hibernate多对一双向关联

关联是类（类的实例）之间的关系，表示有意义和值得关注的连接。

本系列将介绍Hibernate中主要的几种关联映射

Hibernate一对一主键单向关联
Hibernate一对一主键双向关联
Hibernate一对一外键单向关联
Hibernate一对一外键双向关联
Hibernate多对一单向关联
Hibernate多对一双向关联

Hibernate多对多关联

代码都写有注释，主要包括（核心配置文件，实体映射文件，实体类，测试类，数据库）主要操作有增删改查。

本篇主要介绍Hibernate多对一单向关联：

hibernate.cfg.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
      <!-- 配置数据库连接驱动类 -->
        <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
        <!-- 配置数据库连接字符串 -->
        <property name="connection.url"><![CDATA[jdbc:mysql://localhost:3306/test1?useUnicode=true&characterEncoding=utf8]]></property>
        <!-- 配置数据库连接用户名 -->
        <property name="connection.username">root</property>
        <!-- 配置数据库连接密码 -->
        <property name="connection.password">123456</property>
        <!-- 配置数据库方言 -->
        <property name="dialect">org.hibernate.dialect.MySQLDialect</property>
        <!-- 配置数据库表生成策略 -->
        <!--<property name="hibernate.hbm2ddl.auto">update</property> -->
        <!-- 配置是否打印显示SQL语句 -->
        <property name="show_sql">true</property>
        <!-- 配置是否格式化显示SQL语句 -->
        <property name="format_sql">true</property>
        <!-- 配置hibernate是否自动提交事务 -->
        <!--<property name="hibernate.connection.autocommit">true</property> -->
        <!-- 配置实体类对应的映射文件 -->

        <!-- 多对一双向关联配置 -->
        <mapping resource="com/great/entity/Class.hbm.xml"></mapping>
        <mapping resource="com/great/entity/Student.hbm.xml"></mapping>
    </session-factory>
</hibernate-configuration>

Student.hbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" >
<hibernate-mapping package="com.great.entity">
    <!-- 配置实体类与数据库表的映射关系 -->
    <class name="Student" table="student">
        <!-- 配置主键映射关系 -->
        <id name="id" column="sId" type="int">
            <!-- 配置主键生成策略 -->
            <generator class="identity">
            </generator>
        </id>
        <!-- 配置属性和表字段映射关系 -->
        <property name="sName" column="sName" type="string"></property>
        <!-- 多对一关联关系配置 -->
        <!-- many-to-one标签声明多对一关联关系，name指定多方所维护的一方的实例名 -->
        <many-to-one name="classes" class="com.great.entity.Class"
            cascade="save-update">
            <!-- column标签指定关联外键，还是子表的外键列名 -->
            <column name="cId"></column>
        </many-to-one>
    </class>
</hibernate-mapping>

Class.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.great.entity">
    <class name="Class" table="class">
        <!-- 配置主键映射关系 -->
        <id name="cId" column="cId" type="int">
            <!-- 配置主键生成策略 -->
            <generator class="identity">
            </generator>
        </id>
        <!-- 配置属性和表字段映射关系 -->
        <property name="cName" column="cName" type="string"></property>
        <!-- 一对多关联关系配置 -->
        <!-- inverse属性设置为true代表一的一方不在拥有关联关系的控制权，而把控制权交给多的一方 -->
        <!-- fetch属性用来指定子表数据的抓取策略，默认值为"select"，代表对主表子表进行单独查询 ，对主表子表单独发select语句 -->
        <!-- fetch属性用来指定子表数据的抓取策略，设置为"join"时，代表对主表子表进行关联查询，对主表子表发出关联查询语句 -->
        <!-- lazy属性用来指定子表数据是否延迟加载，fetch="select"时起作用，fetch="join"时不起作用 -->
        <!-- fetch="select" lazy="true" -->
        <set name="students" cascade="all" inverse="true">
            <!-- key配置的是子表的外键列名 -->
            <key column="cId"></key>
            <!-- one-to-many标签声明关联关系是一对多，class指定多方的实体类类型 -->
            <one-to-many class="com.great.entity.Student" />
        </set>
    </class>
</hibernate-mapping>

Student.java

package com.great.entity;

import com.great.entity.Class;

public class Student {

    private int id;
    private String sName;
    // 多对一映射时，因为要维护一的一方的实例，所以外键不需要映射
    // private int cId;
    private Class classes;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getsName() {
        return sName;
    }

    public void setsName(String sName) {
        this.sName = sName;
    }

    public Class getClasses() {
        return classes;
    }

    public void setClasses(Class classes) {
        this.classes = classes;
    }

}

Class.java

package com.great.entity;

