Hibernate检索策略学习之--延迟加载
2007-11-28 15:46
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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&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()
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&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()
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