Hibernate 中集合对象的抓取策略(Fetching strategies)
Product.java
代碼；
package com.b510.examples;
/**
?* Product entity. @author MyEclipse Persistence Tools
?*/
public class Product implements java.io.Serializable {
?private static final long serialVersionUID = -1546206493725028472L;
?private Integer id;
?private Category category;
?private String name;
?private String price;
?private String descripton;
?
?public Product() {
?}
?public Integer getId() {
??return this.id;
?}
?public void setId(Integer id) {
??this.id = id;
?}
?public Category getCategory() {
??return this.category;
?}
?public void setCategory(Category category) {
??this.category = category;
?}
?public String getName() {
??return this.name;
?}
?public void setName(String name) {
??this.name = name;
?}
?public String getPrice() {
??return this.price;
?}
?public void setPrice(String price) {
??this.price = price;
?}
?public String getDescripton() {
??return this.descripton;
?}
?public void setDescripton(String descripton) {
??this.descripton = descripton;
?}
}
?
?
?
Category.java
代碼：
package com.b510.examples;
import java.util.HashSet;
import java.util.Set;
public class Category implements java.io.Serializable {
?private static final long serialVersionUID = 3240281547213597385L;
?private Integer id;
?private String name;
?private String description;
?private Set products = new HashSet(0);
?public Category() {
?}
?public Integer getId() {
??return this.id;
?}
?public void setId(Integer id) {
??this.id = id;
?}
?public String getName() {
??return this.name;
?}
?public void setName(String name) {
??this.name = name;
?}
?public String getDescription() {
??return this.description;
?}
?public void setDescription(String description) {
??this.description = description;
?}
?public Set getProducts() {
??return this.products;
?}
?public void setProducts(Set products) {
??this.products = products;
?}
}
?
?
?
?
Product.hbm.xml
代碼：
<?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="com.b510.examples.Product" table="product" catalog="users">
??????? <id name="id" type="java.lang.Integer">
??????????? <column name="id" />
??????????? <generator class="increment" />
??????? </id>
??????? <many-to-one name="category" class="com.b510.examples.Category" fetch="select">
??????????? <column name="category_id" />
??????? </many-to-one>
??????? <property name="name" type="java.lang.String">
??????????? <column name="name" length="500" />
??????? </property>
??????? <property name="price" type="java.lang.String">
??????????? <column name="price" length="10" />
??????? </property>
??????? <property name="descripton" type="java.lang.String">
??????????? <column name="descripton" length="500" />
??????? </property>
??? </class>
</hibernate-mapping>
?
?
Category.hbm.xml
代碼：
<?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="com.b510.examples.Category" table="category" catalog="users">
??????? <id name="id" type="java.lang.Integer">
??????????? <column name="id" />
??????????? <generator class="increment" />
??????? </id>
??????? <property name="name" type="java.lang.String">
??????????? <column name="name" length="500" />
??????? </property>
??????? <property name="description" type="java.lang.String">
??????????? <column name="description" length="500" />
??????? </property>
??????? <set name="products" inverse="true" >
??????????? <key>
??????????????? <column name="category_id" />
??????????? </key>
??????????? <one-to-many class="com.b510.examples.Product" />
??????? </set>
??? </class>
</hibernate-mapping>
?
?
測試代碼：
HibernateTest.java
代碼：
/**
?*
?*/
package com.b510.examples;
import java.util.Set;
import org.hibernate.Session;
/**
?*
?* @author XHW
?*
?* @date 2011-7-18
?*
?*/
public class HibernateTest {
?public static void main(String[] args) {
??new HibernateTest().update();
?}
?public void update(){
??Session session=HibernateSessionFactoryUtil.getSessionFactory().getCurrentSession();
??session.beginTransaction();
??Category category=(Category)session.get(Category.class, 1);
??System.out.println("id:"+category.getId()+"? ,name:"+category.getName()+", description:"+category.getDescription());
??
??Set<Product> products=category.getProducts();
???for(Product p:products){
????System.out.println(p.getName());
???}
??session.getTransaction().commit();??
?}
?
?
}
?
?
?
?
運行效果：
log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment).
log4j:WARN Please initialize the log4j system properly.
Hibernate:
??? select
??????? category0_.id as id1_0_,
??????? category0_.name as name1_0_,
??????? category0_.description as descript3_1_0_
??? from
??????? users.category category0_
??? where
??????? category0_.id=?
id:1? ,name:java, description:java好啊
Hibernate:?
