Hibernate(三)--关联映射

1.多对一

product----category

category.hbm.xml

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.h.pojo">
    <class name="Category" table="category_">
        <id name="id" column="id">
            <generator class="native">
            </generator>
        </id>
        <property name="name" />
    </class>

</hibernate-mapping>

product.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">
<hibernate-mapping package="com.h.pojo">
    <class name="Product" table="product_">
        <id name="id" column="id">
            <generator class="native"></generator>
        </id>
        <property name="name" />
        <property name="price" />

        <many-to-one name="category" class="Category" column="cid"></many-to-one>   product多 对 一category

    </class>
</hibernate-mapping>

hibernate.cfg.xml

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD//EN"
        "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <!-- Database connection settings -->
        <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="connection.url">jdbc:mysql://localhost:3306/test?characterEncoding=UTF-8</property>
        <property name="connection.username">root</property>
        <property name="connection.password">123456</property>
        <!-- SQL dialect -->
        <property name="dialect">org.hibernate.dialect.MySQLDialect</property>
        <property name="current_session_context_class">thread</property>
        <property name="show_sql">true</property>
        <property name="hbm2ddl.auto">update</property>

        <mapping resource="com/h/pojo/Product.hbm.xml" />
        <mapping resource="com/h/pojo/Category.hbm.xml"/>

    </session-factory>
</hibernate-configuration>

测试类：

public class Test1 {
    public static void main(String[] args) {

        SessionFactory factory=new Configuration().configure().buildSessionFactory();
        Session session = factory.openSession();
        session.beginTransaction();

        Category c =new Category();
        c.setName("c1");
        session.save(c);

        Product p = (Product) session.get(Product.class, 2);
        p.setCategory(c);
        session.update(p);

        session.getTransaction().commit();
        session.close();
        factory.close();
    }
}

2.一对多

category 对 product，与多对一相反，就是一对多

测试类：

3.多对多

product  ----  user

一种Product可以被多个User购买
一个User可以购买多种Product
所以Product和User之间的关系是多对多 many-to-many
要实现多对多关系，必须有一张中间表 user_product 用于维护 User和Product之间的关系

测试类：

数据库：