hibernate学习5之one-to-many双向关联

一对多双向关联映射：
* 在一一端的集合上使用<key>，在对方表中加入一个外键指向一一端
* 在多一端采用<many-to-one>

<!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.url">jdbc:mysql://localhost/hibernate_one2many_1</property>
    <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
    <property name="hibernate.connection.username">root</property>
    <property name="hibernate.connection.password">bjsxt</property>
    <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
    <property name="hibernate.show_sql">true</property>
    
    <mapping resource="com/bjsxt/hibernate/Classes.hbm.xml"/>
    <mapping resource="com/bjsxt/hibernate/Student.hbm.xml"/>
  </session-factory>
</hibernate-configuration>

public class Student {
  private int id; 
  private String name;
  private Classes classes;//多对一，多个学生对应一个班级
  //setter，getter
}

import java.util.Set;

public class Classes {
  private int id;
  private String name;
  private Set students; //一对多
  //setter，getter
  
}

Student.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.bjsxt.hibernate.Student" table="t_student">
    <id name="id">
      <generator class="native"/>
    </id>
    <property name="name"/>
    <many-to-one name="classes" column="classesid"/>
  </class>
</hibernate-mapping>

Classes.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 package="com.bjsxt.hibernate">
  <class name="Classes" table="t_classes">
    <id name="id">
      <generator class="native"/>
    </id>
    <property name="name"/>
    <set name="students" inverse="true" cascade="all">
      <key column="classesid"/>
      <one-to-many class="Student"/>
    </set>
  </class>
</hibernate-mapping>

注意：<key>标签指定的外键字段(这里是classesid)必须和<many-to-one>指定的外键字段(这里是classesid)一致，否则引用字段的错误

如果在”一“一端维护一对多关联关系，hibernate会发出多余的udpate语句，所以我们一般在多的一端来维护关联关系。

加上inverse="true"这个属性，就可以强制在多的一端维护关系了。

关于inverse属性：

inverse主要用在一对多和多对多双向关联上，inverse可以被设置到集合标签<set>上，

默认inverse为false，所以我们可以从”一“一端和”多“一端维护关联关系，

如果设置成inverse为true，则我们只能从多一端来维护关联关系

注意：inverse属性，只影响数据的存储，也就是持久化

inverse和cascade

* inverse是关联关系的控制方向

* cascade操作上的连锁反应

测试一下：

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

import junit.framework.TestCase;

import org.hibernate.Session;

public class One2ManyTest extends TestCase {

  public void testSave1() {
    Session session = null;
    try {
      session = HibernateUtils.getSession();
      session.beginTransaction();
      
      Student student1 = new Student();
      student1.setName("10");
      session.save(student1);
      
      Student student2 = new Student();
      student2.setName("祖儿");
      session.save(student2);
      
      Set students = new HashSet();
      students.add(student1);
      students.add(student2);
      
      Classes classes = new Classes();
      classes.setName("尚学堂");
      classes.setStudents(students);
      
      //可以正确保存
      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();
      
      Classes classes = new Classes();
      classes.setName("尚学堂");
      session.save(classes);
      
      Student student1 = new Student();
      student1.setName("10");
      student1.setClasses(classes);
      session.save(student1);
      
      Student student2 = new Student();
      student2.setName("祖儿");
      student2.setClasses(classes);
      session.save(student2);
      
      session.getTransaction().commit();
    }catch(Exception e) {
      e.printStackTrace();
      session.getTransaction().rollback();
    }finally {
      HibernateUtils.closeSession(session);
    }
  }   
  
  public void testSave3() {
    Session session = null;
    try {
      session = HibernateUtils.getSession();
      session.beginTransaction();

      Classes classes = new Classes();
      classes.setName("尚学堂");
      
      Student student1 = new Student();
      student1.setName("10");
      student1.setClasses(classes);
      
      Student student2 = new Student();
      student2.setName("祖儿");
      student2.setClasses(classes);
      
      Set students = new HashSet();
      students.add(student1);
      students.add(student2);
      
      classes.setStudents(students);
      
      //可以正确保存
      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.name=" + student.getName());
      }
      session.getTransaction().commit();
    }catch(Exception e) {
      e.printStackTrace();
      session.getTransaction().rollback();
    }finally {
      HibernateUtils.closeSession(session);
    }
  }   
  
  public void testLoad2() {
    Session session = null;
    try {
      session = HibernateUtils.getSession();
      session.beginTransaction();
      
      Student student = (Student)session.load(Student.class, 1);
      System.out.println("student.name=" + student.getName());
      System.out.println("student.classes.name=" + student.getClasses().getName());
      session.getTransaction().commit();
    }catch(Exception e) {
      e.printStackTrace();
      session.getTransaction().rollback();
    }finally {
      HibernateUtils.closeSession(session);
    }
  }   
  
}