Hibernate系列（六）：多对一关联关系的映射、分析及添加、查询

这里举例子用的是：部门与员工的关系。

一个部门可以对应多个员工，这就是很明显的多对一关联关系。

我们需要建立两个实体类：员工（Employee）、部门（Department）

员工类：Employee.java（在这里边我们增加了一个Department类型的属性，用来做关联）

package cn.itcast.hibernate.domain;

public class Employee {
	private int id;
	private String name;
	private Department 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;
	}
}

package cn.itcast.hibernate.domain;

public class Department {
	private int id;
	private String 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;
	}
}

Employee.hbm.xml（在这里我们增加了一个<many-to-one />标签用来规定映射）

<?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="cn.itcast.hibernate.domain">
	<class name="Employee" table="tb_Employee">
		<id name="id">
			<generator class="native"/> 
		</id>
		<property name="name" />
		<many-to-one name="depart" column="depart_id" />
	</class>
	
</hibernate-mapping>

<?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="cn.itcast.hibernate.domain">

	<class name="Department" table="tb_Department">
		<id name="id">
			<generator class="native"/> 
		</id>
		<property name="name" />
	</class>
	
</hibernate-mapping>

<?xml version='1.0' encoding='utf-8'?> 
<!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
	<session-factory>
		<property name="connection.driver_class">
			com.mysql.jdbc.Driver
		</property>
		<property name="connection.url">
			jdbc:mysql://localhost:3306/test
		</property>
		<property name="connection.username">sa</property>
		<property name="connection.password">123456</property>
		
		<property name="hbm2ddl.auto">create</property>
		<property name="dialect">
			org.hibernate.dialect.MySQLDialect
		</property>
		<property name="show_sql">true</property>
		
		<mapping resource="cn/itcast/hibernate/domain/User.hbm.xml" />
		<mapping resource="cn/itcast/hibernate/domain/Department.hbm.xml" />
		<mapping resource="cn/itcast/hibernate/domain/Employee.hbm.xml" />
	</session-factory>
</hibernate-configuration>

我们建立一个Many2One.java文件：

package cn.itcast.hibernate;

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

import cn.itcast.hibernate.domain.Department;
import cn.itcast.hibernate.domain.Employee;

public class Many2One {
	public static void main(String[] arg){
		add();
		Employee emp = query(1);
	}
	
	static Department add(){
		Session s = null;
		Transaction tx = null;
		try{
			Department depart = new Department();
			depart.setName("depart name");
			
			Employee emp = new Employee();
			emp.setDepart(depart); //建立两个对象的关联关系
			emp.setName("emp name");
			
			s = HibernateUtil.getSession();
			tx = s.beginTransaction();
			s.save(emp);   //emp对象和depart对象保存的先后顺序不同也会有一定的差异
			s.save(depart); //如果员工(emp)先插入，部门(depart)后插入，会生成三条数据库语句，
							//因为在两者都插入之后，emp的depart_id字段还未插入，所以还要有最后一条语句插入
			tx.commit();
			return depart;
		}finally{
			if(s!=null){
				s.close();
			}
		}
	}
	
	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); //根据ID查询
			System.out.println(emp.getName()); //查询员工姓名
			System.out.println(emp.getDepart().getName()); //根据员工查询属于哪个部门
			tx.commit();
			return emp;
		}finally{
			if(s!=null){
				s.close();
			}
		}
	}
}

由此可见，Hibernate可以直接用面向对象的语言直接操作关系型数据库。