hibernate lazy——延迟加载

 hibernate lazy策略可以使用在：
* <class>标签上，可以取值：true/false ，在hibernate3以上版本，默认是true
* <property>标签上，可以取值：true/false 需要类增强工具
* <set><list>标签上，可以取值：true/false/extra
* <one-to-one><many-to-one>单端关联上，可以取值：false/proxy/no-proxy

lazy概念：只有真正使用该对象时，才会创建，对于hibernate而言，正真使用的时候才会发出sql

hibernate支持lazy策略只有在session打开状态下有效



1 <class>标签上:

group.hbm.xml

<?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 package="com.model">
    <class name="Group" table="group5" lazy="true" > //lazy，默认true，可不写
        <id name="id" column="id" type="java.lang.Integer">
            <generator class="native" />        
        </id>
        <property name="name" column="name" length="50" type="java.lang.String" />
    </class>
</hibernate-mapping>

测试用例：

public class LazyTest extends TestCase

{

public void testLoad1(){
   Session session = null;
   Transaction ta = null;
   try{
    session = HibernateUtil.getSession();
    ta = session.beginTransaction();

    //还没发出sql，lazy起延迟作用,若lazy=false,则发出sql

    Group g2 = (Group) session.load(Group.class, 1);
    Group g3 = (Group) session.get(Group.class, 1);   //不支持lazy
    System.out.println("group.id=" + g2.getId());     //还没发出sql，
    System.out.println("group.name=" + g2.getName()); //发出sql
    ta.commit();
   }catch(Exception e){
    e.printStackTrace();
    if(ta != null){
     ta.rollback();
    }
   }finally{
    //关闭session， user变为detached离线对象
    HibernateUtil.closeSession(session);
   }
   // System.out.println("group.name=" + g2.getName());
   // hibernate支持lazy策略只有在session打开状态下有效,所以此出Exception
}

}

<class>标签上的lazy特性只对普通属性起作用

<class>标签上的lazy不会影响到单端关联上的lazy特性




2.<set><list>标签上，可以取值：true/false/extra，默认是true

Classes.hbm.xml

<?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 package="com.zd.model">
    <class name="Classes" table="classes" >
        <id name="id" column="id" type="java.lang.Integer">
            <generator class="native" />
        </id>
        <property name="name" column="name" type="java.lang.String" />
        <set name="students" lazy="true"> //可不配lazy，因默认是true
            <key column="class_id" />
            <one-to-many class="com.zd.model.Student" />
        </set>
    </class>

</hibernate-mapping>

测试用例：

public void testLoad1(){
   Session session = null;
   Transaction ta = null;
   try{
    session = HibernateUtil.getSession();
    ta = session.beginTransaction();
    Classes c = (Classes) session.load(Classes.class, new Integer(2));  //没有sql
    System.out.println("Class.name=" + c.getName());                    //发出一条sql，但不查 set
    Set stuSet = c.getStudents();                                   //没有发出查询sql，不是统计sql
    //System.out.println(stuSet.size());//发出查询sqlsql
    if(stuSet != null && !stuSet.isEmpty()){
     //发出查询sqlsql
     for(Iterator it = stuSet.iterator(); it.hasNext();){
      Student s = (Student) it.next();//若没有.size(),isEmpty(),就在这边发出sql
      System.out.println("student.name=" + s.getName());
     }
    }
    ta.commit();
   }catch(Exception e){
    e.printStackTrace();
    if(ta != null){
     ta.rollback();
    }
   }finally{
    //关闭session， user变为detached离线对象
    HibernateUtil.closeSession(session);
   }
 
}

Hibernate: select classes0_.id as id0_0_, classes0_.name as name0_0_ from classes classes0_ where classes0_.id=?
Class.name=Java Class
Hibernate: select students0_.class_id as class3_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.class_id as class3_1_0_ from student students0_ where students0_.class_id=?
2
student.name=z3
student.name=l4

若<set name="students" lazy="false"> //不延迟加载, 马上加载

则在

System.out.println("Class.name=" + c.getName());// 就发出2条查询语句了。

Hibernate: select classes0_.id as id0_0_, classes0_.name as name0_0_ from classes classes0_ where classes0_.id=?
Hibernate: select students0_.class_id as class3_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.class_id as class3_1_0_ from student students0_ where students0_.class_id=?
Class.name=Java Class
student.name=l4
student.name=z3

若<set name="students" lazy="extra"> //和true差不多，只是在写set.size()时，发出selcet count的sql语句，比true好一些。

测试用例：

public void testLoad1(){
   Session session = null;
   Transaction ta = null;
   try{
    session = HibernateUtil.getSession();
    ta = session.beginTransaction();
    Classes c = (Classes) session.load(Classes.class, new Integer(2));
    System.out.println("Class.name=" + c.getName());
    Set stuSet = c.getStudents();
    System.out.println(stuSet.size());
    if(stuSet != null && !stuSet.isEmpty()){
     for(Iterator it = stuSet.iterator(); it.hasNext();){
      Student s = (Student) it.next();
      System.out.println("student.name=" + s.getName());
     }
    }
    ta.commit();
   }catch(Exception e){
    e.printStackTrace();
    if(ta != null){
     ta.rollback();
    }
   }finally{
    //关闭session， user变为detached离线对象
    HibernateUtil.closeSession(session);
   }
 
}

Hibernate: select classes0_.id as id0_0_, classes0_.name as name0_0_ from classes classes0_ where classes0_.id=?
Class.name=Java Class
Hibernate: select count(id) from student where class_id =?
2
Hibernate: select students0_.class_id as class3_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.name as name1_0_, students0_.class_id as class3_1_0_ from student students0_ where students0_.class_id=?
student.name=z3
student.name=l4




3.<one-to-one><many-to-one>单端关联上，可以取值：false/proxy/no-proxy，默认是proxy(代理），延迟加载作用

User.hbm.xml 多的一端

<?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 package="com.model">
    <class name="User" table="user1" >
        <id name="id" column="user_id" type="java.lang.Integer">
            <generator class="native" />
        </id>
        <property name="name" length="50" type="java.lang.String" />
        <many-to-one name="group" column="group_id" lazy="proxy"></many-to-one>
         //可不写，默认是proxy
    </class>

</hibernate-mapping>

测试用例：

public void testGet1(){
   Session session = null;
   Transaction ta = null;
   User user = null;
   try{
    session = HibernateUtil.getSession();
    ta = session.beginTransaction();
    user = (User)session.load(User.class, new Integer(3)); //无sql
    System.out.println("user.name=" + user.getName()); //有一条sql
    Group group = user.getGroup();//无sql
    System.out.println("group.name=" + group.getName());//有一条sql
    ta.commit();
   }catch(Exception e){
    e.printStackTrace();
    ta.rollback();
   }finally{
    //关闭session， user变为detached离线对象
    HibernateUtil.closeSession(session);
   }
 
}

若<many-to-one name="group" column="group_id" lazy="false"></many-to-one>

不延迟加载，立即加载，

System.out.println("user.name=" + user.getName()); //发出2条sql语句


原帖地址： http://apps.hi.baidu.com/share/detail/38568475