java Hibernate延迟加载
作者：
对one-to-one 关系进行延迟加载和其他关系相比稍微有些不同。many-to-one 的延迟加载是在配置文件的class 标签
示例数据表：team(班级)、certificate(身份证)、student(学生)
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 Persistence Tools
-->
<hibernate-mapping>
<class name="edu.dgut.ke.model.Team" table="TEAM" lazy ="true" ><!-- 多对一的延迟加载设置 -->
<id name="id" type="java.lang.String">
<column name="ID" length="32" />
<generator class="uuid.hex" />
</id>
<property name="teamname" type="java.lang.String">
<column name="TEAMNAME" length="32" not-null="true" unique="true" />
</property>
<set name="students" inverse="true" cascade="all" lazy="true" > <!-- 一对多的延迟加载设置 -->
<key>
<column name="TEAMID" length="32" not-null="true" />
</key>
<one-to-many class="edu.dgut.ke.model.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 Persistence Tools
-->
<hibernate-mapping>
<class name="edu.dgut.ke.model.Certificate" table="CERTIFICATE" lazy="true" ><!-- 一对一的延迟加载设置 -->
<id name="id" type="java.lang.String">
<column name="ID" length="32" />
<generator class="uuid">
</generator>
</id>
<property name="describe" type="java.lang.String">
<column name="`DESCRIBE`" length="50" not-null="true" />
</property>
<one-to-one name="student"
class="edu.dgut.ke.model.Student"
constrained="true" ><!-- 一对一的延迟加载设置 -->
</one-to-one>
</class>
</hibernate-mapping>
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 Persistence Tools
-->
<hibernate-mapping>
<class name="edu.dgut.ke.model.Student" table="STUDENT" lazy="true" >
<id name="id" type="java.lang.String">
<column name="ID" length="32" />
<generator class="uuid.hex" />
</id>
<many-to-one name="certificate"
class="edu.dgut.ke.model.Certificate"
unique="true"
column="cardId"
cascade="all"
>
</many-to-one>
<many-to-one name="team" class="edu.dgut.ke.model.Team">
<column name="TEAMID" length="32" not-null="true" />
</many-to-one>
<property name="studentname" type="java.lang.String">
<column name="STUDENTNAME" length="16" not-null="true" unique="true" />
</property>
</class>
</hibernate-mapping>
注意：对one-to-one 关系进行延迟加载和其他关系相比稍微有些不同。many-to-one 的延迟加载是在配置文件的class 标签
上设置 lazy="true" ，one-to-many 和 many-to-many 的延迟加载是在 set 标签中设置lazy="true"。而one-to-one 不只要在 classs
标签设置 lazy="true"，而且要在one-to-one 标签中设置constrained="true" 。
如果不设置constrained="true"，则一对一查询使用默认的预先抓取策略(fetch="join")。
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 Persistence Tools
-->
<hibernate-mapping>
<class name="edu.dgut.ke.model.Team" table="TEAM" lazy ="true" ><!-- 多对一的延迟加载设置 -->
<id name="id" type="java.lang.String">
<column name="ID" length="32" />
<generator class="uuid.hex" />
</id>
<property name="teamname" type="java.lang.String">
<column name="TEAMNAME" length="32" not-null="true" unique="true" />
</property>
<set name="students" inverse="true" cascade="all" lazy="true" > <!-- 一对多的延迟加载设置 -->
<key>
<column name="TEAMID" length="32" not-null="true" />
</key>
<one-to-many class="edu.dgut.ke.model.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 Persistence Tools
-->
<hibernate-mapping>
<class name="edu.dgut.ke.model.Certificate" table="CERTIFICATE" lazy="true" ><!-- 一对一的延迟加载设置 -->
<id name="id" type="java.lang.String">
<column name="ID" length="32" />
<generator class="uuid">
</generator>
</id>
<property name="describe" type="java.lang.String">
<column name="`DESCRIBE`" length="50" not-null="true" />
</property>
<one-to-one name="student"
class="edu.dgut.ke.model.Student"
constrained="true" ><!-- 一对一的延迟加载设置 -->
</one-to-one>
</class>
</hibernate-mapping>
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 Persistence Tools
-->
<hibernate-mapping>
<class name="edu.dgut.ke.model.Student" table="STUDENT" lazy="true" >
<id name="id" type="java.lang.String">
<column name="ID" length="32" />
<generator class="uuid.hex" />
</id>
<many-to-one name="certificate"
class="edu.dgut.ke.model.Certificate"
unique="true"
column="cardId"
cascade="all"
>
</many-to-one>
<many-to-one name="team" class="edu.dgut.ke.model.Team">
<column name="TEAMID" length="32" not-null="true" />
</many-to-one>
<property name="studentname" type="java.lang.String">
<column name="STUDENTNAME" length="16" not-null="true" unique="true" />
</property>
</class>
</hibernate-mapping>
注意：对one-to-one 关系进行延迟加载和其他关系相比稍微有些不同。many-to-one 的延迟加载是在配置文件的class 标签
上设置 lazy="true" ，one-to-many 和 many-to-many 的延迟加载是在 set 标签中设置lazy="true"。而one-to-one 不只要在 classs
标签设置 lazy="true"，而且要在one-to-one 标签中设置constrained="true" 。
如果不设置constrained="true"，则一对一查询使用默认的预先抓取策略(fetch="join")。