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

import com.great.entity.Student;

public class Class {
    // 班级id
    private int cId;
    // 班级名字
    private String cName;
    // 注意点：必须new一个Set的实现类的实例
    // 原因：子表不一定有主表的关联数据，所以可能取出空集合，引发空指针异常
    private Set<Student> students = new HashSet<Student>();

    public int getcId() {
        return cId;
    }

    public void setcId(int cId) {
        this.cId = cId;
    }

    public String getcName() {
        return cName;
    }

    public void setcName(String cName) {
        this.cName = cName;
    }

    public Set<Student> getStudents() {
        return students;
    }

    public void setStudents(Set<Student> students) {
        this.students = students;
    }

    @Override
    public String toString() {
        return "Class [cId=" + cId + ", cName=" + cName + ", students="
                + students + "]";
    }

}

TestManyToOneBoth.java

package com.great.test;

import java.util.Iterator;
import java.util.Set;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import org.junit.AfterClass;
import org.junit.BeforeClass;
import org.junit.Test;
import com.great.entity.Class;
import com.great.entity.Student;

public class TestManyToOneBoth {
    static Session session;

    // 多对一双向关联
    @BeforeClass
    public static void setUpBeforeClass() throws Exception {
        // 加载hibernate主配置文件
        Configuration cfg = new Configuration().configure();
        // 构建session工厂
        SessionFactory sf = cfg.buildSessionFactory();
        // 打开session
        session = sf.openSession();
    }

    @AfterClass
    public static void tearDownAfterClass() throws Exception {
        // 关闭session，释放资源
        session.close();
    }

    // select1, 通过class查询到student
    @Test
    public void testManyToOneBothSelect1() {
        Class c = (Class) session.get(Class.class, 5);
        System.out.println("班级为：" + c.getcName());
        System.out.println("cId:" + c.getcId());
        Set<Student> students = c.getStudents();
        Iterator<Student> it = students.iterator();
        while (it.hasNext()) {
            Student student = (Student) it.next();
            System.out.println("学生姓名：" + student.getsName());
        }
    }

    // select2, 通过student查询到class
    @Test
    public void testManyToOneBothSelect2() {
        Student s = (Student) session.get(Student.class, 3);
        System.out.println("学生名字为：" + s.getsName());
        System.out.println("班级为：" + s.getClasses().getcName());
    }

    // insert1,通过class保存数据，一条insert class.两条insert student
    @Test
    public void testManyToOneBothSave1() {

        Transaction ts = session.beginTransaction();
        Class c = new Class();
        c.setcName("通信2班");
        Student s1 = new Student();
        Student s2 = new Student();
        s1.setsName("学员1");
        s2.setsName("学员2");
        c.getStudents().add(s1);// 必须加
        c.getStudents().add(s2);// 必须加
        s1.setClasses(c);
        s2.setClasses(c);
        session.save(c);
        ts.commit();
    }

    // insert2,通过student保存数据,一条insert class.两条insert student
    @Test
    public void testManyToOneBothSave2() {
        Transaction ts = session.beginTransaction();
        Class c = new Class();
        c.setcName("硬件3班");
        Student s1 = new Student();
        Student s2 = new Student();
        s1.setsName("学员3");
        s2.setsName("学员4");
        c.getStudents().add(s1);// 可加可不加
        c.getStudents().add(s2);// 可加可不加
        s1.setClasses(c);
        s2.setClasses(c);
        session.save(s1);
        session.save(s2);
        ts.commit();
    }

    // delete1数据，仅仅删除student,cascade="save-update"[因为是多对一] 发出一条select ,一条delete
    @Test
    public void testManyToOneDelete1() {
        Transaction ts = session.beginTransaction();
        Student student = (Student) session.get(Student.class, 17);
        session.delete(student);
        ts.commit();
    }

    /*
     * delete2数据，通过删除class,把与class以及class相关的student全部删除，先删student,再删class,发出两条select
     * ,两条delete，在class配置文件中配置cascade="all"
     */
    @Test
    public void testManyToOneDelete2() {
        Transaction ts = session.beginTransaction();
        Class classes = (Class) session.get(Class.class, 2);
        session.delete(classes);
        ts.commit();
    }

    // 更新数据1，通过student进行更新
    @Test
    public void testManyToOneUpdate1() {
        Transaction ts = session.beginTransaction();
        Student student = (Student) session.get(Student.class, 2);
        student.setsName("小哥");
        student.getClasses().setcName("设计二班");
        session.saveOrUpdate(student);
        ts.commit();
    }

    // 更新数据2，通过class进行更新
    @Test
    public void testOneToOneUpdate2() {
        Transaction ts = session.beginTransaction();
        Class classes = (Class) session.get(Class.class, 11);
        classes.setcName("工业设计五班");
        Set<Student> students = classes.getStudents();
        Iterator<Student> it = students.iterator();
        while (it.hasNext()) {
            Student student = (Student) it.next();
            System.out.println("学生姓名：" + student.getsName());
            if ("吴邪".equals(student.getsName())) {
                student.setsName("吴小邪");
            }
        }
        ts.commit();
    }
}

数据库：

student表

三个字段 sId sName cId(外键)

class表

两个字段  cId cName

注意：在student表的cId上建立外键关系。