JSP页面中的PropertyNotFoundException

Question

我有一个Java bean类如下

public class Users extends dbConnect
{
   private int UserId;  

   public int getUserId() 
   {
    return UserId;
   }

   public void setUserId(int userId) 
   {
     UserId = userId;
   }
}

我在我的Servlet中有一个doGet方法,它获取javabean类中的值并在JSP页面中显示.ListUsers方法将从数据库返回一个列表.

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException 
    {
        List<Users> arrUserList = new ArrayList<Users>();

        Users objUser = new Users();
        arrUserList   = objUser.listUsers();

        for(Users Userobj : arrUserList)
        {
            System.out.println(Userobj.getUserId());
        }

        request.setAttribute("arrUserDetails", arrUserList);

        RequestDispatcher rst = request.getRequestDispatcher("ListUsers.jsp");
        rst.forward(request, response);
    }

当我在for循环上面打印UserId的值时,显示UserId.But它没有显示在我的JSP页面中.

<%@ page  import="com.acme.users.Users"%>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>

<c:forEach items="${arrUserDetails}" var="account">
<tr>
      <td>
         <c:out value="${account.UserId}" />
      </td>
    </tr>
</c:forEach>

它显示javax.el.PropertyNotFoundException.我检查了堆栈的异常,并确保bean名称和getters方法是相同的.但它仍然显示错误.

控制台中的消息如下

Feb 16, 2013 2:44:19 PM org.apache.catalina.core.ApplicationDispatcher invoke
SEVERE: Servlet.service() for servlet jsp threw exception
javax.el.PropertyNotFoundException: Property 'UserId' not found on type com.acme.users.Users
    at javax.el.BeanELResolver$BeanProperties.get(BeanELResolver.java:223)
    at javax.el.BeanELResolver$BeanProperties.access$400(BeanELResolver.java:200)
    at javax.el.BeanELResolver.property(BeanELResolver.java:311)
    at javax.el.BeanELResolver.getValue(BeanELResolver.java:85)
    at javax.el.CompositeELResolver.getValue(CompositeELResolver.java:67)
    at org.apache.el.parser.AstValue.getValue(AstValue.java:169)
    at org.apache.el.ValueExpressionImpl.getValue(ValueExpressionImpl.java:189)
    at org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate(PageContextImpl.java:985)
    at org.apache.jsp.ListUsers_jsp._jspx_meth_c_005fout_005f0(ListUsers_jsp.java:170)
    at org.apache.jsp.ListUsers_jsp._jspx_meth_c_005fforEach_005f0(ListUsers_jsp.java:124)
    at org.apache.jsp.ListUsers_jsp._jspService(ListUsers_jsp.java:84)
    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:419)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:391)
    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:304)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:684)
    at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:471)
    at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:402)
    at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:329)
    at com.acme.users.ListUsers.doGet(ListUsers.java:36)
    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:304)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:240)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:164)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:462)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:164)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:562)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:395)
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:250)
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:188)
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:166)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:302)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)

为什么会这样？

Answer 1

您正在使用UserId哪个是字段的名称(顺便说一句,它违反了Java命名约定 - 就像dbConnect类名一样).

JSP想要使用属性 - getter和setter.您尝试调用它的方法是getUserId,但属性名称(在JavaBeans术语中)只是userId.因此,如果您将代码更改为:

 <c:out value="${account.userId}" />

它应该工作.您还应该更改您的类以使用名为just的字段userId,或者甚至id(假设它已经是用户 - 该部分是隐式的).

值得回顾JSP/JSTL/JavaBeans教程 - 您会注意到它们都使用以小写字母开头的属性名称,例如${foo.age}调用该getAge()方法.

从PropertyDescriptor构造函数的文档(它也出现在JavaBeans规范中):

通过使用getFoo和setFoo访问器方法为遵循标准Java约定的属性构造PropertyDescriptor.因此,如果参数名称是"fred",则它将假定writer方法是"setFred"并且reader方法是"getFred"(或者对于布尔属性是"isFred").请注意,属性名称应以小写字符开头,该字符将在方法名称中大写.

(强调我的.)

目前尚不清楚是否有任何方法名称可以让${account.UserId}工作 - 如果没有,那么我认为错误信息可能会更加清晰.(或者它可能只是更宽容一点.)

附加要点:

• 目前还不清楚为什么你的类Users在代表单个用户时被调用
• 目前尚不清楚它应该扩展一个与数据库连接有关的类 - 毕竟用户不是数据库连接.我怀疑你应该更努力地让每个班级只对一个概念负责.