HIbernate的“1+N”问题

HibernateTest.java

代码：

/**  *  */ package com.b510.examples;

import java.util.Iterator;

import org.hibernate.Query; 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();

  Query query = session.createQuery("from Product");   Iterator it=query.iterate();   while(it.hasNext()){    Product p=(Product)it.next();    System.out.println("id:"+p.getId()+"   name:"+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         product0_.id as col_0_0_     from         users.product product0_ Hibernate:     select         product0_.id as id0_0_,         product0_.category_id as category2_0_0_,         product0_.name as name0_0_,         product0_.price as price0_0_,         product0_.descripton as descripton0_0_     from         users.product product0_     where         product0_.id=? id:1   name:java SE应用程序设计 Hibernate:     select         product0_.id as id0_0_,         product0_.category_id as category2_0_0_,         product0_.name as name0_0_,         product0_.price as price0_0_,         product0_.descripton as descripton0_0_     from         users.product product0_     where         product0_.id=? id:3   name:java  web Hibernate Hibernate:     select         product0_.id as id0_0_,         product0_.category_id as category2_0_0_,         product0_.name as name0_0_,         product0_.price as price0_0_,         product0_.descripton as descripton0_0_     from         users.product product0_     where         product0_.id=? id:4   name:HIbernate Hibernate:     select         product0_.id as id0_0_,         product0_.category_id as category2_0_0_,         product0_.name as name0_0_,         product0_.price as price0_0_,         product0_.descripton as descripton0_0_     from         users.product product0_     where         product0_.id=? id:5   name:Hibernate2 Hibernate:     select         product0_.id as id0_0_,         product0_.category_id as category2_0_0_,         product0_.name as name0_0_,         product0_.price as price0_0_,         product0_.descripton as descripton0_0_     from         users.product product0_     where         product0_.id=? id:6   name:hibernate3 Hibernate:     select         product0_.id as id0_0_,         product0_.category_id as category2_0_0_,         product0_.name as name0_0_,         product0_.price as price0_0_,         product0_.descripton as descripton0_0_     from         users.product product0_     where         product0_.id=? id:7   name:hibernate4 Hibernate:     select         product0_.id as id0_0_,         product0_.category_id as category2_0_0_,         product0_.name as name0_0_,         product0_.price as price0_0_,         product0_.descripton as descripton0_0_     from         users.product product0_     where         product0_.id=? id:8   name:hibernate5 Hibernate:     select         product0_.id as id0_0_,         product0_.category_id as category2_0_0_,         product0_.name as name0_0_,         product0_.price as price0_0_,         product0_.descripton as descripton0_0_     from         users.product product0_     where         product0_.id=? id:2   name:java WEB开发与实战

我们明明要查询7条记录，但是hibernate用的是8条selelct语句

为什么会出现这种情况呢？

这个主要是hibernate考虑到了一级和二级缓存的效果， 如果缓存中有我们要查询的数据

那么这时我们就不用到数据库中去查询数据，而是直接从缓存中读取我们需要的数据。这就大大

提高了我们程序的运行效率。

但是一旦缓存中没有我们需要的数据，那么这时hibernate就会向数据库发出请求，所以我们看到上面的

运行效果会出现“1+N”的效果；（说明缓存中并没有我们要查询的 数据）

我们要避免这种问题，方法：

测试代码：

HibernateTest.java

代码：

/**  *  */ package com.b510.examples;

import java.util.List;

import org.hibernate.Query; 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();

  Query query = session.createQuery("from Product");  List<Product> list=query.list();   for(Product p:list){    System.out.println("id:"+p.getId()+"  name :"+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         product0_.id as id0_,         product0_.category_id as category2_0_,         product0_.name as name0_,         product0_.price as price0_,         product0_.descripton as descripton0_     from         users.product product0_ id:1  name :java SE应用程序设计 id:2  name :java WEB开发与实战 id:3  name :java  web Hibernate id:4  name :HIbernate id:5  name :Hibernate2 id:6  name :hibernate3 id:7  name :hibernate4 id:8  name :hibernate5

我们用List的时候，”1+N“的方法，就会解决，而List是只从数据库中读取数据，不会

到缓存中读取数据，但是list会把从数据库中读取的数据放入到缓存中，而Iterator则是先查询缓存

，如果缓存中有我们需要的数据，那么，这时就读取缓存。如果缓存中没有我们需要的数据，即：没有命中

这时再向数据库发出请求。所以会有”1+N“的运行效果。