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，在对方表中加入一个外键指向一一端
在多一端采用

<!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：标签指定的外键字段(这里是classesid)必须和指定的外键字段(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);
        }
    }       

}

原文链接：https://blog.csdn.net/gundumw100/article/details/69962262

hibernate学习5之one-to-many双向关联

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