??? select
??????? products0_.category_id as category2_1_,
??????? products0_.id as id1_,
??????? products0_.id as id0_0_,
??????? products0_.category_id as category2_0_0_,
??????? products0_.name as name0_0_,
??????? products0_.price as price0_0_,
??????? products0_.descripton as descripton0_0_
??? from
??????? users.product products0_
??? where
??????? products0_.category_id=?
java SE應用程序設計
這里我們看到 的是，我們在讀取一個集合對象的 時候，hibernate用了一條selelct語句；
?
1、查詢抓取(Select fetching)
修改：Category.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="com.b510.examples.Category" table="category" catalog="users">
??????? <id name="id" type="java.lang.Integer">
??????????? <column name="id" />
??????????? <generator class="increment" />
??????? </id>
??????? <property name="name" type="java.lang.String">
??????????? <column name="name" length="500" />
??????? </property>
??????? <property name="description" type="java.lang.String">
??????????? <column name="description" length="500" />
??????? </property>
??????? <set name="products" inverse="true" fetch="select">
??????????? <key>
??????????????? <column name="category_id" />
??????????? </key>
??????????? <one-to-many class="com.b510.examples.Product" />
??????? </set>
??? </class>
</hibernate-mapping>
測試代碼：
HibernateTest.java
代碼：
/**
?*
?*/
package com.b510.examples;
import java.util.Set;
import org.hibernate.Session;
/**
?*
?* @author XHW
?*
?* @date 2011-7-18
?*
?*/
public class HibernateTest {
?public static void main(String[] args) {
??new HibernateTest().update();
?}
?public void update(){
??Session session=HibernateSessionFactoryUtil.getSessionFactory().getCurrentSession();
??session.beginTransaction();
??Category category=(Category)session.get(Category.class, 1);
??System.out.println("id:"+category.getId()+"? ,name:"+category.getName()+", description:"+category.getDescription());
??
??Set<Product> products=category.getProducts();
???for(Product p:products){
????System.out.println(p.getName());
???}
??session.getTransaction().commit();??
?}
?
}
運行效果：
log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment).
log4j:WARN Please initialize the log4j system properly.
Hibernate:
??? select
??????? category0_.id as id1_0_,
??????? category0_.name as name1_0_,
??????? category0_.description as descript3_1_0_
??? from
??????? users.category category0_
??? where
??????? category0_.id=?
id:1? ,name:java, description:java好啊
Hibernate:?
??? select
??????? products0_.category_id as category2_1_,
??????? products0_.id as id1_,
??????? products0_.id as id0_0_,
??????? products0_.category_id as category2_0_0_,
??????? products0_.name as name0_0_,
??????? products0_.price as price0_0_,
??????? products0_.descripton as descripton0_0_
??? from
??????? users.product products0_
??? where
??????? products0_.category_id=?
java SE應用程序設計
這種方法是查詢抓取，他是hibernate默認值，所以我們會看到和上面的運行效果一摸一樣。
測試代碼二：
HibernateTest.java
代碼；
/**
?*
?*/
package com.b510.examples;
import java.util.Set;
import org.hibernate.Query;
import org.hibernate.Session;
/**
?*
?* @author XHW
?*
?* @date 2011-7-18
?*
?*/
public class HibernateTest {
?public static void main(String[] args) {
??new HibernateTest().update();
?}
?public void update(){
??Session session=HibernateSessionFactoryUtil.getSessionFactory().getCurrentSession();
??session.beginTransaction();
??Query query=session.createQuery("select c from Category c inner join fetch c.products where c.id=?");
??query.setInteger(0, 1);
??Category category=(Category)query.uniqueResult();
??System.out.println("id:"+category.getId()+"? ,name:"+category.getName()+", description:"+category.getDescription());
??
??Set<Product> products=category.getProducts();
???for(Product p:products){
????System.out.println(p.getName());
???}
??session.getTransaction().commit();??
?}?
}
運行效果：
log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment).
log4j:WARN Please initialize the log4j system properly.
Hibernate:?
??? select
??????? category0_.id as id1_0_,
??????? products1_.id as id0_1_,
??????? category0_.name as name1_0_,
??????? category0_.description as descript3_1_0_,
??????? products1_.category_id as category2_0_1_,
??????? products1_.name as name0_1_,
??????? products1_.price as price0_1_,
??????? products1_.descripton as descripton0_1_,
??????? products1_.category_id as category2_0__,
??????? products1_.id as id0__
??? from
??????? users.category category0_
??? inner join
??????? users.product products1_
??????????? on category0_.id=products1_.category_id
??? where
??????? category0_.id=?
id:1? ,name:java, description:java好啊
java SE應用程序設計
我們發現現在hibernate只是用了一個select語句。這是我們通過hql語句重載底層設置的抓取策略
hql的優先級很好。盡管你我們在配置文件中設置了fetch=“select”，就是說hibernate要用兩個selelct語句
去執行這樣的行為，但是，我們在hql語句中設置了急迫內連接查詢，這樣hibernate就必須只用一條select語句去查詢
這樣就等于把我們底層設置進行了重載，或者說是覆蓋。
?
