Hibernate的N+1条SQL查询问题-------Iterate

hibernate使用session.createQuery(hql)查询数据的时候，有两种查询方式：

1、一种是只查询一次，将所有要查询的数据都查询出来，后面直接取数据就可以了；

      获取方式：session.createQuery(hql).list()。

      一次性加载的数据多，效率低，用于例如商品信息展示。适用于展示所有信息。

2、另一种是先查询一次，将主键查询出来，后面需要数据的时候，根据主键一条一条的取数据。

       获取方式：session.createQuery(hql).iterate()。

      用于新闻列表展示。当需要查看新闻内容时，在去根据主键查询具体内容数据。

      适用于仅仅展示某一条或几条信息。

下面用一个例子来演示一下

新建一个java项目，项目结构如下：

实体类Book代码：

package com.myeclipse.pojo;import java.util.Date;public class Book {private int id;private String author;private String name;private double price;private Date pubDate;public int getId() {return id;}public void setId(int id) {this.id = id;}public String getAuthor() {return author;}public void setAuthor(String author) {this.author = author;}public String getName() {return name;}public void setName(String name) {this.name = name;}public double getPrice() {return price;}public void setPrice(double price) {this.price = price;}public Date getPubDate() {return pubDate;}public void setPubDate(Date pubDate) {this.pubDate = pubDate;}@Overridepublic String toString() {return "Book [id=" + id + ", author=" + author + ", name=" + name+ ", price=" + price + ", pubDate=" + pubDate + "]";}}

Book.hbm.xml代码：

<?xml version="1.0" encoding="utf-8"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN""http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"><hibernate-mapping package="com.myeclipse.pojo"><class name="Book" table="t_book" lazy="false"><id name="id"><generator class="identity" /></id><property name="author" /><property name="name" column="book_name" /><property name="price" /><property name="pubDate" /></class></hibernate-mapping>

HibernateUtil代码：

package com.robert.util;import org.hibernate.Session;import org.hibernate.SessionFactory;import org.hibernate.boot.registry.StandardServiceRegistryBuilder;import org.hibernate.cfg.Configuration;/** * hibernate工具类 */public class HibernateUtil {private static Configuration cfg = null;private static SessionFactory factory = null;private static Session session = null ;static {init();}/** * 初始化获得Configuration和SessionFacroty对象 */public static void init() {cfg = new Configuration().configure();factory = cfg.buildSessionFactory(new StandardServiceRegistryBuilder().applySettings(cfg.getProperties()).build());}/** * 获得Session对象 * @return */public static Session getSession() {if (factory != null){return session = factory.openSession();}init();return session = factory.openSession();}/** * 关闭Session */public static void closeSession() {if(session!=null && session.isOpen())session.close();}}

hibernate.cfg.xml代码：

<!DOCTYPE hibernate-configuration PUBLIC"-//Hibernate/Hibernate Configuration DTD 3.0//EN""http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd"><hibernate-configuration><session-factory><!-- 配置数据库连接信息 --><property name="connection.driver_class">com.mysql.jdbc.Driver</property><property name="connection.url">jdbc:mysql:///hibernate4</property><property name="connection.username">root</property><property name="connection.password">root</property><!-- 数据库方言 --><property name="hibernate.dialect">org.hibernate.dialect.MySQL5Dialect</property><!-- 是否打印sql语句 --><property name="show_sql">true</property><!-- 格式化sql语句 --><property name="format_sql">true</property><!-- 数据库更新方式： 1、create：每次更新都先把原有数据库表删除，然后创建该表；2、create-drop:使用create-drop时，在显示关闭SessionFacroty时（sessionFactory.close()），将drop掉数据库Schema（表） 3、validate：检测；4、update（常用）：如果表不存在则创建，如果存在就不创建--><property name="hbm2ddl.auto">update</property><!-- hbm映射文件 --><mapping resource="com/myeclipse/pojo/Book.hbm.xml" /></session-factory></hibernate-configuration>

