Hibernate fetch 抓取策略
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Hibernate fetch抓取策略定义了当抓取关联对象时，以怎样的一种方式进行抓取。当抓取策略为join时，抓取关联对象是通过连接表来抓取的，只会发送一条sql语句，即可完成主对象与关联对象的抓取，而当抓取策略为select时，抓取关联对象则会发送两条sql语句来完成关联对象的抓取，一条是抓取主对象，而另一条则是抓取关联对象。抓取策略通常会与延迟加载策略合并使用，这是因为当采用延迟加载策略时，如果抓取策略的设置不适当，会造成想象之外的异常，而当采用非延迟加载策略时，即即时加载策略，通常的建议是采用join即连接抓取策略，这样避免了向数据库发送多条sql语句。需要注意的是，只有当使用get或load方法来获取对象时，抓取策略才会起作用，如果查找对象采用的是自定义的hql或sql语句，抓取策略不会起作用。其中get方法是即时加载对象，而load则是延迟加载对象，只有当使用到该对象时，才会向数据库发送查询语句，以查找出该对象。
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一。Husband
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package com.dream.model.join;import java.util.Set;/** * Created by IntelliJ IDEA. * User: Zhong Gang * Date: 9/26/11 * Time: 5:46 PM */public class Husband { private Integer id; private String name; private Set<Wife> wifes; public Husband(String name) { this.name = name; } public Husband() { } public Husband(String name, Set<Wife> wifes) { this.name = name; this.wifes = wifes; } public Set<Wife> getWifes() { return wifes; } public Integer getId() { return id; } public String getName() { return name; } public void setName(String name) { this.name = name; }}?
<?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"><hibernate-mapping default-access="field"> <class name="com.dream.model.join.Husband" table="husband" dynamic-insert="true" dynamic-update="true"> <id name="id" column="id" type="java.lang.Integer"> <generator column="name" type="java.lang.String"/> <set name="wifes" table="wife" cascade="all" lazy="false" fetch="join"> <key column="husbandid"/> <one-to-many name="code">package com.dream.model.join;/** * Created by IntelliJ IDEA. * User: Zhong Gang * Date: 9/26/11 * Time: 5:47 PM */public class Wife { private Integer id; private String name; private Husband husband; public Wife(String name) { this.name = name; } public Wife() { } public Wife(String name, Husband husband) { this.name = name; this.husband = husband; } public String getName() { return name; } public Husband getHusband() { return husband; }}?
<?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"><hibernate-mapping default-access="field"> <class name="com.dream.model.join.Wife" table="wife" dynamic-insert="true" dynamic-update="true"> <id name="id" column="id" type="java.lang.Integer"> <generator column="name" type="java.lang.String"/> <many-to-one name="husband" column="husbandid"/> </class></hibernate-mapping>
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三。CoupleDao
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package com.dream.dao.standard;import com.dream.model.join.Husband;import com.dream.model.join.Wife;import java.util.Set;/** * Created by IntelliJ IDEA. * User: Zhong Gang * Date: 9/26/11 * Time: 5:51 PM */public interface CoupleDao { Husband findHusbandById(Integer id); void saveOrUpdateHusband(Husband husband);}?
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package com.dream.dao;import com.dream.dao.standard.CoupleDao;import com.dream.model.join.Husband;import org.springframework.orm.hibernate3.support.HibernateDaoSupport;import java.util.List;/** * Created by IntelliJ IDEA. * User: Zhong Gang * Date: 9/26/11 * Time: 5:52 PM */public class CoupleDaoImpl extends HibernateDaoSupport implements CoupleDao { public Husband findHusbandById(Integer id) { return getHibernateTemplate().get(Husband.class, id); } public void saveOrUpdateHusband(Husband husband) { getHibernateTemplate().saveOrUpdate(husband); }}?
