Hibernate检索策略学习之--立即加载
2007-11-28 14:58
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所谓立即加载，是在读取查询对象时，把和查询对象所有有关联的数据全部查出来（包括有关联的对象的关联对象），这种方式是效率最慢的，但好处是只需要一次数据库连接，就可以读取出全部数据，如果数据库连接资源紧张，可以考虑使用
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;
}
}
建立对应的hbm文件
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.immediately" >
<!-- 设置非延迟加载 -->
<class name="Student" table="student" lazy="false">
<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.immediately.Certificate"
constrained="true"
outer-join="false"
cascade="all">
</one-to-one>
<!-- hibernate会自动使用外连接获取学生所属的team,为测试立即加载，关闭外连接 -->
<many-to-one name="team"
column="team_id"
outer-join="false"
class="Search.immediately.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.immediately" >
<class name="Team" table="team" lazy="false">
<id name="id" column="id">
<generator class="uuid.hex"></generator>
</id>
<property name="teamName" column="teamName"></property>
<set name="students" lazy="false" inverse="true" outer-join="false">
<key column="team_id"></key>
<one-to-many class="Search.immediately.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.immediately" >
<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.immediately.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/immediately/Certificate.hbm.xml" />
<mapping resource="Search/immediately/Student.hbm.xml" />
<mapping resource="Search/immediately/Team.hbm.xml" />
</session-factory>
</hibernate-configuration>
测试程序
package Search.immediately;
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/immediately"+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");
}
}
hibernate sql如下，我们开始分析
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
Hibernate: select team0_.id as id2_0_, team0_.teamName as teamName2_0_ from team team0_ where team0_.id=?
由于Student对象关联着其所属的team，这里取回team对象
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=?
team对象持有一个其所有学生的集合，所以，取回team对象后，继续查询student,把另一个（id为2）的studet对象取回，由于id为1的student已经在hibernate缓存中，所以，不会再进行一次数据库查询
Hibernate: select certificat0_.id as id0_0_, certificat0_.description as descript2_0_0_ from certificate certificat0_ where certificat0_.id=?
由于有了一个新取回的student对象，所以，立即加载其certificate对象
综上所述，简单的一个get操作，会引发5个SQL
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;
}
}
建立对应的hbm文件
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.immediately" >
<!-- 设置非延迟加载 -->
<class name="Student" table="student" lazy="false">
<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.immediately.Certificate"
constrained="true"
outer-join="false"
cascade="all">
</one-to-one>
<!-- hibernate会自动使用外连接获取学生所属的team,为测试立即加载，关闭外连接 -->
<many-to-one name="team"
column="team_id"
outer-join="false"
class="Search.immediately.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.immediately" >
<class name="Team" table="team" lazy="false">
<id name="id" column="id">
<generator class="uuid.hex"></generator>
</id>
<property name="teamName" column="teamName"></property>
<set name="students" lazy="false" inverse="true" outer-join="false">
<key column="team_id"></key>
<one-to-many class="Search.immediately.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.immediately" >
<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.immediately.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/immediately/Certificate.hbm.xml" />
<mapping resource="Search/immediately/Student.hbm.xml" />
<mapping resource="Search/immediately/Team.hbm.xml" />
</session-factory>
</hibernate-configuration>
测试程序
package Search.immediately;
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/immediately"+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");
}
}
hibernate sql如下，我们开始分析
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
Hibernate: select team0_.id as id2_0_, team0_.teamName as teamName2_0_ from team team0_ where team0_.id=?
由于Student对象关联着其所属的team，这里取回team对象
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=?
team对象持有一个其所有学生的集合，所以，取回team对象后，继续查询student,把另一个（id为2）的studet对象取回，由于id为1的student已经在hibernate缓存中，所以，不会再进行一次数据库查询
Hibernate: select certificat0_.id as id0_0_, certificat0_.description as descript2_0_0_ from certificate certificat0_ where certificat0_.id=?
由于有了一个新取回的student对象，所以，立即加载其certificate对象
综上所述，简单的一个get操作，会引发5个SQL
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