Hibernate 关联映射之一对多

Hibernate的关联映射包括：

一对一（Persion - IDCard）

一对多（Department - Employee）

多对一（Employee - Department）

多对多（Teacher - Student）

组件映射（User - Name）

集合映射（Set、List、Map）

inverse和cascade（Employee - Department）

本文以一对多为例：

一个部门对应多个员工（Department - Employee）

映射文件 <many-to-one name="depart" column="depart_id">

实体类：Department.java 和 Employee.java

package com.cos.entity;

import java.util.Set;

/**
 * 部门
 * @author wangjy
 */
public class Department {
	private int id;
	private String name;
	private Set<Employee> emps;//一对多
	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 Set<Employee> getEmps() {
		return emps;
	}
	public void setEmps(Set<Employee> emps) {
		this.emps = emps;
	}
}

package com.cos.entity;
/**
 * 员工
 * @author wangjy
 */
public class Employee {
	private int id;
	private String name;
	private Department dept;//多对一
	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 getDept() {
		return dept;
	}
	public void setDept(Department dept) {
		this.dept = dept;
	}
}

 映射文件：Department.hbm.xml 和 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>
	<class name="com.cos.entity.Department" lazy="true">
		<id name="id">
			<generator class="native"/>
		</id>	
		<property name="name"/>
		<set name="emps">
			<key column="dept_id"></key>
			<one-to-many class="com.cos.entity.Employee"/>
		</set>
	</class>
</hibernate-mapping>

 set标签中的name=emps是Department对象中的属性值。

 key标签中的column="dept_id"是Department对应表中的字段值。

 one-to-many标签中的class值是具体的对象。

<?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>
	<class name="com.cos.entity.Employee" lazy="true">
		<id name="id">
			<generator class="native"/>
		</id>	
		<property name="name"/>
		<many-to-one name="dept" column="dept_id"/>
	</class>
</hibernate-mapping>

Hibernate配置文件：hibernate.cfg.xml

<!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="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="hibernate.connection.url">jdbc:mysql:///hi</property>
        <property name="hibernate.connection.username"></property>
        <property name="hibernate.connection.password"></property>
        <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
        <property name="hibernate.hbm2ddl.auto">validate</property>
        <property name="show_sql">true</property>
		<mapping resource="com/cos/entity/Employee.hbm.xml"/>
		<mapping resource="com/cos/entity/Department.hbm.xml"/>
	</session-factory>
</hibernate-configuration>

工具类：HibernateUtil.java

package com.cos.util;

import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public final class HibernateUtil {

	private static SessionFactory sesseionFactory;

	static {
		Configuration conf = new Configuration();
		conf.configure();
		sesseionFactory = conf.buildSessionFactory();
	}

	public static SessionFactory getSesseionFactory() {
		return sesseionFactory;
	}
}

测试类：

package com.cos.main;

import java.util.HashSet;
import java.util.Set;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import com.cos.entity.Department;
import com.cos.entity.Employee;
import com.cos.util.HibernateUtil;

public class One2Many {

	public static void main(String[] args) {
		add();
	}
	public static void add(){
		SessionFactory  sessionFactory = null;
		Session s = null;
		Transaction t = null;
		try{
			sessionFactory = HibernateUtil.getSesseionFactory();
			s = sessionFactory.openSession();
			t = s.beginTransaction();
			Department dept = new Department();
			dept.setName("dept name 3");
			Employee emp1 = new Employee();
			emp1.setName("emp name 3");
			emp1.setDept(dept);
			Employee emp2 = new Employee();
			emp2.setName("emp name 33");
			emp2.setDept(dept);
			s.save(emp1);
			s.save(emp2);
			Set<Employee> set = new HashSet<Employee>();
			set.add(emp1);
			set.add(emp2);
			dept.setEmps(set);
			s.save(dept);
			t.commit();
			s.close();
			sessionFactory.close();
		}catch(Exception e){
			e.printStackTrace();
		}
	}
}

表结构：department表 和employee表

CREATE TABLE `department` (   
  `id` int(11) NOT NULL auto_increment,   
  `name` varchar(255) default NULL,   
  PRIMARY KEY  (`id`)   
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=latin1   
  
CREATE TABLE `employee` (   
  `id` int(11) NOT NULL auto_increment,   
  `name` varchar(255) default NULL,   
  `dept` int(11) default NULL,   
  PRIMARY KEY  (`id`),   
  KEY `FK4AFD4ACE66A70F4E` (`dept`),   
  CONSTRAINT `FK4AFD4ACE66A70F4E` FOREIGN KEY (`dept`) REFERENCES `department` (`id`)   
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=latin1