Hibernate检索策略学习之--延迟加载

所谓延迟加载，是在读取查询对象时，不立即取回其所关联的对象，只返回其对象的一个代理，并没有实际数据，当需要用到关联对象的属性时，这是再发送SQL进行数据库查询，减少了不必要的数据库资源我们有三个实体对象Student,Team,Cerificate

其中Student和Team是1对1的关系，采用主键关联的方式（Student的id和Certificate的id保持一致） 
        Team和Student是一对多的关系（反之为1对多）

在Mysql中运行以下脚本，建立数据库

CREATE   TABLE  certificate (
  id  varchar ( 100 )  NOT   NULL   default   '' ,
  description  varchar ( 100 )  default   '' ,
   PRIMARY   KEY   (id)
);





CREATE   TABLE  student (
  team_id  varchar ( 100 )  default   '' ,
  id  varchar ( 100 )  NOT   NULL   default   '' ,
  name  varchar ( 20 )  default   '' ,
  cardId  varchar ( 20 )  NOT   NULL   default   '' ,
  age  int ( 11 )  default   ' 0 ' ,
   PRIMARY   KEY   (id)
);






CREATE   TABLE  team (
  id  varchar ( 100 )  NOT   NULL   default   '' ,
  teamName  varchar ( 100 )  default   '' ,
   PRIMARY   KEY   (id)
);



INSERT   INTO  certificate  VALUES  ( ' 1 ' , ' 110108 ' );
INSERT   INTO  certificate  VALUES  ( ' 2 ' , ' 110109 ' );

INSERT   INTO  student  VALUES  ( ' 1 ' , ' 1 ' , ' tomclus ' , ' 2006m ' , 33 );
INSERT   INTO  student  VALUES  ( ' 2 ' , ' 2 ' , ' tom ' , ' 2007m ' , 22 );
INSERT   INTO  team  VALUES  ( ' 1 ' , ' team1 ' );

建立对应的Pojo对象

package  Search.immediately;

public   class  Certificate ... ... {
    private String id;
    private String description;
    private Student stu;

    public Student getStu() ......{
        return stu;
    }

    public void setStu(Student stu) ......{
        this.stu = stu;
    }


    public String getDescription() ......{
        return description;
    }

    public void setDescription(String description) ......{
        this.description = description;
    }

    public String getId() ......{
        return id;
    }

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



package  Search.immediately;

import  java.util.HashSet;
import  java.util.Set;


public   class  Team ... ... {
    private String id;
    private Set students=new HashSet();
    private String teamName;
    private Set tests;
  
    public Set getTests() ......{
        return tests;
    }
 
    public void setTests(Set tests) ......{
        this.tests = tests;
    }

    public String getId() ......{
        return id;
    }

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

    public String getTeamName() ......{
        return teamName;
    }

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

    public Set getStudents() ......{
        return students;
    }

    public void setStudents(Set students) ......{
        this.students = students;
    }
}



package  Search.immediately;

public   class  Certificate ... ... {
    private String id;
    private String description;
    private Student stu;

    public Student getStu() ......{
        return stu;
    }

    public void setStu(Student stu) ......{
        this.stu = stu;
    }


    public String getDescription() ......{
        return description;
    }

    public void setDescription(String description) ......{
        this.description = description;
    }

    public String getId() ......{
        return id;
    }

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

本文中，Student和Certificate为还为立即加在，student和team双向1对多，多对1采用延迟加载

Student.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" >
<!--  
    Mapping file autogenerated by MyEclipse - Hibernate Tools
-->
< hibernate-mapping  package ="Search.lazy"   >
     <!--  设置非延迟加载，由于many-to-one不能设置lazy属性，所以我们在class上
          设置，但one-to-one的cer属性不需要延迟，怎么办呢？我们可以在one-to-one上
          再设置一下lazy="true",这样one-to-one就会使用自己的lazy设置了 -->
     < class  name ="Student"  table ="student"  lazy ="true" >
        < id  name ="id"  column ="id"  unsaved-value ="null" >
          < generator  class ="uuid.hex" ></ generator >
        </ id >

        < property  name ="cardId"  column ="cardId" ></ property >
        < property  name ="name"  column ="name" ></ property >
        < property  name ="age"  column ="age" ></ property >
        <!--  为测试立即加载，关闭外连接  -->
        < one-to-one  name ="cer"  
                   class ="Search.lazy.Certificate"
                   constrained ="true"  
                   outer-join ="false"  
                   cascade ="all" >
        </ one-to-one >
        <!--  关闭外连接,并设置lazy="true",表示读取student对象后，并不立即把其所属的team信息取回 
              注意这里并不是设置lazy="true"而是设置lazy="no-proxy"(针对hibernate3),而且还必须在
              其关联的Team的class标签中设置lazy="true"方能实现student对team的延迟加载，
              这个知识点也可以参考http://blog.csdn.net/daryl715/archive/2007/11/28/1905989.aspx
        -->
        < many-to-one  name ="team"  
                    column ="team_id"
                    outer-join ="false"  
                    lazy ="no-proxy"
                    class ="Search.lazy.Team" ></ many-to-one >
       </ class >
</ hibernate-mapping >

Team.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" >
<!--  
    Mapping file autogenerated by MyEclipse - Hibernate Tools
-->
< hibernate-mapping  package ="Search.lazy"   >
     < class  name ="Team"  table ="team"  lazy ="true" >
        < id  name ="id"  column ="id" >
          < generator  class ="uuid.hex" ></ generator >
        </ id >
        < property  name ="teamName"  column ="teamName" ></ property >
        <!--  关闭外连接,并设置lazy="true",表示读取team对象后，并不立即把其所有的student信息取回  -->
        < set  name ="students"  lazy ="true"  inverse ="true"  outer-join ="false" >
          < key  column ="team_id" ></ key >
          < one-to-many  class ="Search.lazy.Student" />
        </ set >
       </ class >
</ hibernate-mapping >

