（十一）Hibernate中的多表操作（1）：单向一对多

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一、单向一对多（）

• 案例一（用XML文件配置）： 一个班级有多个学生，班级可以查看所有学生的信息。
• ClassBean.java

package bean;

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

/**
 * ClassBean entity. @author MyEclipse Persistence Tools
 */

public class ClassBean implements java.io.Serializable {

    // Fields

    private Integer classid;
    private String classname;

    private Set<StudentBean> stuSet=new HashSet<StudentBean>();

    // Constructors

    /** default constructor */
    public ClassBean() {
    }

    /** minimal constructor */
    public ClassBean(Integer classid) {
        this.classid = classid;
    }

    /** full constructor */
    public ClassBean(Integer classid, String classname) {
        this.classid = classid;
        this.classname = classname;
    }

    // Property accessors

    public Integer getClassid() {
        return this.classid;
    }

    public void setClassid(Integer classid) {
        this.classid = classid;
    }

    public String getClassname() {
        return this.classname;
    }

    public void setClassname(String classname) {
        this.classname = classname;
    }

    public Set<StudentBean> getStuSet() {
        return stuSet;
    }

    public void setStuSet(Set<StudentBean> stuSet) {
        this.stuSet = stuSet;
    }

}

StudentBean.java

package bean;

/**
 * StudentBean entity. @author MyEclipse Persistence Tools
 */

public class StudentBean implements java.io.Serializable {

    // Fields

    private Integer stuid;
    private String stuname;
    private Integer classId;

    // Constructors

    /** default constructor */
    public StudentBean() {
    }

    /** minimal constructor */
    public StudentBean(Integer stuid) {
        this.stuid = stuid;
    }

    /** full constructor */
    public StudentBean(Integer stuid, String stuname, Integer classId) {
        this.stuid = stuid;
        this.stuname = stuname;
        this.classId = classId;
    }

    // Property accessors

    public Integer getStuid() {
        return this.stuid;
    }

    public void setStuid(Integer stuid) {
        this.stuid = stuid;
    }

    public String getStuname() {
        return this.stuname;
    }

    public void setStuname(String stuname) {
        this.stuname = stuname;
    }

    public Integer getClassId() {
        return this.classId;
    }

    public void setClassId(Integer classId) {
        this.classId = classId;
    }

}

• 单向一对多的映射文件 ：

ClassBean.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">
<!--
    Mapping file autogenerated by MyEclipse Persistence Tools
-->
<hibernate-mapping>
    <class name="bean.ClassBean" table="t_class" catalog="test">
        <id name="classid" type="java.lang.Integer">
            <column name="classid" />
            <generator class="assigned"></generator>
        </id>
        <property name="classname" type="java.lang.String">
            <column name="classname" />
        </property>

         <!--set元素，就是定义一个集合，它的name属性值是对应的POJO中的相关属性名称-->
        <set name="stuSet" cascade="all">
            <key column="class_id"></key>  <!--指定“多”的一段的外键，与“一”端得主键相关联，这里的column=“class_id”是指数据库中的字段，而非bean中的属性,注意这里不会自动生成
                                                                class_id属性，所以在bean.StudentBean类中要有一个外键的映射属性-->
            <one-to-many class="bean.StudentBean"/>  <!--指定了“多”端对应的类-->
        </set>
    </class>
</hibernate-mapping>

• StudentBean.hbml.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">
<!--
    Mapping file autogenerated by MyEclipse Persistence Tools
-->
<hibernate-mapping>
    <class name="bean.StudentBean" table="t_student" catalog="test">
        <id name="stuid" type="java.lang.Integer">
            <column name="stuid" />
            <generator class="assigned"></generator>
        </id>
        <property name="stuname" type="java.lang.String">
            <column name="stuname" />
        </property>
        <property name="classId" type="java.lang.Integer">
            <column name="class_id" />
        </property>
    </class>
</hibernate-mapping>

• 测试类

package action;

import java.util.Set;

import org.hibernate.Session;
import org.hibernate.Transaction;

import bean.ClassBean;
import bean.StudentBean;
import util.HibernateUtil;

/**
 * 本类测试一对多的操作
 * @author 半颗柠檬、
 *
 */
public class Test_ManytoOne {
    public static void main(String[] args) {
//        Test_ManytoOne.save();

//        Test_ManytoOne.select();

        Test_ManytoOne.delete();
    }

    public static void save(){
        Session session = null;
        Transaction tran = null;
        try {
            session = HibernateUtil.getSessionFactory().getCurrentSession();
            tran=session.beginTransaction();

            ClassBean class1=new ClassBean();
            class1.setClassid(1);
            class1.setClassname("精英班");

