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。