我使用hibernate(带ehcache)/spring/mysql/jsf。我从mysql中的两个表中获取数据。这些表格如下:
用户
id int(主键)
姓名等
课程
id int(主键)
课程varchar
teacher_id int(来自用户(id)的外键)
和课程模型类s:
@Entity
@Table(name = "oys_lesson")
@Cache(usage = CacheConcurrencyStrategy.READ_WRITE, region = "myregion")
public class Lesson {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@Column(name = "id", unique = true, nullable = false)
private Integer id;
@Column(name="lesson")
private String lessonName;
@ManyToOne(cascade=CascadeType.ALL,fetch=FetchType.LAZY)
@JoinColumn(name="teacher_id",nullable=false)
private User user;
//Getters and Setters
用户模型类
@Entity
@Table(name = "oys_user")
@Cache(usage = CacheConcurrencyStrategy.READ_WRITE, region = "myregion")
public class User implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@Column(name = "id", unique = true, nullable = false)
private Integer id;
private String username;
etc....
@OneToOne(cascade = CascadeType.REMOVE)
@JoinTable(name = "oys_user_role", joinColumns = { @JoinColumn(name = "user_id", referencedColumnName = "id") }, inverseJoinColumns = { @JoinColumn(name = "role_id", referencedColumnName = "id") })
private Role role;
@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "user")
private Set<Lesson> teacherLessons;
//Getters and Setters
我正在用这个daimpl获取课程表
public List<Lesson> getLessonList() {
// TODO Auto-generated method stub
String username = SecurityContextHolder.getContext()
.getAuthentication().getName();
Query query = openSession()
.createQuery(
"from Lesson l where l.user in (select id from User u where u.username=:username)");
query.setParameter("username", username);
// query.setCacheable(true);
List<Lesson> lessonList = query.list();
if (lessonList.size() > 0)
return lessonList;
return null;
}
此查询正在获取当前用户的课程。我的jsf页面如下:
<h:form id="form">
<p:dataTable var="lesson" value="#{lessonManagedBean.lessonList}"
paginator="true" rows="10" rowKey="#{lesson.id}"
paginatorTemplate="{RowsPerPageDropdown} {FirstPageLink} {PreviousPageLink} {CurrentPageReport} {NextPageLink} {LastPageLink}"
rowsPerPageTemplate="5,10,15" selectionMode="single"
selection="#{lessonManagedBean.selectedLesson}" id="carTable" lazy="true">
<p:ajax event="rowSelect" listener="#{lessonManagedBean.onRowSelect}"
update=":form:lessonDetail" oncomplete="PF('lessonDialog').show()" />
<p:column headerText="id" sortBy="#{lesson.id}"
filterBy="#{lesson.id}">
<h:outputText value="#{lesson.id}" />
</p:column>
<p:column headerText="Lesson Name" sortBy="#{lesson.lessonName}"
filterBy="#{lesson.lessonName}">
<h:outputText value="#{lesson.lessonName}" />
</p:column>
</p:dataTable>
<p:dialog header="Ders Detayları" widgetVar="lessonDialog" showEffect="fade"
hideEffect="fade" resizable="false">
<p:outputPanel id="lessonDetail" style="text-align:center;">
<p:panelGrid columns="2"
rendered="#{not empty lessonManagedBean.selectedLesson}"
columnClasses="label,value">
<h:outputText value="Id:" />
<h:outputText value="#{lessonManagedBean.selectedLesson.id}" />
<h:outputText value="Lesson Name" />
<h:outputText value="#{lessonManagedBean.selectedLesson.lessonName}" />
</p:panelGrid>
</p:outputPanel>
</p:dialog>
</h:form>
当我打开这个页面时,我在控制台中得到了以下输出
休眠:从oys_class lesson0中选择lesson0_id作为id0_,lesson0.session作为lesson0_,lesson0.tacher_id作为teacher_3_0__其中lesson0_teacher_id在(从oys_user user1中选择user1_id_左外加入user1_上的oys_user_role user1_1_。id=user1_user_id其中user1_username=?)
当页面打开时,此查询将运行3次。3 of the same query
。但当我选择row.dialog窗口打开并运行相同的查询时,更多的是19 times
managedBean.class
@ManagedBean
@SessionScoped
public class LessonManagedBean implements Serializable {
private Lesson selectedLesson=null;
List<Lesson> lessonList;
//Getters and Setters
为什么要重复运行同一个查询?当对话框窗口打开时,是否需要运行任何查询?为什么它能工作19次?提前谢谢。。
我解决了我的问题。Hibernate发生的查询太多。因为JSF多次调用getter。所以我的getList方法可以工作多次
这通常不被视为主要问题。因为getter方法是一种非常廉价的操作
但是,如果您在getter方法中执行昂贵的业务逻辑(db操作)(就像我一样),那么每次都会重新执行。因此,相同的查询(在getter方法中)一次又一次地工作
最简单的解决方案:
public List<Object> getPropList(){
if(propList==null){
propList=loadListFromDb();
}
return propList;
}
有关更多详细信息:
- JSF为什么多次调用getter
查询数取决于FetchMode,默认情况下为FetchMode.SELECT,因此它将激发SELECT语句,将其更改为FetchMode.SUBSELECT或FetchMode.JOIN并检查查询。
@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "user")
@Fetch(FetchMode.SUBSELECT)
private Set<Lesson> teacherLessons;