四。CoupleService
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package com.dream.service.standard;import com.dream.model.join.Husband;/** * Created by IntelliJ IDEA. * User: Zhong Gang * Date: 9/26/11 * Time: 5:53 PM */public interface CoupleService { Husband loadHusbandById(Integer id); void saveOrUpdateHusband(Husband husband);}?package com.dream.service;import com.dream.dao.standard.CoupleDao;import com.dream.exception.DataNotExistException;import com.dream.model.join.Husband;import com.dream.service.standard.CoupleService;/** * Created by IntelliJ IDEA. * User: Zhong Gang * Date: 9/26/11 * Time: 5:53 PM */public class CoupleServiceImpl implements CoupleService { private CoupleDao coupleDao; public Husband loadHusbandById(Integer id) { Husband husband = coupleDao.findHusbandById(id); if (husband == null) { throw new DataNotExistException(); } return husband; } public void saveOrUpdateHusband(Husband husband) { coupleDao.saveOrUpdateHusband(husband); } public void setCoupleDao(CoupleDao coupleDao) { this.coupleDao = coupleDao; }}?
package com.dream.exception;/** * Created by IntelliJ IDEA. * User: Zhong Gang * Date: 9/26/11 * Time: 6:01 PM */public class DataNotExistException extends RuntimeException { public DataNotExistException() { super("The data you find does not exist in the database."); }}??
五。testDatasource
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<?xml version="1.0" encoding="UTF-8"?><beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:context="http://www.springframework.org/schema/context" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.5.xsdhttp://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsdhttp://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd" default-autowire="byName"> <context:property-placeholder location="classpath:testDB.properties"/> <bean id="dataSource" value="${db.driver}"/> <property name="url" value="${db.url}"/> <property name="username" value="${db.username}"/> <property name="password" value="${db.password}"/> </bean> <bean id="sessionFactory" ref="dataSource"/> <property name="mappingLocations"> <list> <value>/hibernate_mappings/Husband.hbm.xml</value> <value>/hibernate_mappings/Wife.hbm.xml</value> </list> </property> <property name="hibernateProperties"> <props> <prop key="hibernate.dialect">${hibernate.dialect}</prop> <prop key="hibernate.show_sql">${hibernate.show_sql}</prop> <prop key="hibernate.hbm2ddl.auto">${hibernate.hbm2ddl.auto}</prop> <prop key="hibernate.jdbc.batch_size">${hibernate.jdbc.batch_size}</prop> </props> </property> </bean> <bean id="hibernateTemplate" ref="sessionFactory"/> </bean> <bean id="transactionManager" ref="sessionFactory"/> </bean> <tx:advice id="txAdvice" transaction-manager="transactionManager"> <tx:attributes> <tx:method name="*" propagation="REQUIRED"/> </tx:attributes> </tx:advice> <aop:config> <aop:pointcut id="service" expression="execution(* com.dream.service..*.*(..))"/> <aop:advisor advice-ref="txAdvice" pointcut-ref="service"/> </aop:config> <bean id="coupleDao" ref="sessionFactory"/> </bean> <bean id="coupleService" ref="coupleDao"/> </bean></beans>?
六。testDB
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db.url=jdbc:mysql://localhost:3306/test_fetchdb.driver=com.mysql.jdbc.Driverdb.username=rootdb.password=roothibernate.dialect=org.hibernate.dialect.MySQL5Dialecthibernate.show_sql=truehibernate.hbm2ddl.auto=updatehibernate.jdbc.batch_size=100
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七。TestCase
