Classes类:
package com.cnblogs.hibernate_first;import java.util.Set;public class Classes { ???private int id; ???private String name; ???????//hibernate对Set进行了扩展，可以延时加载，在需要的时候才需要查询SET。比如有1000个学生，当需要的时候才进行加载 ???private Set students; ???public Set getStudents() { ???????return students; ???} ???public void setStudents(Set students) { ???????this.students = students; ???} ???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; ???}}Student:
public class Student { ???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; ???}}classes.hbm.xml
<?xml version="1.0" encoding="UTF-8"?><!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.cnblogs.hibernate_first"> ???<class name="Classes" table="t_classes"> ???????<id name="id" column="t_id"> ???????????<generator class="native"></generator> ???????</id> ???????<property name="name" column="t_name" length="30"/> ???????<set name="students"> ???????????<key column="classesid"></key> ???????????<one-to-many class="com.cnblogs.hibernate_first.Student"/> ???????</set> ???</class></hibernate-mapping>
student.hbm.xml:
<?xml version="1.0" encoding="UTF-8"?><!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.cnblogs.hibernate_first"> ???<class name="Student" table="t_student"> ???????<id name="id" column="t_id"> ???????????<generator class="native"></generator> ???????</id> ???????<property name="name" column="t_name" length="30"/> ???</class></hibernate-mapping>
测试类：
package com.cnblogs.hibernate_first;import java.util.HashSet;import java.util.Iterator;import java.util.Set;import org.hibernate.Session;import junit.framework.TestCase;public class One2ManyTest extends TestCase { ???public void testSave1() { ???????Session session = null; ???????try { ???????????session = HibernateUtils.getSession(); ???????????session.beginTransaction(); ???????????// 建立关联 ???????????Student student1 = new Student(); ???????????student1.setName("张三"); ???????????????????????Student student2 = new Student(); ???????????student2.setName("李四"); ???????????Classes classes = new Classes(); ???????????classes.setName("动力节点"); ???????????Set<Student> students = new HashSet<Student>(); ???????????students.add(student1); ???????????students.add(student2); ???????????classes.setStudents(students); ???????????//因为student不是持久化状态，他是Transient状态 ???????????session.save(classes); ???????????session.getTransaction().commit(); ???????} catch (Exception e) { ???????????e.printStackTrace(); ???????????session.getTransaction().rollback(); ???????} finally { ???????????HibernateUtils.closeSession(session); ???????} ???} ???????public void testSave2() { ???????Session session = null; ???????try { ???????????session = HibernateUtils.getSession(); ???????????session.beginTransaction(); ???????????// 建立关联 ???????????Student student1 = new Student(); ???????????student1.setName("张三"); ???????????session.save(student1); ???????????????????????Student student2 = new Student(); ???????????student2.setName("李四"); ???????????session.save(student2); ???????????????????????Classes classes = new Classes(); ???????????classes.setName("动力节点"); ???????????Set<Student> students = new HashSet<Student>(); ???????????students.add(student1); ???????????students.add(student2); ???????????classes.setStudents(students); ???????????//因为student不是持久化状态，他是Transient状态 ???????????session.save(classes); ???????????session.getTransaction().commit(); ???????} catch (Exception e) { ???????????e.printStackTrace(); ???????????session.getTransaction().rollback(); ???????} finally { ???????????HibernateUtils.closeSession(session); ???????} ???} ???public void testLoad1() { ???????Session session = null; ???????try { ???????????session = HibernateUtils.getSession(); ???????????session.beginTransaction(); ???????????Classes classes = (Classes)session.load(Classes.class, 1); ???????????System.out.println("Classes.name = " + classes.getName()); ???????????Set students = classes.getStudents(); ???????????for(Iterator iter = students.iterator();iter.hasNext();){ ???????????????Student student = (Student)iter.next(); ???????????????System.out.println("Student in Classes " + student.getName()); ???????????} ???????????session.getTransaction().commit(); ???????} catch (Exception e) { ???????????e.printStackTrace(); ???????????session.getTransaction().rollback(); ???????} finally { ???????????HibernateUtils.closeSession(session); ???????} ???}}hibernate 一对多的关联映射（单向 Classes ——》Student）
hibernate对Set进行了扩展，可以延时加载，在需要的时候才需要查询SET。比如有1000个学生，当需要的时候才进行加载
一对多和多对一的映射原理是一样的，都是在多的一端加入一个外键指向一的一端
他们的区别在于维护的关系不用
多对一维护的关系：多指向一的关系，如果维护了多指向一的关系，那么加载多的时候会把一加载上来。
一对多维护的关系：一指向多的关系，如果维护了一指向多的关系，那么加载一的时候会把一加载上来。
在一的一端维护关系存在缺陷
*因为多的一端Student不知道Classes的存在（也就是Student没有维护与Classes的关系），所以在保存Student的时候关系字段Classid为null，
如果将该关系字段设置为非空，则将无法保存数据。
*另外因为Student不维护关系，而Classes维护关系，Classes就会发出多余的update语句，保证Classes与Student有关系，，这样加载Classes的
时候，才能把该Classes对应的学生集合加载上来。
hibernate one2many (单向关联)
原文地址：http://www.cnblogs.com/Juli/p/7743938.html