hibernate set映射

Hibernate之容器映射
Set映射   两张表对应一个类

CREATE TABLE person
(
 id INT identity(1,1) NOT NULL PRIMARY KEY,
 name VARCHAR(100) NOT NULL
);
CREATE TABLE emails
(
 id INT NOT NULL,
 address VARCHAR(100) NOT NULL
);

 

一条person记录对应多条email记录,我们用set,因为元素不重复.

public class Person
{
 private Integer id;
 private String name;
 //一个人拥有多个不同的email地址
 private Set emails;
 getter和setter
}

 

从person表创建映射
映射文件为

<hibernate-mapping>
    <class name="com.zhv.pojo.Person" table="person" catalog="bbs">
        <id name="id" type="integer">
            <column name="id" />
            <generator class="native"></generator>
        </id>
        <property name="name" type="string">
            <column name="name" length="100" not-null="true" />
        </property>
    </class>
</hibernate-mapping>

 

应当对这个文件进行修改
在person表中加入Set集合,集合内容为emails表中全部数据

<set name="emails" table="eails">//对应Person类中字段,表是emails表
    <key column="id"></key> //按照id关联
   <element type="java.lang.String" column="address"></element>
   //集合email中的元素是对应address字段,为String类型
</set>

 

set中映射到class

<set name="images" table="IMAGES" lazy="true" order-by="IMAGE_NAME asc">
			<key column="CUSTOMER_ID"></key>
			<composite-element class="mypack.Image">
				<parent name="customer"/>
				<property name="name" column="IMAGE_NAME"></property>
				<property name="filename" column="FILENAME"></property>
			</composite-element>
		</set>

 

这样就可以通过person一个类来操作两张表

public static void main(String[] args)
 {
  Person p = new Person();
  p.setName("李杏花");
  Set set = new TreeSet();
  set.add("[e-mail]");
  set.add("[e-mail]");
  set.add("[e-mail]");
  p.setEmails(set);
  PersonOper po = new PersonOper();
  po.insert(p);
 }

Hibernate: insert into bbs.person (name) values (?)
Hibernate: insert into emails (id, address) values (?, ?)
Hibernate: insert into emails (id, address) values (?, ?)
Hibernate: insert into emails (id, address) values (?, ?)

在 Hibernate3中,有延迟加载机制.所谓的延迟加载就是指当程序用到数据的时候才自行加载.

public Person queryById(int id)
 {
  Person p = null;
  String hql = "FROM Person as p where p.id=?";
  Query q = this.session.createQuery(hql);
  q.setInteger(0,id);
  Iterator itr = q.list().iterator();
  if(itr.hasNext())
  {
   p = (Person)itr.next();  
  }
 
  this.session.close();
  return p;
 }
 public static void main(String[] args)
 {
  PersonOper po = new PersonOper();
  Person p = po.queryById(1);
  System.out.println(p.getName());
  System.out.println(p.getEmails());

 }

 

报错！
failed to lazily initialize a collection of role: com.zhv.pojo.Person.emails - no session or session was closed
就是因为延迟加载
实际执行SQL语句为
Hibernate: select person0_.id as id, person0_.name as name0_ from bbs.person person0_ where person0_.id=?
根本就没有执行对emails表的查询。
设置就在 set的lazy属性中，默认是true。
如果不关闭session就没有问题了。

当设置为false的时候，就可以查出来
Hibernate: select person0_.id as id, person0_.name as name0_ from bbs.person person0_ where person0_.id=?
Hibernate: select emails0_.id as id0_, emails0_.address as address0_ from emails emails0_ where emails0_.id=?
上来就卡插卡插都给查出来了。

Hibernate中提供的delete方法最大缺点就是必须首先把数据查询出来在进行删除，最好的做法是将数据按照id进行删除，but按照id删除，必须使用hql，但是hql能删除person，不能删除 emails，最好的方法是在数据库里面加入外键foreign key，让数据库自己进行表的维护。