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

<hibernate-mapping package="com.zd.model">

    <class name="classes" table="classes" >

            <generator class="native" />

        <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>

    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());

     }

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。

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。

    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());

      student s = (student) it.next();

hibernate: select count(id) from student where class_id =?

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

    <class name="user" table="user1" >

        <id name="id" column="user_id" type="java.lang.integer">

        <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

public void testget1(){

   user user = null;

    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.rollback();

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