2、子查詢抓取(Subselect fetching)
?Category.hbm.xml
代碼：
<?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="com.b510.examples.Category" table="category" catalog="users">
??????? <id name="id" type="java.lang.Integer">
??????????? <column name="id" />
??????????? <generator class="increment" />
??????? </id>
??????? <property name="name" type="java.lang.String">
??????????? <column name="name" length="500" />
??????? </property>
??????? <property name="description" type="java.lang.String">
??????????? <column name="description" length="500" />
??????? </property>
??????? <set name="products" inverse="true" fetch="subselect">
??????????? <key>
??????????????? <column name="category_id" />
??????????? </key>
??????????? <one-to-many class="com.b510.examples.Product" />
??????? </set>
??? </class>
</hibernate-mapping>
測試代碼：
HibernateTest.java
代碼：
/**
?*
?*/
package com.b510.examples;
import java.util.Iterator;
import java.util.List;
import java.util.Set;
import org.hibernate.Session;
import org.hibernate.Transaction;
/**
?*
?* @author XHW
?*
?* @date 2011-7-18
?*
?*/
public class HibernateTest {
?public static void main(String[] args) {
??new HibernateTest().update();
?}
?public void update() {
??Session session = HibernateSessionFactoryUtil.getSessionFactory()
????.getCurrentSession();
??Transaction tx = session.beginTransaction();
??List categorys = session.createQuery("from Category").list();
??for (Iterator<Category> it = categorys.iterator(); it.hasNext();) {
???Category category = it.next();
???System.out.println("id:" + category.getId() + "? ,name:"
?????+ category.getName() + ", description:"
?????+ category.getDescription());
???for (Iterator<Product> it2 = category.getProducts().iterator(); it2
?????.hasNext();) {
????Product p = it2.next();
????System.out.println(p.getName());
???}
??}
??tx.commit();
?}
}
運行效果：
log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment).
log4j:WARN Please initialize the log4j system properly.
Hibernate:
??? select
??????? category0_.id as id1_,
??????? category0_.name as name1_,
??????? category0_.description as descript3_1_
??? from
??????? users.category category0_
id:1? ,name:java, description:java好啊
Hibernate:?
??? select
??????? products0_.category_id as category2_1_,
??????? products0_.id as id1_,
??????? products0_.id as id0_0_,
??????? products0_.category_id as category2_0_0_,
??????? products0_.name as name0_0_,
??????? products0_.price as price0_0_,
??????? products0_.descripton as descripton0_0_
??? from
??????? users.product products0_
??? where
??????? products0_.category_id in (
??????????? select
??????????????? category0_.id
??????????? from
??????????????? users.category category0_
??????? )
java SE應用程序設計
id:2? ,name:Hibernate, description:Hibernate好啊
java WEB開發與實戰
我們看到，下面的selelct語句中又嵌套了一個select語句
3、連接抓取(Join fetching)
Category.hbm.xml
代碼；
<?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="com.b510.examples.Category" table="category" catalog="users">
??????? <id name="id" type="java.lang.Integer">
??????????? <column name="id" />
??????????? <generator class="increment" />
??????? </id>
??????? <property name="name" type="java.lang.String">
??????????? <column name="name" length="500" />
??????? </property>
??????? <property name="description" type="java.lang.String">
??????????? <column name="description" length="500" />
??????? </property>
??????? <set name="products" inverse="true" fetch="join">
??????????? <key>
??????????????? <column name="category_id" />
??????????? </key>
??????????? <one-to-many class="com.b510.examples.Product" />
??????? </set>
??? </class>
</hibernate-mapping>
?
測試代碼；
HibernateTest.java
代碼：
/**
?*
?*/
package com.b510.examples;
import java.util.Iterator;
import java.util.List;
import org.hibernate.Session;
import org.hibernate.Transaction;
/**
?*
?* @author XHW
?*
?* @date 2011-7-18
?*
?*/
public class HibernateTest {
?public static void main(String[] args) {
??new HibernateTest().example();
?}
?public void example(){
??Session session =HibernateSessionFactoryUtil.getSessionFactory().getCurrentSession();
??Transaction tx=session.beginTransaction();
??
??Category category=(Category)session.get(Category.class, 1);
??System.out.println("id:" + category.getId() + "? ,name:"
????+ category.getName() + ", description:"
????+ category.getDescription());
??