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package com.fetch;import com.dream.model.join.Husband;import com.dream.model.join.Wife;import com.dream.service.standard.CoupleService;import junit.framework.TestCase;import org.springframework.context.ApplicationContext;import org.springframework.context.support.ClassPathXmlApplicationContext;import java.util.HashSet;import java.util.Set;/** * Created by IntelliJ IDEA. * User: Zhong Gang * Date: 9/26/11 * Time: 6:05 PM */public class HibernateFetchTest extends TestCase { private CoupleService coupleService; @Override public void setUp() throws Exception { ApplicationContext context = new ClassPathXmlApplicationContext("classpath:testDatasource.xml"); coupleService = (CoupleService) context.getBean("coupleService"); } public void testCascadeAllDelete() throws Exception { Wife wife1 = new Wife("Wife1"); Wife wife2 = new Wife("Wife2"); Set<Wife> wifes = new HashSet<Wife>(); wifes.add(wife1); wifes.add(wife2); Husband husband = new Husband("Husband1", wifes); coupleService.saveOrUpdateHusband(husband); Husband husbandAfterFound = coupleService.loadHusbandById(husband.getId()); Set<Wife> wifeSet = husbandAfterFound.getWifes(); assertEquals(2, wifeSet.size()); }} Scenario 1：lazy="true" fetch="join" Hibernate: insert into husband (name) values (?)Hibernate: insert into wife (name) values (?)Hibernate: insert into wife (name) values (?)Hibernate: update wife set husbandid=? where id=?Hibernate: update wife set husbandid=? where id=?Hibernate: select husband0_.id as id0_1_, husband0_.name as name0_1_, wifes1_.husbandid as husbandid0_3_, wifes1_.id as id3_, wifes1_.id as id1_0_, wifes1_.name as name1_0_, wifes1_.husbandid as husbandid1_0_ from husband husband0_ left outer join wife wifes1_ on husband0_.id=wifes1_.husbandid where husband0_.id=?只发送了一条sql语句便查找出了主对象及关联对象。? Scenario 2：lazy="true" fetch="select"
Hibernate: insert into husband (name) values (?)Hibernate: insert into wife (name) values (?)Hibernate: insert into wife (name) values (?)Hibernate: update wife set husbandid=? where id=?Hibernate: update wife set husbandid=? where id=?Hibernate: select husband0_.id as id0_0_, husband0_.name as name0_0_ from husband husband0_ where husband0_.id=?org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.dream.model.join.Husband.wifes, no session or session was closedat org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:383)at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationExceptionIfNotConnected(AbstractPersistentCollection.java:375)at org.hibernate.collection.AbstractPersistentCollection.readSize(AbstractPersistentCollection.java:122)at org.hibernate.collection.PersistentSet.size(PersistentSet.java:162)at com.fetch.HibernateFetchTest.testCascadeAllDelete(HibernateFetchTest.java:42)at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)at com.intellij.junit3.JUnit3IdeaTestRunner.doRun(JUnit3IdeaTestRunner.java:139)at com.intellij.junit3.JUnit3IdeaTestRunner.startRunnerWithArgs(JUnit3IdeaTestRunner.java:52)at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:199)at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:62)at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)? 当采用延迟加载策略的同时，使用select抓取策略时，因为只有当对象使用时才会发起sql语句，这时因为没有了session的管理，会造成异常。
Scenario 3：lazy="false" fetch="join"
Hibernate: insert into husband (name) values (?)Hibernate: insert into wife (name) values (?)Hibernate: insert into wife (name) values (?)Hibernate: update wife set husbandid=? where id=?Hibernate: update wife set husbandid=? where id=?Hibernate: select husband0_.id as id0_1_, husband0_.name as name0_1_, wifes1_.husbandid as husbandid0_3_, wifes1_.id as id3_, wifes1_.id as id1_0_, wifes1_.name as name1_0_, wifes1_.husbandid as husbandid1_0_ from husband husband0_ left outer join wife wifes1_ on husband0_.id=wifes1_.husbandid where husband0_.id=?使用即时加载策略的同时，使用连接抓取策略，只会发送一条连接sql语句便会查找出主对象及关联对象。
Scenario 4：lazy="false" fetch="select"
Hibernate: insert into husband (name) values (?)Hibernate: insert into wife (name) values (?)Hibernate: insert into wife (name) values (?)Hibernate: update wife set husbandid=? where id=?Hibernate: update wife set husbandid=? where id=?Hibernate: select husband0_.id as id0_0_, husband0_.name as name0_0_ from husband husband0_ where husband0_.id=?Hibernate: select wifes0_.husbandid as husbandid0_1_, wifes0_.id as id1_, wifes0_.id as id1_0_, wifes0_.name as name1_0_, wifes0_.husbandid as husbandid1_0_ from wife wifes0_ where wifes0_.husbandid=?? 使用即时加载策略的同时，使用select抓取策略，会发送两条sql语句，第一条用于查询出主对象，而第二条则会查询出关联对象。
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