            StudentBean stu1=new StudentBean();
            stu1.setClassId(class1.getClassid());
            stu1.setStuid(1);
            stu1.setStuname("学生1");

            StudentBean stu2=new StudentBean();
            stu2.setStuid(2);
            stu2.setStuname("学生2");
            stu2.setClassId(class1.getClassid());

            session.save(class1);
            session.save(stu1);
            session.save(stu2);

            tran.commit();
        } catch (Exception e) {
            e.printStackTrace();
            tran.rollback();
        }
    }

    /**
     * 测试“一”方能够读取“多”方的信息
     */
    private static void select() {
        Session session = null;
        Transaction tran = null;
        try {
            session = HibernateUtil.getSessionFactory().getCurrentSession();
            tran=session.beginTransaction();

            /**
             * 由“一”方得知“多”方信息
             */

           ClassBean cla=(ClassBean)session.get(ClassBean.class, new Integer(1));

           Set<StudentBean> stuSet=cla.getStuSet();

           for(StudentBean stu:stuSet){

               System.out.println(stu.getStuname());
               System.out.println(stu.getClassId());
               System.out.println(stu.getStuid());
           }

            tran.commit();
        } catch (Exception e) {
            e.printStackTrace();
            tran.rollback();
        }
    }

    /**
     * 删除One一方时，默认不会级联删除Many的一方。
     *
     * 如果要级联删除：，一般使用代码手动级联更新
     *
     * A：使用代码来控制级联删除。也是推荐使用的方式，手动进行级联删除，
     *
     * B：配置级联关系，让Hibernate自动执行。
     *
     * 在关联关系中配置cascade属性。
     *
     * @OneToMany(cascade = { CascadeType.REMOVE, CascadeType.MERGE })
     *
     */
    private static void delete() {
        Session session = null;
        Transaction tx = null;
        try {
            session = HibernateUtil.getSession();
            tx = session.beginTransaction();

            /**
             * B  在关联关系中配置cascade属性之后
             */
            ClassBean class1 = (ClassBean) session.get(ClassBean.class,
                    new Integer(1));
            session.delete(class1);

            tx.commit();
        } catch (Exception e) {
            e.printStackTrace();
            tx.rollback();
        } finally {
            HibernateUtil.closeSession();
        }
    }

}

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案例二：使用注解测试单向一对多操作

• 一个班级有多个学生，班级可以查看所有学生的信息，但是学生不能查看班级的信息。
• ClassBean.java

package bean;

import java.io.Serializable;
import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;
import javax.persistence.Table;

@Entity
@Table(name = "t_class")
public class ClassBean implements Serializable {
    @Id
    private Integer classId;

    private String className;

    @OneToMany
    @JoinColumn(name = "class_id")   //不会在本类中自动创建名为classes_id的属性因为只有“多”方有外键，Hibernate会自动把本类中的所有属性都映射为数据库中的字段，所以该属性要
                          //和表中的外键名一致，且Hibernate默认与StudentBean的主键相关联
    private Set<StudentBean> stuSet=new HashSet<StudentBean>();

    public ClassBean() {

    }

    public ClassBean(Integer classId, String className) {
        super();
        this.classId = classId;
        this.className = className;
    }

    public Integer getClassId() {
        return classId;
    }

    public void setClassId(Integer classId) {
        this.classId = classId;
    }

    public String getClassName() {
        return className;
    }

    public void setClassName(String className) {
        this.className = className;
    }

    public Set<StudentBean> getStuSet() {
        return stuSet;
    }

    public void setStuSet(Set<StudentBean> stuSet) {
        this.stuSet = stuSet;
    }
}

• StudentBean.java

package bean;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name = "t_student")
public class StudentBean implements Serializable {

    @Id
    @Column(name = "stuid")
    private Integer studentId;

    @Column(name = "stuname")
    private String studentName;

    @Column(name="class_id")
    private Integer classid;

    public StudentBean() {

    }

    public StudentBean(Integer studentId, String studentName) {
        super();
        this.studentId = studentId;
        this.studentName = studentName;
    }

    public Integer getStudentId() {
        return studentId;
    }

    public void setStudentId(Integer studentId) {
        this.studentId = studentId;
    }

    public String getStudentName() {
        return studentName;
    }

    public void setStudentName(String studentName) {
        this.studentName = studentName;
    }

    public Integer getClassid() {
        return classid;
    }

    public void setClassid(Integer classid) {
        this.classid = classid;
    }

}

• 设置总配置文件的映射关系

<mapping class="bean.ClassBean" />
    <mapping class="bean.StudentBean" />

• 测试一对多的操作

package action;

import java.util.List;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.OneToMany;

import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.Transaction;

import bean.ClassBean;
import bean.StudentBean;
import util.HibernateUtil;

