java之hibernate之单向的一对多关联映射

这篇主要讲hiberante中的 单向一对多关联映射

1.在应用中，有时候需要从一的一端获取多的一端的数据。比如：查看某个分类下的所有书籍信息；查看某个订单下的所有商品等。

2.在一对多的关联关系中，表的设计为：

从表的设计中可以看出，表结构和多对一的表结构相同

3.类的设计

Book.java

public class Book implements Serializable{
    private int id;
    private String name;
    private String author;
    private double price;
    private Date pubDate;
    public Book() {
    }
    public Book(String name, String author, double price, Date pubDate) {
        super();
        this.name = name;
        this.author = author;
        this.price = price;
        this.pubDate = pubDate;
    }

    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;
    }
}

Category.java

public class Category implements Serializable{
    private int id;
    private String name;
    private Set<Book> books = new HashSet<>();
    public Category() {
    }
    
    public Category(String name) {
        super();
        this.name = 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;
    }

    public Set<Book> getBooks() {
        return books;
    }

    public void setBooks(Set<Book> books) {
        this.books = books;
    }
}

4.映射文件

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"/>
    </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"/>
        <!-- 一对多的关联映射 -->
        <set name="books">
            <!-- 指定外键的名称 -->
            <key column="cid"></key>
            <!-- 指定多的一端的类型 -->
            <one-to-many class="Book"/>
        </set>
    </class>
</hibernate-mapping>

此处，如果package没写，class name=”cn.swy.pojo.Category”并且<one-to-many class=”cn.swy.pojo.Book”

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
     * 使用一对多的方式来保存数据，会执行update语句来更新外键
     * 使得效率会比多对一的方式效率低
     */
    @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"));
            Book b2 = new Book("struts","apache",40,df.parse("2006-09-12"));
            Book b3 = new Book("明朝那些事儿","当年明月",70,df.parse("2008-05-23"));
            Book b4 = new Book("水浒传","老撕",20,df.parse("1985-05-23"));
            //设置关系
            c1.getBooks().add(b1);
            c1.getBooks().add(b2);
            c2.getBooks().add(b4);
            c3.getBooks().add(b3);
            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();
        }
    }
    /**
     * 在查询一的一端数据时可以获取多的一端的数据
     */
    @Test
    public void testGetData(){
        Session session = HibernateUtil.getSession();
        Category c1 = (Category)session.get(Category.class, 1);
        System.out.println(c1.getId()+"----"+c1.getName());
        System.out.println("-----------------");
        for(Book book:c1.getBooks()){
            System.out.println(book);
        }
        HibernateUtil.close();
    }
}