) {
       add();
    }
//保存操作
    static Department add() {
       Session session = null;
       Transaction transaction = null;
       try {
           session = HibernateUtil.getSession();
           transaction = session.beginTransaction();
           // 添加部门
           Department department = new Department();
           department.setName("老王公司 one");

           Employee employee1 = new Employee();
           employee1.setName("老王");
           employee1.setDepart(department);// 对象模型：对象建立关联关系

           //保存操作
           session.save(department);
           session.save(employee1);

           transaction.commit();
           return department;
       } finally {
           if (session != null) {
              session.close();
           }
       }
    }
}
注：
需要注意的是下面两句话顺序不同，执行的结果不同，
（1）如果是先保存employee，即操作为：
session.save(employee1);
session.save(department);
结果为
Hibernate: insert into employee (name, depart_id) values (?, ?)
    Hibernate:insert into department (name) values (?)
    Hibernate: update employee set name=?, depart_id=? where id=?

（2）如果一的一方先执行,即操作的是
session.save(department);
    session.save(employee1);
执行的语句为：
    Hibernate: insert into department (name) values (?)
    Hibernate: insert into employee (name, depart_id) values (?, ?)


//通过查询员工表的id获取到部门表的name

    static Employee query(int empId) {
       Session session = null;
       try {
           session = HibernateUtil.getSession();
           Employee employee = (Employee) session.get(Employee.class, empId);
    System.out.println(employee.getName()+"=="+employee.getDepart().getName());
           return employee;
       } finally {
           if (session != null) {
              session.close();
           }
       }
    }

注：
(1)如果把输出的语句

    System.out.println(employee.getName()+"=="+employee.getDepart().getName());
放到主方法中会抛异常:
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=?
Exception in thread "main" org.hibernate.LazyInitializationException: could not initialize proxy - no Session
    at org.hibernate.proxy.AbstractLazyInitializer.initialize(AbstractLazyInitializer.java:167)
    at org.hibernate.proxy.AbstractLazyInitializer.getImplementation(AbstractLazyInitializer.java:215)
    at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.invoke(JavassistLazyInitializer.java:190)
    at com.hbsi.domain.Department_$$_javassist_1.getName(Department_$$_javassist_1.java)
    at com.hbsi.test.Many2One.main(Many2One.java:16)

原因是懒加载超时异常,因为可以看出只输出一条语句,没有查找出DEepartment，因为它就相当于懒加载
(2)如果非要把输出语句放到方法中，比避免抛异常，可以在执行的时候写一条语句用来初始化Department：Hibernate.initialize(employee.getDepart());

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标签： hibernate session string constructor generator insert

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