/**
 * 本类测试一对多的操作
 *
 * @author 半颗柠檬、
 *
 */
public class Test_ManytoOne {
    public static void main(String[] args) {
//         Test_ManytoOne.save();

//        Test_ManytoOne.select();

         Test_ManytoOne.delete();
    }

    public static void save() {
        Session session = null;
        Transaction tran = null;
        try {
            session = HibernateUtil.getSessionFactory().getCurrentSession();
            tran = session.beginTransaction();

            ClassBean class1 = new ClassBean();
            class1.setClassid(1);
            class1.setClassname("精英班");

            StudentBean stu1 = new StudentBean();
            stu1.setClassid(class1.getClassid());
            stu1.setStuid(1);
            stu1.setStuname("学生1");

            StudentBean stu2 = new StudentBean();
            stu2.setStuid(2);
            stu2.setStuname("学生2");
            stu2.setClassid(class1.getClassid());

            session.save(class1);
            session.save(stu1);
            session.save(stu2);

            tran.commit();
        } catch (Exception e) {
            e.printStackTrace();
            tran.rollback();
        }
    }

    /**
     * 测试“一”方能够读取“多”方的信息
     */
    private static void select() {
        Session session = null;
        Transaction tran = null;
        try {
            session = HibernateUtil.getSessionFactory().getCurrentSession();
            tran = session.beginTransaction();

            /**
             * 由“一”方得知“多”方信息
             */

            ClassBean cla = (ClassBean) session.get(ClassBean.class,
                    new Integer(1));
            Set<StudentBean> stuSet = cla.getStuSet();

            for (StudentBean stu : stuSet) {

                System.out.println(stu.getStuname() + "\t" + stu.getStuid()
                        + "\t" + stu.getClassid());

            }
            // 过滤后的数据 ,这种方式会把排序注解忽略掉，即排序注解不会生效
            System.out.println("过滤后的数据.......");
            Query query = session.createFilter(stuSet,
                    "  where  stuid in (2,3)");
            List<StudentBean> stuList = query.list();
            for (StudentBean stu : stuList) {

                System.out.println(stu.getStuname() + "\t" + stu.getStuid()
                        + "\t" + stu.getClassid());

            }

            /**
             * 使用HQL语句。
             */

            System.out.println("使用HQL语句");
            String hql = " select u from StudentBean u where stuid in(2,3) order by stuid desc ";
            query = session.createQuery(hql);
            stuList = query.list();

            for (StudentBean stu : stuList) {

                System.out.println(stu.getStuname() + "\t" + stu.getStuid()
                        + "\t" + stu.getClassid());

            }

            tran.commit();
        } catch (Exception e) {
            e.printStackTrace();
            tran.rollback();
        }
    }

    /**
     * 删除one一方的时候，一般不删除多方，Hibernate会自动把多方的外键置空
     */
    private static void delete() {
        Session session = null;
        Transaction tx = null;
        try {
            session = HibernateUtil.getSession();
            tx = session.beginTransaction();

            /**
             * B 在关联关系中配置cascade属性之后
             */
            ClassBean class1 = (ClassBean) session.get(ClassBean.class,
                    new Integer(1));
            session.delete(class1);

            tx.commit();
        } catch (Exception e) {
            e.printStackTrace();
            tx.rollback();
        } finally {
            HibernateUtil.closeSession();
        }
    }

}

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• 对于项目中大量使用@Entity注册在类头上,将一个类声明为一个实体bean(即一个持久化POJO类) , @Table注册在类头上,注解声明了该实体bean映射指定的表,来体现实体与表的这种映射关系。

  对于属性字段和表的字段关系对应的注解属性的位置，一般我们采用以下两种方式:

  第一种：

  是把注解@Column(name ="xx")放在field上，一种是把注解放在get方法上一般放在field上看起来比较集中、清晰；

  第二种：

  是把注解@Column(name= "xx")放在get方法上，这种方式看起来比较散漫、不很清楚；

  但是第一种方式这样做实际上破坏了Java面向对象的封装性，原因是一般我们写javaBean，成员变量通常定义为private，目的就是不让别人来直接访问的私有属性，而我们把注解放在私有成员的变量上，就是默认hibernate可以直接访问我们的私有的成员变量，所以我们定义属性为private，就实际没有多大意义，至于hibernate为什么能访问，hibernate采用java的反射机制完全可以访问私有成员变量！所以应该放在get方法上，第二种方式这个时候就显得更加合理。

• 注意 ：注意的一点,在一个实体类中，要么将映射注解全部放在成员变量上，要么全部放在成员方法上，不能各放一部分