Hibernate HQL示例二:list与iterate的区别
示例演示查询list与iterate的区别
1、实体对象的查询，查询的是实体对象的数据【重要】 * n+1问题，在默认配置的情况下，使用query.iterate()操作，有可能有n＋1问题，所谓 n+1，指在查询对象数据的时候，发出了n＋1条查询语句。 1：首先发出了一条查询语句，查询对象的id列表 n:在迭代访问每个对象的时候，如果缓存中没有对象数据，Hibernate会在此发出一条查询语句， 查询相应的对象 *List操作与Iterate操作的区别 list,每次都会发出一条查询语句，查询所有的对象 iterate,首先发出一条查询语句，查询对象的id列表，然后根据缓存情况，决定 是否发出更多的查询语句，来查询对象数据
?查看下面的代码:
package com.bjsxt.hibernate;import java.util.Iterator;import java.util.List;import org.hibernate.Query;import org.hibernate.Session;import junit.framework.TestCase;/** * 对象查询中的list操作和iterator操作的差异 * @author Administrator * */public class SimpleObjectQueryTest2 extends TestCase {public void testQueryWithListMethod() {Session session = null;try {session = HibernateUtils.getSession();/** * 将发出一条查询语句，获取Student的集合数据 * select student0_.id as id1_, student0_.name as name1_, * student0_.createTime as createTime1_, student0_.classid as classid1_ * from t_student student0_ */List students = session.createQuery("from Student").list();for (Iterator iter = students.iterator();iter.hasNext();) {Student student = (Student)iter.next();System.out.println(student.getName());}}catch(Exception e) {e.printStackTrace();}finally {HibernateUtils.closeSession(session);}}public void testQueryWithIterateMethod() {Session session = null;try {session = HibernateUtils.getSession();//先发出查询id的列表语句//select student0_.id as col_0_0_ from t_student student0_//再依次发出查询对象的sql（根据id）//select student0_.id as id1_0_, student0_.name as name1_0_, //student0_.createTime as createTime1_0_, student0_.classid as classid1_0_ //from t_student student0_ where student0_.id=?Query query = session.createQuery("from Student");Iterator students = query.iterate();while (students.hasNext()) {Student student = (Student)students.next();System.out.println(student.getName());}}catch(Exception e) {e.printStackTrace();}finally {HibernateUtils.closeSession(session);}}public void testQueryWithListAndIterate() {Session session = null;try {session = HibernateUtils.getSession();Query query = session.createQuery("from Student");List students = query.list();for (Iterator iter = students.iterator();iter.hasNext();) {Student student = (Student)iter.next();System.out.println(student.getName());}//如果使用iterate进行查询//因为list操作已经将对象加载到了session的一级缓存，所以//再使用iterate操作的时候，它先会发出查询id列表的查询语句//再根据id到缓存中获取相关的数据//只有再缓存中找不到相关数据的情况下，才会再次发出sql进行查询Iterator studentsIter = query.iterate();while (studentsIter.hasNext()) {Student student = (Student)studentsIter.next();System.out.println(student.getName());}}catch(Exception e) {e.printStackTrace();}finally {HibernateUtils.closeSession(session);}}public void testQueryWithListAndList() {Session session = null;try {session = HibernateUtils.getSession();Query query = session.createQuery("from Student");List students = query.list();for (Iterator iter = students.iterator();iter.hasNext();) {Student student = (Student)iter.next();System.out.println(student.getName());}//再次发出发出sql//在默认情况下，list每次都会向数据库发出查询对象数据的sql，//除非配置了查询缓存，所以下面的list()操作，虽然在session已经有了//对象缓存数据，但list()并不理会这个中缓存，而再次发出查询语句进行查询students = query.list();for (Iterator iter = students.iterator();iter.hasNext();) {Student student = (Student)iter.next();System.out.println(student.getName());}}catch(Exception e) {e.printStackTrace();}finally {HibernateUtils.closeSession(session);}}}?
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详细查看注释部分代码.
以后对查询的性能优化..