java之hibernate之关联映射之多对一单向关联

1.在之前学习了单表的crud操作。在实际应用中，大都是多表关联操作，这篇会学习如何处理多表之间的关系。

2.考察书籍表和书籍分类表的关系。书籍表和书籍分类表之间是多对一的关系。数据库的表设计为：

3.在java中，如何使用类的关系来表明表与表之间关系

Book.java

public class Book implements Serializable{
    private int id;
    private String name;
    private String author;
    private double price;
    private Date pubDate;
    private Category category;//在多的一端对一的一端的引用
   //也就是说在多的一端可以获取到一的一端信息
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getAuthor() {
        return author;
    }
    public void setAuthor(String author) {
        this.author = author;
    }
    public double getPrice() {
        return price;
    }
    public void setPrice(double price) {
        this.price = price;
    }
    public Date getPubDate() {
        return pubDate;
    }
    public void setPubDate(Date pubDate) {
        this.pubDate = pubDate;
    }
    public Category getCategory() {
        return category;
    }
    public void setCategory(Category category) {
        this.category = category;
    }
}

Category.java

public class Category implements Serializable{
    private int id;
    private String name;
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
}

4.映射文件关系

Book.hbm.xml

<hibernate-mapping package="cn.sxt.pojo">
    <class name="Book" table="t_book">
        <id name="id">
            <generator class="native"></generator>
        </id>
        <property name="name"/>
        <property name="author"/>
        <property name="price"/>
        <property name="pubDate"/>
        <!-- 多对一的关联关系设置   column指定外键的名称 -->
        <many-to-one name="category" column="cid"/>
    </class>
</hibernate-mapping>

Category.hbm.xml

<hibernate-mapping package="cn.sxt.pojo">
    <class name="Category" table="t_category">
        <id name="id">
            <generator class="native"></generator>
        </id>
        <property name="name"/>
    </class>
</hibernate-mapping>

5.测试

public class HibernateTest {
    /**
     * 生成数据库表的工具方法
     * */
    @Test
    public void testCreateDB(){
        Configuration cfg = new Configuration().configure();
        SchemaExport se = new SchemaExport(cfg);
        //第一个参数  是否打印sql脚本
        //第二个参数 是否将脚本导出到数据库中执行
        se.create(true, true);
    }
    /**
     * 初始化表数据
     */
    @Test
    public void testInit(){
        Session session = null;
        Transaction tx = null;
        try {
            session = HibernateUtil.getSession();
            tx = session.beginTransaction();
            Category c1 = new Category("计算机类");
            Category c2 = new Category("文学");
            Category c3 = new Category("历史");
            SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd");
            Book b1 = new Book("java","sun",30,df.parse("1995-05-23"));
            b1.setCategory(c1);
            Book b2 = new Book("struts","apache",40,df.parse("2006-09-12"));
            b2.setCategory(c1);
            Book b3 = new Book("明朝那些事儿","当年明月",70,df.parse("2008-05-23"));
            b3.setCategory(c3);
            Book b4 = new Book("水浒传","老撕",20,df.parse("1985-05-23"));
            b4.setCategory(c2);
            session.save(c1);
            session.save(c2);
            session.save(c3);
            session.save(b1);
            session.save(b2);
            session.save(b3);
            session.save(b4);
            tx.commit();
            
        } catch (Exception e) {
            if(tx!=null)
                tx.rollback();
        }finally {
            HibernateUtil.close();
        }
    }
    /**
     * 在查询多的一端数据时可以获取一的一端的数据
     */
    @Test
    public void testGetData(){
        Session session = HibernateUtil.getSession();
        Book book = (Book)session.get(Book.class, 3);
        System.out.println(book.getId()+"--"+book.getName()+"---"+book.getAuthor()+
                "---"+book.getPrice()+"---"+book.getPubDate()+"---"+book.getCategory().getName());
        
        HibernateUtil.close();
    }
}

注意：初始化数据时，保存顺序，先保存的分类，再保存的书籍