n+1问题：在默认情况下，使用query.iterator()查询，有可能有n+1问题，所谓n+1是指在查询对象的时候发出n+1条查询语句。
1：先发出查询id列表的sql语句。
N：再发出根据id到缓存中查询，如果缓存中有与之匹配的数据，就从缓存中取得数据，否则依次根据id发出sql语句。
list和iterator到区别：
list：在默认情况下，list每次都会发出sql查询实体对象，list会向缓存里放数据，但是不会利用缓存中的数据。
iterator：首先发出一条查询id列表的sql语句，如果缓存中有与之匹配的数据，就从缓存中取得数据，否则依次根据id发出sql语句。
import java.util.Iterator;import java.util.List;import junit.framework.TestCase;import org.hibernate.Session;import org.hibernate.Transaction;//...public class QueryTest extends TestCase { public void testQuery1() { Session session = null; Transaction t = null; try { session = HibernateUtils.getSession(); t = session.beginTransaction(); /** * 采用list查询，将发出一条sql语句来获取student数据 * Hibernate: select student0_.id as id1_, student0_.name as name1_ from Student student0_ 张三 李四 */ List<Student> list = session.createQuery("from Student").list(); for (Iterator<Student> iter = list.iterator(); iter.hasNext();) { Student student = iter.next(); System.out.println(student.getName()); } t.commit(); } catch (Exception e) { e.printStackTrace(); t.rollback(); } finally { HibernateUtils.closeSession(session); } } public void testQuery2() { Session session = null; Transaction t = null; try { session = HibernateUtils.getSession(); t = session.beginTransaction(); /** * N+1问题： * 使用iterator，先发出查询id列表的sql语句， * Hibernate: select student0_.id as col_0_0_ from Student student0_ * 再发出根据id查询实体对象的sql * Hibernate: select student0_.id as id1_0_, student0_.name as name1_0_ from Student student0_ where student0_.id=? 张三 Hibernate: select student0_.id as id1_0_, student0_.name as name1_0_ from Student student0_ where student0_.id=? 李四 × */ Iterator<Student> it = session.createQuery("from Student").iterate(); while(it.hasNext()){ Student student = it.next(); System.out.println(student.getName()); } t.commit(); } catch (Exception e) { e.printStackTrace(); t.rollback(); } finally { HibernateUtils.closeSession(session); } } public void testQuery3() { Session session = null; Transaction t = null; try { session = HibernateUtils.getSession(); t = session.beginTransaction(); List<Student> list = session.createQuery("from Student").list(); for (Iterator<Student> iter = list.iterator(); iter.hasNext();) { Student student = iter.next(); System.out.println(student.getName()); } System.out.println("======================="); /** * 不会出现n+1问题 * 因为list操作已经将对象加入到一级缓存，所以在使用iterator的时候, * 他首先发出查询id列表的sql，再根据id到缓存中获取数据， * 只有在缓存中找不到，才再次发出sql语句Hibernate: select student0_.id as id1_, student0_.name as name1_ from Student student0_张三李四=======================Hibernate: select student0_.id as col_0_0_ from Student student0_张三李四 */ Iterator<Student> it = session.createQuery("from Student").iterate(); while(it.hasNext()){ Student student = it.next(); System.out.println(student.getName()); } t.commit(); } catch (Exception e) { e.printStackTrace(); t.rollback(); } finally { HibernateUtils.closeSession(session); } }}
