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.youkuaiyun.com/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()