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jsp标记文件：如何访问不同类型的session属性集合？

[英]jsp tag file: how to access different types of collections of a session attribute?

 原文  2012-11-29 16:44:21  1   1    java/ jsp/ java-ee/ servlets/ jsp-tags

问题描述

I have a jsp to which a servlet passes an object (a collection). 我有一个jsp，servlet将一个对象传递给它（一个集合）。 the object becomes a session attribute: 该对象成为会话属性： 

session.setAttribute("groupList", request.getAttribute("groups"));

I use a tag to recursively generate a menu from the content of this collection: 我使用标签根据此集合的内容递归生成菜单： 

<c:forEach items="${groupList}" var="emp" begin="0" end="10">
        <tag:menuRenderer groupList="${groupList}" />
    </c:forEach>

Those items in the are objects of a class Group . 中的那些项目是Group类的对象。 This class contains a list of subgroups subGroups and a list of objects of a different class called students . 这个类包含分组列表subGroups和不同的类中调用的对象的列表students 。

My problem is that I can not access this list of students in my tag. 我的问题是我无法访问标签中的此学生列表。 This is the tag code: 这是标记代码： 

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<%@ taglib tagdir="/WEB-INF/tags" prefix="tag"%>
<%@ taglib uri='http://java.sun.com/jsp/jstl/functions' prefix='fn' %>
<%@ attribute name="groupList" type="java.util.Collection"%>

<c:forEach var="item" items="${groupList}">

    <li><c:out value="${item.name}" /></li>
    <br>

    <!-- ##### here #### -->
    <li><c:out value="${fn:length(item.students)}" />   <!-- #### doesn't work ####-->
    <li><c:out value="${fn:length(item.subGroups)}" />  <!-- ####     works    #### -->

    <ul>
        <tag:menuRenderer groupList="${item.subGroups}" />
    </ul>
</c:forEach>

The call to item.students causes an exception, but I can't see what exactly it is because the strack trace only shows where the exception is thrown in the jsp: 对item.students的调用会导致异常，但是我无法确切知道它是什么，因为strack跟踪仅显示在jsp中引发异常的位置： 

SEVERE: Servlet.service() for servlet jsp threw exception
org.hibernate.LazyInitializationException: could not initialize proxy - no Session
    at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:186)
    at org.hibernate.collection.internal.AbstractPersistentCollection.readSize(AbstractPersistentCollection.java:137)
    at org.hibernate.collection.internal.PersistentSet.size(PersistentSet.java:156)
    at org.apache.taglibs.standard.functions.Functions.length(Functions.java:202)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.apache.el.parser.AstFunction.getValue(AstFunction.java:112)
    at org.apache.el.ValueExpressionImpl.getValue(ValueExpressionImpl.java:185)
    at org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate(PageContextImpl.java:986)
    at org.apache.jsp.tag.webmenuRenderer_tag._jspx_meth_c_005fout_005f1(webmenuRenderer_tag.java:202)
    at org.apache.jsp.tag.webmenuRenderer_tag._jspx_meth_c_005fforEach_005f0(webmenuRenderer_tag.java:143)
    at org.apache.jsp.tag.webmenuRenderer_tag.doTag(webmenuRenderer_tag.java:97)
    at org.apache.jsp.tag.webmenuRenderer_tag._jspx_meth_tag_005fmenuRenderer_005f0(webmenuRenderer_tag.java:240)
    at org.apache.jsp.tag.webmenuRenderer_tag._jspx_meth_c_005fforEach_005f0(webmenuRenderer_tag.java:153)
    at org.apache.jsp.tag.webmenuRenderer_tag.doTag(webmenuRenderer_tag.java:97)
    at org.apache.jsp.testMenu2_jsp._jspx_meth_tag_005fmenuRenderer_005f0(testMenu2_jsp.java:173)
    at org.apache.jsp.testMenu2_jsp._jspx_meth_c_005fforEach_005f0(testMenu2_jsp.java:138)
    at org.apache.jsp.testMenu2_jsp._jspService(testMenu2_jsp.java:90)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:432)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:749)
    at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:487)
    at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:412)
    at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:339)
    at myPackage.controller.ControllerServlet.doGet(ControllerServlet.java:38)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:168)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:99)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:929)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:407)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1002)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:585)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:310)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

Nov 29, 2012 5:50:36 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [Hello] in context with path [/HibernateTest] threw exception [An exception occurred processing JSP page /testMenu2.jsp at line 19

16: 
17: <ul>
18:     <c:forEach items="${treeMap}" var="emp" begin="0" end="10">
19:         <tag:menuRenderer treeMap="${treeMap}" />
20:     </c:forEach>
21: </ul>
22: 


Stacktrace:] with root cause
org.hibernate.LazyInitializationException: could not initialize proxy - no Session
    at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:186)
    at org.hibernate.collection.internal.AbstractPersistentCollection.readSize(AbstractPersistentCollection.java:137)
    at org.hibernate.collection.internal.PersistentSet.size(PersistentSet.java:156)
    at org.apache.taglibs.standard.functions.Functions.length(Functions.java:202)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.apache.el.parser.AstFunction.getValue(AstFunction.java:112)
    at org.apache.el.ValueExpressionImpl.getValue(ValueExpressionImpl.java:185)
    at org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate(PageContextImpl.java:986)
    at org.apache.jsp.tag.webmenuRenderer_tag._jspx_meth_c_005fout_005f1(webmenuRenderer_tag.java:202)
    at org.apache.jsp.tag.webmenuRenderer_tag._jspx_meth_c_005fforEach_005f0(webmenuRenderer_tag.java:143)
    at org.apache.jsp.tag.webmenuRenderer_tag.doTag(webmenuRenderer_tag.java:97)
    at org.apache.jsp.tag.webmenuRenderer_tag._jspx_meth_tag_005fmenuRenderer_005f0(webmenuRenderer_tag.java:240)
    at org.apache.jsp.tag.webmenuRenderer_tag._jspx_meth_c_005fforEach_005f0(webmenuRenderer_tag.java:153)
    at org.apache.jsp.tag.webmenuRenderer_tag.doTag(webmenuRenderer_tag.java:97)
    at org.apache.jsp.testMenu2_jsp._jspx_meth_tag_005fmenuRenderer_005f0(testMenu2_jsp.java:173)
    at org.apache.jsp.testMenu2_jsp._jspx_meth_c_005fforEach_005f0(testMenu2_jsp.java:138)
    at org.apache.jsp.testMenu2_jsp._jspService(testMenu2_jsp.java:90)
    // and so on

I'd be very happy if somebody could tell me why this doesn't work! 如果有人可以告诉我为什么这行不通，我将非常高兴！ Do I have to import something? 我必须导入什么吗？

Thanks in advance! 提前致谢！ :) :)

1 个解决方案

解决方案1
 0 已采纳  2012-11-29 17:34:24

The students collection probably comes from a lazy one-to-many relationship and you're trying to access it in the web tier in which no Hibernate session is active. students集合可能来自懒惰的一对多关系，您正试图在没有Hibernate会话处于活动状态的Web层中访问它。 You should declare the relationship as EAGER in order to have the collection loaded at query time. 您应将关系声明为EAGER以便在查询时加载集合。

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