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）
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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">
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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 name="code"><?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 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
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