在學著編寫OA 許可權管理時，在jsp  頁面中通過已取得的 父模組來獲取該模組的所有子模組

JSP 程式碼片段：

     <c:forEach  items="${modules}" var="module">

   <c:forEach items="${module.children}" var="child">

 <td>    ${child.name }</td>

java 程式碼：

@OneToMany(mappedBy="parent")
    public Set<Module> getChildren() {
        return children;
    }

在這段程式碼中沒有對延遲載入進行設定，在JSP頁面顯示時報了failed to lazily initialize a collection of role: no session or session was closed (Hibernate) 的錯誤。

通過BAIdu 一下，發現我犯了延遲hibernate 延遲載入方面的錯誤。

修改程式碼後

    @OneToMany(mappedBy="parent", fetch = FetchType.EAGER)
    public Set<Module> getChildren() {
        return children;
    }

Hibernate: select module0_.id as id3_, module0_.name as name3_, module0_.orderNum as orderNum3_, module0_.parentId as parentId3_, module0_.sn as sn3_, module0_.url as url3_ from Module module0_ where module0_.parentId is null limit ?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?
Hibernate: select children0_.parentId as parentId1_, children0_.id as id1_, children0_.id as id3_0_, children0_.name as name3_0_, children0_.orderNum as orderNum3_0_, children0_.parentId as parentId3_0_, children0_.sn as sn3_0_, children0_.url as url3_0_ from Module children0_ where children0_.parentId=?

雖然這種方式在查詢模組時會進行大量的SQL操作，但在本次實踐中作為一個練習是可行的。

頁面顯示正常，能夠獲取到該模組的子模組。通過檢視hibernate打印出的sql 語句，可以看出hibernate在取出父模組的同時也將其關聯的子模組全部取出。

為了驗證  @OneToMany(mappedBy="parent", fetch = FetchType.LAZY)
    public Set<Module> getChildren() {
        return children;
    }

是否能夠取得理想效果，我修改了程式碼後，JSP 頁面仍然報同樣的錯誤，提示不能載入子模組。

雖然設定了延遲載入，如果是在java 程式碼中應該可以實現延遲載入功能。但是在這裡，使用jsp 頁面呼叫的方式，Action 將參賽傳遞到JSP頁面中後，session  已經關閉，而JSP頁面中的modules 是通過session 從Action中獲取的。在session 已經關閉時，希望通過modules 延遲載入（也就是重新發SQL 語句到資料庫中取到其子模組的實體類） 是不可能實現的。

檢視hibernate 列印的sql 語句證實了其只能查詢出該實體類本身的屬性，而不能查詢出與其關聯的屬性

Hibernate: select module0_.id as id3_, module0_.name as name3_, module0_.orderNum as orderNum3_, module0_.parentId as parentId3_, module0_.sn as sn3_, module0_.url as url3_ from Module module0_ where module0_.parentId is null limit ?

為了弄清楚延遲載入，又baidu了一把，總結下延遲載入：

A，實體物件的延遲載入： 通常在用  load（）  方法時使用。此時返回的是實體類的代理類。而在對該實體類進行操作時，hibernate才會去資料庫獲取真正的實體類物件

B,集合型別的延遲載入：  就是本文提到的父模組與子模組之間的關係，在這裡提醒一下，在實體類中定義子物件集時，必須使用set 集合。

C, 屬性延遲載入：  通常對某個實體類進行細分，設定該實體類的某些屬性為延遲載入，但需要對這些屬性進行操作時，才去讀取這些屬性在資料庫中的column 值