Hibernate一对多自身映射(参考张龙老师的视频的^

本文介绍了一个使用Hibernate框架实现的目录类关联映射案例，包括多对一和一对多的关系处理，通过Category类展示了如何建立目录间的层级关系，并演示了如何在Java中进行目录结构的保存。

//目录类：

//Category文件：

package yang.www.hibernate;

import java.util.Set;

public class Category
{

//主键标识符，也是OID

private Long id;
private String name;

//父目录

private Category parentCategory;

//子目录

    private Set<Category> childCategories;

   public Category(){}




   public Category(String name, Category parentCategory, Set<Category> childCategories)
   {
       super();
       this.name = name;
       this.parentCategory = parentCategory;
       this.childCategories = childCategories;
   }

   public Long getId() {
       return id;
   }

   public String getName() {
       return name;
   }

   public Category getParentCategory() {
       return parentCategory;
   }

   public Set<Category> getChildCategories() {
       return childCategories;
   }

   public void setId(Long id) {
       this.id = id;
   }

   public void setName(String name) {
       this.name = name;
   }

   public void setParentCategory(Category parentCategory) {
       this.parentCategory = parentCategory;
   }

   public void setChildCategories(Set<Category> childCategories) {
       this.childCategories = childCategories;
   }



}

//测试Java文件：

package yang.www.hibernate;

import java.util.HashSet;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;

public class Test2
{
       private static SessionFactory sessionFactory;
       static
       {
           try

{

//加载hibernate.cfg.xml文件并且创建一个session工厂.

//因为工厂一般情况下只需一个，所以只在初始化时初始化一次，以后就可以直接使用它来产生session

               sessionFactory = new Configuration().configure().buildSessionFactory();
           }
           catch(Exception ex)
           {
               ex.printStackTrace();
           }
       }

       public static void main(String[] args)

{

//产生一次与数据库的连接:

           Session session = sessionFactory.openSession();
           Transaction tx = null;
           try

{

//开始事务:

               tx = session.beginTransaction();
               Category category1 = new Category("Level1", null, new HashSet<Category>());
               Category category2 = new Category("Level2", null, new HashSet<Category>());
               Category category3 = new Category("Level2", null, new HashSet<Category>());
               Category category4 = new Category("Level3", null, new HashSet<Category>());
               Category category5 = new Category("Level3", null, new HashSet<Category>());
               Category category6 = new Category("Level3", null, new HashSet<Category>());
               Category category7 = new Category("Level3", null, new HashSet<Category>());
                //设置category2和3的父目录为category1,以下的set方法类似
               category2.setParentCategory(category1);
               category3.setParentCategory(category1);
                //不要忘记还要为父目录添加子目录,以下的get方法类似
               category1.getChildCategories().add(category2);
               category1.getChildCategories().add(category3);


               category4.setParentCategory(category2);
               category5.setParentCategory(category2);

               category2.getChildCategories().add(category4);
               category2.getChildCategories().add(category5);


               category6.setParentCategory(category3);
               category7.setParentCategory(category3);

               category3.getChildCategories().add(category6);
               category3.getChildCategories().add(category7);

　　　　//因为在Category.hbm.xml中已经设置好cascade="all"了，

//所以只需要保存根目录就可以联级保存了。同样的道理，删除全部时，只需要删除根目录就可以了。

               session.save(category1);

               tx.commit();

           }
           catch(Exception ex)
           {
               if (null != tx)
               {
                   tx.rollback();
               }

               ex.printStackTrace();
           }
           finally
           {
               session.close();
           }

       }
}

//Category.hbm.xml文件：

<?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>
   <class name="yang.www.hibernate.Category" table="categories">
       <id name="id" column="id" type="long">
           <generator class="increment"></generator>
       </id>

       <property name="name" type="string">
           <column name="name" length="13"></column>
       </property>

//多对一，即多个子目录，对应一个父目录
<many-to-one name="parentCategory" column="category_id" class="yang.www.hibernate.Category"></many-to-one>

//key为对应的父目录的ID，即表中的外键

//hibernate.cfg.xml文件

<?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>
        <property name="connection.driver_class">org.postgresql.Driver</property>
        <property name="connection.url">jdbc:postgresql://localhost:5432/myhibernate</property>
        <property name="hibernate.connection.username">postgres</property>
        <property name="hibernate.connection.password">yang</property>
        <property name="hibernate.dialect">org.hibernate.dialect.PostgreSQLDialect</property>
        <property name="current_session_context_class">thread</property>
        <property name="show_sql">true</property>
        <property name="format_sql">true</property>

        <mapping resource="Category.hbm.xml" />
    </session-factory>
</hibernate-configuration>

//SQL文件：

create database myhibernate;

create table categories
(
   id bigint not null,
   name varchar(15),
   category_id bigint,
   primary key(id)
);
alter table categories add constraint fk_category_id foreign key(category_id) references categories(id);