【Hibernate】(4)Hibernate一对多映射

1. 一对多关联

(1). 创建工程与数据库

打开MyEclipse，创建工程导入所需要的jar包，配置hibernate.cfg.xml文件：

<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
	<session-factory>
		<property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
		<property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
		<property name="hibernate.connection.username">root</property>
		<property name="hibernate.connection.password">123465</property>
		<property name="hibernate.connection.url">
        	<![CDATA[
        		jdbc:mysql://localhost:3306/one2many:useUnicode=true&characterEncoding=utf8
        	]]>
		</property>
		<property name="show_sql">true</property>
		<property name="hbm2ddl.auto">update</property>
	</session-factory>
</hibernate-configuration>

创建MySQL中数据库one2many。

(2). 创建工具类：

public class HibernateUtil {
	private static SessionFactory sessionFactory;
	private static Session session;

	static {
		// 创建configuration对象，读取hibernate.cfg.xml配置
		Configuration config = new Configuration().configure();
		StandardServiceRegistryBuilder builder = new StandardServiceRegistryBuilder()
				.applySettings(config.getProperties());
		StandardServiceRegistry registry = builder.build();
		sessionFactory = config.buildSessionFactory(registry);
	}

	public static SessionFactory getSessionFactory() {
		return sessionFactory;
	}

	public static Session getSession() {
		session = sessionFactory.openSession();
		return session;
	}

	public static void closeSession(Session session) {
		if (session != null) {
			session.close();
		}
	}
}

(3). 创建表

create table grade(
gid int primary key,
gname varchar(20) not null,
gdesc varchar(50)
);
create table student(
sid int primary key,
sname varchar(20) not null,
sex char(2),
gid int
);
alter table student add constraint fk_student_gid foreign key (gid) references grade(gid);

(4). 创建Student和Grade类以及他们的关联映射文件

<hibernate-mapping>

	<class name="com.thr.entity.Student" table="student">
		<id name="sid" column="sid" type="java.lang.Integer">
			<generator class="increment"></generator>
		</id>
		<property name="sname" type="java.lang.String">
			<column name="sname" length="20" not-null="true"></column>
		</property>
		<property name="sex">
			<column name="sex"></column>
		</property>
	</class>
</hibernate-mapping>

<hibernate-mapping>
	<class name="com.thr.entity.Grade" table="grade">
		<id name="gid" column="gid" type="java.lang.Integer">
			<generator class="increment"></generator>
		</id>
		<property name="gname" type="java.lang.String">
			<column name="gname" length="20" not-null="true"></column>
		</property>
		<property name="gdesc">
			<column name="gdesc"></column>
		</property>
		<!-- 配置单向一对多的关联关系 -->
		<set name="students" table="student">
			<!-- 指定关联的外键列 -->
			<key column="gid"></key>
			<one-to-many class="com.thr.entity.Student" />
		</set>
	</class>
</hibernate-mapping>

然后在hibernate.cfg.xml配置文件中添加这两个文件的路径：

		<!-- 指定关联映射文件的路径 -->
		<mapping resource="com/thr/entity/Grade.hbm.xml"/>
		<mapping resource="com/thr/entity/Student.hbm.xml"/>

(5). 编写测试类

测试添加和查询：

	/**
	 * 学生添加到班级
	 */
	@Test
	public void add() {
		Grade g = new Grade("三年二班", "哎呦不错哦");
		Student s1 = new Student("张三", "男");
		Student s2 = new Student("李四", "女");
		g.getStudents().add(s1);
		g.getStudents().add(s2);

		Session session = HibernateUtil.getSession();
		Transaction transaction = session.beginTransaction();
		session.save(g);
		session.save(s1);
		session.save(s2);
		transaction.commit();
		HibernateUtil.closeSession(session);
	}

	/**
	 * 查询班级中包含的学生
	 */
	@Test
	public void findStudentByGrade() {
		Session session = HibernateUtil.getSession();
		Grade g = (Grade) session.get(Grade.class, 1);
		System.out.println(g.toString());
		Set<Student> students = g.getStudents();
		for (Student s : students) {
			System.out.println(s);
		}
	}

修改和删除测试：

	/**
	 * 修改学生信息
	 */
	@Test
	public void update() {
		Grade g = new Grade("三年三班", "还行吧");
		Session session = HibernateUtil.getSession();
		Transaction transaction = session.beginTransaction();
		Student s = (Student) session.get(Student.class, 1);
		g.getStudents().add(s);
		session.save(g);
		transaction.commit();
		HibernateUtil.closeSession(session);
	}

	/**
	 * 删除学生信息
	 */
	@Test
	public void delete() {
		Session session = HibernateUtil.getSession();
		Transaction transaction = session.beginTransaction();
		Student s = (Student) session.get(Student.class, 1);
		session.delete(s);
		transaction.commit();
		HibernateUtil.closeSession(session);
	}

(6). set元素的常用属性

2. 多对一关联

给原来的Student类添加一个Grade成员，设置set和get方法，修改Student.hbm.xml配置，添加：

		<!-- 配置多对一关联配置 -->
		<many-to-one name="grade" class="com.thr.entity.Grade"
			column="gid">
		</many-to-one>

编写测试类：

	@Test
	public void save() {
		Grade g = new Grade("四年三班", "四年三班");
		Student s1 = new Student("Jerry", "男");
		Student s2 = new Student("kate", "女");
		// 设置关联关系
		s1.setGrade(g);
		s2.setGrade(g);

		Session session = HibernateUtil.getSession();
		Transaction transaction = session.beginTransaction();
		session.save(g);
		session.save(s1);
		session.save(s2);
		transaction.commit();
		HibernateUtil.closeSession(session);
	}

3. inverse和cascade属性

(1). inverse属性

<set>节点的inverse属性指定关联关系的控制方向，默认由one方来维护。

关联关系中，inverse="false"则为主动方，由主动方负责维护关联关系。

在一对多关联中，只能设置one方的inverse为true，这将有助于性能的改善。

(2). cascade属性

当设置了cascade属性不为none时，Hibernate会自动持久化所关联的对象

cascade属性的设置会带来性能上的变动，需谨慎设置

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