问题描述
只有用户，和留言两个表，POJO如下： public class Article {private Integer id;private String title;private String content;private User user; //省略setter,getter} public class User {private Integer id;private String username;private String password;private Set<Article> articles = new HashSet<Article>();} Article 的XML映射文件 <hibernate-mapping package="com.minibbs.domain"><class name="Article" table="T_ARTICLE"><cache usage="read-only"/><id column="article_id" name="id"><generator class="sequence" /></id><property name="title" column="title" type="string" length="20" not-null="true"/><property name="content" column="content" type="text" /><many-to-one name="user" column="user_id" class="User" not-null="true" lazy="false" /></class></hibernate-mapping> 进入留言列表，读取所有留言需要的信息有Article的title和content，以及User的username <s:iterator value="#request.articles" id="a" status="s"> <tr> <td><s:property value="#s.index"/></td> <td class="title"><s:property value="#a.title"/></td> <td class="content"><s:property value="#a.content"/> </td> <td class="username"><s:property value="#a.user.username"/></td></tr></s:iterator> 查询语句 public List<Article> findAll() {return getHibernateTemplate().find("from Article");} 此时，进入文章列表会产生多条SQL语句 Hibernate: /* from Article */ select article0_.article_id as article1_0_, article0_.title as title0_, article0_.content as content0_, article0_.user_id as user4_0_ from T_ARTICLE article0_Hibernate: /* load com.minibbs.domain.User */ select user0_.user_id as user1_1_0_, user0_.username as username1_0_, user0_.password as password1_0_ from T_USER user0_ where user0_.user_id=?Hibernate: /* load com.minibbs.domain.User */ select user0_.user_id as user1_1_0_, user0_.username as username1_0_, user0_.password as password1_0_ from T_USER user0_ where user0_.user_id=?Hibernate: /* load com.minibbs.domain.User */ select user0_.user_id as user1_1_0_, user0_.username as username1_0_, user0_.password as password1_0_ from T_USER user0_ where user0_.user_id=?Hibernate: /* load com.minibbs.domain.User */ select user0_.user_id as user1_1_0_, user0_.username as username1_0_, user0_.password as password1_0_ from T_USER user0_ where user0_.user_id=? 从User表中读取用户对象的SQL语句条数和发帖人的总个数有关一样。 请问这样是正常的么？我感觉用户的信息可以在一条SQL语句中同时可以拿出来？有必要么？
解决方案
正常的，这是因为你用的配置的fetch是select就是关联的时候，hibernate可以配置你可以N+1次查询获取关联对象，或者是直接用join一次连接查询出关联对象。默认fetch=select<many-to-one name="user" column="user_id" class="User" not-null="true" lazy="false" fetch="select"/> 你可以改成join 就不会N+1次查询了<many-to-one name="user" column="user_id" class="User" not-null="true" lazy="false" fetch="join"/>