Hibernate 发生太多查询

Question

我使用休眠（带 ehcache）/spring/mysql/jsf。我从 mysql.And 中的两个表中获取数据，这些表如下：
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Class

@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

我正在获取这个 daoimpl 的课程列表。

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;
}

此查询正在获取以下当前 user.My jsf 页面的课程：

<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，我在控制台中得到以下输出。

Hibernate: select lesson0_.id as id0_, lesson0_.lesson as lesson0_, lesson0_.teacher_id as teacher3_0_ from oys_lesson lesson0_ where lesson0_.teacher_id in (select user1_.id from oys_user user1_ left outer join oys_user_role user1_1_ on user1_.id=user1_1_.user_id where user1_.username=?)

当页面打开时，这个查询是运行ning 3 次。3 of the same query。但是当我选择 row.dialog window 打开并运行ning相同的查询更多 19 times.

managedBean.class

@ManagedBean
@SessionScoped
public class LessonManagedBean implements Serializable {
    private Lesson selectedLesson=null;
    List<Lesson> lessonList;
//Getters and Setters

为什么运行重复同一个查询？当对话框 window 打开时，不需要运行任何查询？为什么它有效 19 次？提前致谢。 .

Answer 1

查询数量取决于FetchMode, by default it is FetchMode.SELECT, so it will fire select statements, change it to FetchMode.SUBSELECT or FetchMode.JOIN并检查查询。

@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "user")
@Fetch(FetchMode.SUBSELECT)
private Set<Lesson> teacherLessons;

Answer 2

我解决了我的 problem.Hibernate 发生太多 queries.Because JSF 调用 getter 多次 times.So 我的 getList 方法多次工作。

这通常不被视为主要 problem.Because getter 方法是一种非常便宜的操作。

但是，如果你在 getter 方法中执行昂贵的业务逻辑（数据库操作）（像我一样），这将被重新执行 everytime.So，相同的查询（在 getter方法）一次又一次地工作。

最简单的方案：

 public List<Object> getPropList(){
    if(propList==null){
        propList=loadListFromDb();
    }
    return propList;
}

更多详情：

Why JSF calls getters multiple times?

Hibernate 发生太多查询

Hibernate occurs too many queries

spring

hibernate

nhibernate-mapping

hibernate-criteria