hibernate中主表和从表查询懒加载

解决懒加载的问题

Exception in thread "main" org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role

----主表

<?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">
<!-- 
    Mapping file autogenerated by MyEclipse Persistence Tools
-->
<hibernate-mapping>
    <class name="com.po.Sortp" table="sortp" >
        <id name="id" type="java.lang.Integer">
            <column name="id" />
            <generator class="assigned" />
        </id>
        <property name="name" type="java.lang.String">
            <column name="name" length="40" not-null="true" />
        </property>
        <set name="productses" inverse="true" cascade="all" lazy="false">
            <key>
                <column name="sortid" not-null="true" />
            </key>
            <one-to-many class="com.po.Products" />
        </set>
    </class>
</hibernate-mapping>

---从表

<?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">
<!-- 
    Mapping file autogenerated by MyEclipse Persistence Tools
-->
<hibernate-mapping>
    <class name="com.po.Products" table="products" >
        <id name="id" type="java.lang.Integer">
            <column name="id" />
            <generator class="assigned" />
        </id>
        <many-to-one name="sortp" class="com.po.Sortp" fetch="select" cascade="all">
            <column name="sortid" not-null="true" />
        </many-to-one>
        <property name="name" type="java.lang.String">
            <column name="name" length="50" not-null="true" />
        </property>
        <property name="price" type="java.lang.Double">
            <column name="price" precision="22" scale="0" not-null="true" />
        </property>
        <property name="saleprice" type="java.lang.Double">
            <column name="saleprice" precision="22" scale="0" not-null="true" />
        </property>
        <property name="descript" type="java.lang.String">
            <column name="descript" length="65535" not-null="true" />
        </property>
        <property name="contents" type="java.lang.String">
            <column name="contents" length="65535" not-null="true" />
        </property>
        <property name="saledate" type="java.util.Date">
            <column name="saledate" length="10" not-null="true" />
        </property>
        <property name="salecount" type="java.lang.Integer">
            <column name="salecount" />
        </property>
        <property name="image" type="java.lang.String">
            <column name="image" length="50" />
        </property>
    </class>
</hibernate-mapping>

在主表映射中需要设置懒加载，默认是为true 的

<set name="productses" inverse="true" cascade="all" lazy="true">

还有一种方式就是用代码处理

public static void main(String[] args) {
   // TODO Auto-generated method stub
   SortpDAO dao = new SortpDAO();
   List<Sortp> list = dao.getAllSortp();
  
   for(Sortp s :list){
    if(!Hibernate.isInitialized(s.getProductses())){
     Hibernate.initialize(s.getProductses());
    }
   }
   dao.closeSession();
   for(Sortp s :list){
      System.out.println(s.getName());
      Set<Products> set =s.getProductses();
      Iterator<Products> it = set.iterator();
      while(it.hasNext()){
      System.out.println("-----------"+it.next().getDescript());
      }
   }
}

评论

1 楼 lizhihui19871127 2012-05-08  

楼主：和你探讨一下
一、我觉得你的第一种方式，不好。为什么呢，因为是一对多的关系，如果lazy设置为false的话，加载主对象的时候，会把所有的依赖关系的对象全部读取出来。你想想，如果是数百万个实例对象，系统性能如何？考虑过没有？所以一般都是多对一的时候才设置layz为true。
二、代码处理的话，也不对，如果是懒加载，都没有加载对应的关联对象，你的Set<Products> set =s.getProductses();
还能获取到值？应该是空的吧。
我也是今天研究了一下，欢迎一起研讨一下，一起进步~
不敬之处，敬请谅解~