??Iterator<Product> it=category.getProducts().iterator();
??Product p=null;
??while(it.hasNext()){
???p=it.next();
???System.out.println(p.getName());
??}
??tx.commit();
?}
}
運行效果：
log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment).
log4j:WARN Please initialize the log4j system properly.
Hibernate:
??? select
??????? category0_.id as id1_1_,
??????? category0_.name as name1_1_,
??????? category0_.description as descript3_1_1_,
??????? products1_.category_id as category2_3_,
??????? products1_.id as id3_,
??????? products1_.id as id0_0_,
??????? products1_.category_id as category2_0_0_,
??????? products1_.name as name0_0_,
??????? products1_.price as price0_0_,
??????? products1_.descripton as descripton0_0_
??? from
??????? users.category category0_?
??? left outer join
??????? users.product products1_
??????????? on category0_.id=products1_.category_id
??? where
??????? category0_.id=?
id:1? ,name:java, description:java好啊
java SE應用程序設計
4、批量抓取(Batch fetching)
?Category.xml
代碼：
<?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="com.b510.examples.Category" table="category" catalog="users">
??????? <id name="id" type="java.lang.Integer">
??????????? <column name="id" />
??????????? <generator class="increment" />
??????? </id>
??????? <property name="name" type="java.lang.String">
??????????? <column name="name" length="500" />
??????? </property>
??????? <property name="description" type="java.lang.String">
??????????? <column name="description" length="500" />
??????? </property>
??????? <set name="products" inverse="true" batch-size="3">
??????????? <key>
??????????????? <column name="category_id" />
??????????? </key>
??????????? <one-to-many class="com.b510.examples.Product" />
??????? </set>
??? </class>
</hibernate-mapping>
?測試代碼：
HibernateTest.java
代碼：
/**
?*
?*/
package com.b510.examples;
import java.util.Iterator;
import java.util.List;
import org.hibernate.Session;
import org.hibernate.Transaction;
/**
?*
?* @author XHW
?*
?* @date 2011-7-18
?*
?*/
public class HibernateTest {
?public static void main(String[] args) {
??new HibernateTest().update();
?}
?public void update() {
??Session session = HibernateSessionFactoryUtil.getSessionFactory()
????.getCurrentSession();
??Transaction tx = session.beginTransaction();
??List<Category> categorys = session.createQuery("from Category").list();
??for(Category category:categorys){
???System.out.println(category.getProducts());
??}??
??tx.commit();
?}
}
運行效果：
log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment).
log4j:WARN Please initialize the log4j system properly.
Hibernate:
??? select
??????? category0_.id as id1_,
??????? category0_.name as name1_,
??????? category0_.description as descript3_1_
??? from
??????? users.category category0_
Hibernate:
??? select
??????? products0_.category_id as category2_1_,
??????? products0_.id as id1_,
??????? products0_.id as id0_0_,
??????? products0_.category_id as category2_0_0_,
??????? products0_.name as name0_0_,
??????? products0_.price as price0_0_,
??????? products0_.descripton as descripton0_0_
??? from
??????? users.product products0_
??? where
??????? products0_.category_id in (
??????????? ?, ?, ?
??????? )
[com.b510.examples.Product@d7b7d9, com.b510.examples.Product@93df2c, com.b510.examples.Product@3a1834, com.b510.examples.Product@190a0d6, com.b510.examples.Product@2f729e, com.b510.examples.Product@10deb5f, com.b510.examples.Product@b1cd0]
[com.b510.examples.Product@18488ef]
Hibernate:
??? select
??????? products0_.category_id as category2_1_,
??????? products0_.id as id1_,
??????? products0_.id as id0_0_,
??????? products0_.category_id as category2_0_0_,
??????? products0_.name as name0_0_,
??????? products0_.price as price0_0_,
??????? products0_.descripton as descripton0_0_
??? from
??????? users.product products0_
??? where
??????? products0_.category_id in (
??????????? ?, ?, ?
??????? )
[]
Hibernate:
??? select
??????? products0_.category_id as category2_1_,
??????? products0_.id as id1_,
??????? products0_.id as id0_0_,
??????? products0_.category_id as category2_0_0_,
??????? products0_.name as name0_0_,
??????? products0_.price as price0_0_,
??????? products0_.descripton as descripton0_0_
??? from
??????? users.product products0_
??? where
??????? products0_.category_id in (
??????????? ?, ?
??????? )
[]
[]
[]
[]
[]
batch-size="3"所以查詢的時候是一次查詢3條記錄。
總結
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