Hibernate检索策略学习之--预先抓取

所谓预先加载，是指hibernate通过select语句使用outer join(一般为左外连接left outer join)来获取对象的关联实例或者关联集合（集合被初始化了，这是重点，和延迟加载不同）

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;
    }
}

 

<!--推荐1对多，多对多使用延迟加载，而多1对1，多对1采用预先抓取-->

设置student对team及certificate进行预先抓取

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.fetch" >
   
    <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>
       <!--推荐1对多，多对多使用延迟加载，而多1对1，多对1采用预先抓取-->
       <one-to-one name="cer" 
                   class="Search.fetch.Certificate"
                   constrained="true" 
                   outer-join="true" 
                   cascade="all">
       </one-to-one>
      
      <!--推荐1对多，多对多使用延迟加载，而多1对1，多对1采用预先抓取-->
       <many-to-one name="team" 
                    column="team_id"
                    outer-join="true" 
                    class="Search.fetch.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.fetch" >
    <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.fetch.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.fetch" > 
    <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>
       <!--推荐1对多，多对多使用延迟加载，而多1对1，多对1采用预先抓取-->
       <one-to-one name="stu" 
                   class="Search.fetch.Student"
                   outer-join="true" 
                   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/fetch/Certificate.hbm.xml" />
    <mapping resource="Search/fetch/Student.hbm.xml" />
    <mapping resource="Search/fetch/Team.hbm.xml" />

</session-factory>

</hibernate-configuration>

 

测试代码：

 

package Search.fetch;

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/fetch"+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());

    }

}

 

结果：

Hibernate: select student0_.id as id1_3_, student0_.cardId as cardId1_3_, student0_.name as name1_3_, student0_.age as age1_3_, student0_.team_id as team5_1_3_, certificat1_.id as id0_0_, certificat1_.description as descript2_0_0_, student2_.id as id1_1_, student2_.cardId as cardId1_1_, student2_.name as name1_1_, student2_.age as age1_1_, student2_.team_id as team5_1_1_, team3_.id as id2_2_, team3_.teamName as teamName2_2_ from student student0_ inner join certificate certificat1_ on student0_.id=certificat1_.id left outer join student student2_ on certificat1_.id=student2_.id left outer join team team3_ on student0_.team_id=team3_.id where student0_.id=?
使用外连接读取student对象，以及和他关联的team对象，certificate对象
tomclus
-------------------------
team1
-------------------------
Hibernate: select students0_.team_id as team5_3_, students0_.id as id3_, students0_.id as id1_2_, students0_.cardId as cardId1_2_, students0_.name as name1_2_, students0_.age as age1_2_, students0_.team_id as team5_1_2_, certificat1_.id as id0_0_, certificat1_.description as descript2_0_0_, student2_.id as id1_1_, student2_.cardId as cardId1_1_, student2_.name as name1_1_, student2_.age as age1_1_, student2_.team_id as team5_1_1_ from student students0_ inner join certificate certificat1_ on students0_.id=certificat1_.id left outer join student student2_ on certificat1_.id=student2_.id where students0_.team_id=?
由于team对students采用了延迟加载，所以在调用System.out.println(stu.getTeam().getStudents().size());的时候，进行SQL执行，同样使用了预先抓取，抓取对象是team中student对象所关联的certificate对象
2