Certificate.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" >
<!--  
    Mapping file autogenerated by MyEclipse - Hibernate Tools
-->
< hibernate-mapping  package ="Search.lazy"   >  
     < class  name ="Certificate"  table ="certificate"  lazy ="false" >
        <!--  共享主键关联，在子方配置constrained=true  -->
        < id  name ="id"  column ="id" >
          < generator  class ="foreign" >
            < param  name ="property" > stu </ param >
          </ generator >
        </ id >


        < property  name ="description"  column ="description" ></ property >
        <!--  为测试立即加载，关闭外连接  -->
        < one-to-one  name ="stu"  
                   class ="Search.lazy.Student"
                   outer-join ="false"  
                   constrained ="true" >
        </ one-to-one >
       </ class >
</ hibernate-mapping >

hibernate.cfg.xml

<? 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" >

<!--  Generated by MyEclipse Hibernate Tools.                    -->
< hibernate-configuration >

< session-factory >
     < property  name ="connection.username" > root </ property >
     < property  name ="connection.url" >
        jdbc:mysql://localhost:3306/hibernateconformity?characterEncoding=gb2312 &amp; useUnicode=true
     </ property >
     < property  name ="dialect" >
        org.hibernate.dialect.MySQLDialect
     </ property >
     < property  name ="myeclipse.connection.profile" > mysql </ property >
     < property  name ="connection.password" > 1234 </ property >
     < property  name ="connection.driver_class" >
        com.mysql.jdbc.Driver
     </ property >
     < property  name ="hibernate.dialect" >
        org.hibernate.dialect.MySQLDialect
     </ property >
     < property  name ="hibernate.show_sql" > true </ property >
     < property  name ="current_session_context_class" > thread </ property >
     < mapping  resource ="Search/lazy/Certificate.hbm.xml"   />
     < mapping  resource ="Search/lazy/Student.hbm.xml"   />
     < mapping  resource ="Search/lazy/Team.hbm.xml"   />

</ session-factory >

</ hibernate-configuration >

测试代码：

package  Search.lazy;

import  java.io.File;
import  java.io.FileInputStream;

import  org.hibernate.Session;
import  org.hibernate.SessionFactory;
import  org.hibernate.cfg.Configuration;

public   class  Test  ... {


    public static void main(String[] args) ...{
        String filePath=System.getProperty("user.dir")+File.separator+"src/Search/lazy"+File.separator+"hibernate.cfg.xml";
        File file=new File(filePath);
        SessionFactory sessionFactory=new Configuration().configure(file).buildSessionFactory();
        Session session=sessionFactory.openSession();
        Student stu=(Student)session.get(Student.class, "1");
        System.out.println(stu.getName());
        System.out.println("-------------------------");
        System.out.println(stu.getTeam().getTeamName());
        System.out.println("-------------------------");
        System.out.println(stu.getTeam().getStudents().size());

    }

}

运行结果：

Hibernate: select student0_.id as id1_0_, student0_.cardId as cardId1_0_, student0_.name as name1_0_, student0_.age as age1_0_, student0_.team_id as team5_1_0_ from student student0_ where student0_.id=?
这条语句是因为我们调用了get方式，取id为1的student对象而产生的
Hibernate: select certificat0_.id as id0_0_, certificat0_.description as descript2_0_0_ from certificate certificat0_ where certificat0_.id=?
由于是立即加载，hibernate会自动根据取回的student对象来取得和他对应的Certificate
tomclus
-------------------------
打印学生的名字，可以砍到，这里只执行2条sql
Hibernate: select team0_.id as id2_0_, team0_.teamName as teamName2_0_ from team team0_ where team0_.id=?
当运行System.out.println(stu.getTeam().getTeamName());取student关联的team的时候，这时候才执行sql取team对象，但只是取team对象而已，其关联的student集合并不取回
team1
-------------------------
打印班级的名字
Hibernate: select students0_.team_id as team5_1_, students0_.id as id1_, students0_.id as id1_0_, students0_.cardId as cardId1_0_, students0_.name as name1_0_, students0_.age as age1_0_, students0_.team_id as team5_1_0_ from student students0_ where students0_.team_id=?

Hibernate: select certificat0_.id as id0_0_, certificat0_.description as descript2_0_0_ from certificate certificat0_ where certificat0_.id=?
当运行System.out.println(stu.getTeam().getStudents().size());时，发送sql指令，取回team关联的student集合，并根据student和certificate之间的立即加载策略，取回对应的certificate,由于hibernate session的缓存，这里不用再取一边id为1的的student

2

特别注意：

如果在System.out.println(stu.getTeam().getTeamName());之前关闭了session,这是运行
System.out.println(stu.getTeam().getTeamName());会出现could not initialize proxy-thw owning Session was close的异常错误，这是因为关闭session对象后，取回的student对象已经从持久太转变成游离态，这时候，如果再进行读取，当然会提示没有session,如果我们这时候重新建立一个session2,则需要把student对象和新session绑定，重新转变成持久抬，方法如下:

session2.update(stu.getTeam());
session2.initialize(stu.getTeam());
session2.initialize(stu.getTeam().getStudents());这一句非常重要，因为initialize方法只处理代理对象和集合，对其再嵌套的对象不起作用，所以，为了能读出team中的student对象，我们需要两次执行initialize()