Hibernate 中集合对象的抓取策略(Fetching strategies)

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Hibernate 中集合对象的抓取策略(Fetching strategies)

Product.java

代码；

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

代码：

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

代码：

<?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>
    <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

代码：

<?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>
    <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>

测试代码：

HibernateTest.java

代码：

/**
*
*/
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();
}

}

运行效果：

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应用程序设计
这里我们看到的是，我们在读取一个集合对象的时候，hibernate用了一条selelct语句；

1、查询抓取(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">

<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>

测试代码：

HibernateTest.java

代码：

/**
*
*/
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();
}

}

运行效果：

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应用程序设计
这种方法是查询抓取，他是hibernate默认值，所以我们会看到和上面的运行效果一摸一样。

测试代码二：

HibernateTest.java

代码；

/**
*
*/
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();
}
}

运行效果：

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应用程序设计

我们发现现在hibernate只是用了一个select语句。这是我们通过hql语句重载底层设置的抓取策略

hql的优先级很好。尽管你我们在配置文件中设置了fetch=“select”，就是说hibernate要用两个selelct语句

去执行这样的行为，但是，我们在hql语句中设置了急迫内连接查询，这样hibernate就必须只用一条select语句去查询

这样就等于把我们底层设置进行了重载，或者说是覆盖。

2、子查询抓取(Subselect fetching)

Category.hbm.xml

代码：

<?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>
    <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>

测试代码：

HibernateTest.java

代码：

/**
*
*/
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();
}

}

运行效果：

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应用程序设计
id:2 ,name:Hibernate, description:Hibernate好啊
java WEB开发与实战

我们看到，下面的selelct语句中又嵌套了一个select语句

3、连接抓取(Join fetching)

Category.hbm.xml

代码；

<?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>
    <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>

测试代码；

HibernateTest.java

代码：

/**
*
*/
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();
}
}

运行效果：

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应用程序设计

4、批量抓取(Batch fetching)

Category.xml

代码：

<?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>
    <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>

测试代码：

HibernateTest.java

代码：

/**
*
*/
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();
}

}

运行效果：

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"所以查询的时候是一次查询3条记录。

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原文地址：https://www.cnblogs.com/hongten/p/2111635.html