HibernateTest测试类代码：

package com.ghibernate.test;import java.util.Date;import java.util.Iterator;import java.util.List;import org.hibernate.Query;import org.hibernate.Session;import org.hibernate.Transaction;import org.hibernate.cfg.Configuration;import org.hibernate.tool.hbm2ddl.SchemaExport;import org.junit.Test;import com.myeclipse.pojo.Book;import com.robert.util.HibernateUtil;public class HibernateTest {@Testpublic void testCreateDB() {Configuration cfg = new Configuration().configure();SchemaExport se = new SchemaExport(cfg);// 第一个参数：是否生成ddl脚本// 第二个参数：是否执行到数据库中se.create(true, true);}@Testpublic void testSave() {Session session = HibernateUtil.getSession();Transaction tx = session.beginTransaction();Book book = new Book();book.setName("读者");book.setPrice(5.6);book.setAuthor("众人");book.setPubDate(new Date());Book book1 = new Book();book1.setName("傲慢与偏见");book1.setPrice(80.0);book1.setAuthor("简.奥斯汀");book1.setPubDate(new Date());Book book2 = new Book();book2.setName("中国历史");book2.setPrice(30.0);book2.setAuthor("人民出版社");book2.setPubDate(new Date());Book book3 = new Book();book3.setName("翩眇之旅");book3.setPrice(70.0);book3.setAuthor("萧鼎");book3.setPubDate(new Date());Book book4 = new Book();book4.setName("蓝血人");book4.setPrice(60.0);book4.setAuthor("卫斯理");book4.setPubDate(new Date());Book book5 = new Book();book5.setName("我的大学");book5.setPrice(60.5);book5.setAuthor("高尔基");book5.setPubDate(new Date());session.save(book);session.save(book1);session.save(book2);session.save(book3);session.save(book4);session.save(book5);tx.commit();HibernateUtil.closeSession();}@Testpublic void testIterator() {Session session = HibernateUtil.getSession();Transaction tx = session.beginTransaction();List<Book> list = session.createQuery("from Book").list() ;for(Book book : list) {System.out.println("书名："+book.getName());}tx.commit();HibernateUtil.closeSession();System.out.println("-----------------上面是list方式-------------------------");System.out.println("-----------------下面是iterator方式---------------------");session = HibernateUtil.getSession();tx = session.beginTransaction();Iterator<Book> iter = session.createQuery("from Book").iterate() ;for (; iter.hasNext();) {Book book = (Book) iter.next();System.out.println("iterator遍历的书名："+book.getName());}tx.commit();HibernateUtil.closeSession();}}

先运行testCreateDB()，重新生成数据库表；

执行testSave(),保存数据；

执行testIterator()方法，生成的sql语句如下：

Hibernate:     select        book0_.id as id1_0_,        book0_.author as author2_0_,        book0_.book_name as book_nam3_0_,        book0_.price as price4_0_,        book0_.pubDate as pubDate5_0_     from        t_book book0_书名：读者书名：傲慢与偏见书名：中国历史书名：翩眇之旅书名：蓝血人书名：我的大学-----------------上面是list方式------------------------------------------下面是iterator方式---------------------Hibernate:     select        book0_.id as col_0_0_     from        t_book book0_Hibernate:     select        book0_.id as id1_0_0_,        book0_.author as author2_0_0_,        book0_.book_name as book_nam3_0_0_,        book0_.price as price4_0_0_,        book0_.pubDate as pubDate5_0_0_     from        t_book book0_     where        book0_.id=?iterator遍历的书名：读者Hibernate:     select        book0_.id as id1_0_0_,        book0_.author as author2_0_0_,        book0_.book_name as book_nam3_0_0_,        book0_.price as price4_0_0_,        book0_.pubDate as pubDate5_0_0_     from        t_book book0_     where        book0_.id=?iterator遍历的书名：傲慢与偏见Hibernate:     select        book0_.id as id1_0_0_,        book0_.author as author2_0_0_,        book0_.book_name as book_nam3_0_0_,        book0_.price as price4_0_0_,        book0_.pubDate as pubDate5_0_0_     from        t_book book0_     where        book0_.id=?iterator遍历的书名：中国历史Hibernate:     select        book0_.id as id1_0_0_,        book0_.author as author2_0_0_,        book0_.book_name as book_nam3_0_0_,        book0_.price as price4_0_0_,        book0_.pubDate as pubDate5_0_0_     from        t_book book0_     where        book0_.id=?iterator遍历的书名：翩眇之旅Hibernate:     select        book0_.id as id1_0_0_,        book0_.author as author2_0_0_,        book0_.book_name as book_nam3_0_0_,        book0_.price as price4_0_0_,        book0_.pubDate as pubDate5_0_0_     from        t_book book0_     where        book0_.id=?iterator遍历的书名：蓝血人Hibernate:     select        book0_.id as id1_0_0_,        book0_.author as author2_0_0_,        book0_.book_name as book_nam3_0_0_,        book0_.price as price4_0_0_,        book0_.pubDate as pubDate5_0_0_     from        t_book book0_     where        book0_.id=?iterator遍历的书名：我的大学