hibernate多对一关联映射两种形式的理解

<?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/test?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 -->
        <mapping resource="com/great/entity/Class.hbm.xml"></mapping>
        <mapping resource="com/great/entity/Student.hbm.xml"></mapping>
        <mapping resource="com/great/entity1/Product.hbm.xml"></mapping>
        <mapping resource="com/great/entity1/Factory.hbm.xml"></mapping>
    </session-factory>
</hibernate-configuration>

package com.great.entity1;

public class Factory {
    // 厂商类
    // 厂商id
    private int fId;
    // 厂商名字
    private String fName;

    public int getfId() {
        return fId;
    }

    public void setfId(int fId) {
        this.fId = fId;
    }

    public String getfName() {
        return fName;
    }

    public void setfName(String fName) {
        this.fName = fName;
    }

}

package com.great.entity1;

public class Product {
    // 多对一单向关联
    // 产品类
    // 产品id
    private int pId;
    // 产品名字
    private String pName;
    // 厂商
    private Factory factory;

    public int getpId() {
        return pId;
    }

    public void setpId(int pId) {
        this.pId = pId;
    }

    public String getpName() {
        return pName;
    }

    public void setpName(String pName) {
        this.pName = pName;
    }

    public Factory getFactory() {
        return factory;
    }

    public void setFactory(Factory factory) {
        this.factory = factory;
    }

}

<?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.entity1">
<class name="Factory" table="factory">
    <!-- 配置主键映射关系 -->
    <id name="fId" column="fId" 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="fName" column="fName" type="string"></property>
  </class>
</hibernate-mapping>

<?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.entity1">
  <!-- 配置实体类与数据库表的映射关系 -->
  <class name="Product" table="Product">
    <!-- 配置主键映射关系 -->
    <id name="pId" column="pId" 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="pName" column="pName" 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="factory" class="com.great.entity1.Factory" cascade="save-update">
      <!-- column标签指定关联外键，还是子表的外键列名 -->
      <column name="fId"></column>
    </many-to-one>
  </class>
</hibernate-mapping>

package com.great.test;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
import org.junit.AfterClass;
import org.junit.BeforeClass;
import org.junit.Test;
import com.great.entity1.Product;

public class TestManyToOneUnidirectional {
    static Session session;

    // 多对一单向关联
    @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();
    }

    // 查询产品名字和厂家
    @Test
    public void testManyToOneSelect() {
        Product p = (Product) session.get(Product.class, 1);
        System.out.println("产品名字为：" + p.getpName());
        System.out.println("产品厂家为：" + p.getFactory().getfName());
    }
}

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 + "]";
    }

}

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;
    }

}

<?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>

<?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>