在JSP中无法正确读取JSTL

问题描述：

美好的一天！

我尝试在Java中使用JSTL，但出现错误:

I tried using JSTL in java but there's an error:

exception
javax.servlet.ServletException: java.lang.InstantiationException: class session.Item : java.lang.InstantiationException: session.Item

root cause
java.lang.InstantiationException: class session.Item : java.lang.InstantiationException: session.Item

我的代码如下:

<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
    "http://www.w3.org/TR/html4/loose.dtd">

<html>
    <head>
        <title>DISPLAY ITEM</title>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    </head>
    <body>

    <jsp:useBean id="invenItem" class="session.Item" scope="request"/>  
    <c:choose>
        <c:when test="${invenItem != null}">
             <h1>Inventory Item Details: JSP Tester</h1>
             Stock ID  : <jsp:getProperty name="invenItem" property="stockID" /><br/>
             Name      : <jsp:getProperty name="invenItem" property="itemName" /><br/>
             Unit Price: <jsp:getProperty name="invenItem" property="unitPrice" /><br/>
             On Stock  : <jsp:getProperty name="invenItem" property="onStock" />
            <h1> Inventory Item Details Tester: EL Method </h1>
            Stock ID  : ${invenItem.stockID} <br/>
            Name      : ${invenItem.itemName}<br/>
            Unit Price: ${invenItem.unitPrice}<br/>
            On Stock  : ${invenItem.onStock}
        </c:when>
        <c:otherwise>
            <%@ include file ="DataForm.html" %><br>
            Item not existing!<br>
            <%@ include file ="ItemEntry.html" %>
        </c:otherwise>
    </c:choose>

</body>
</html>

我已经阅读了 JLPT ，并按照那里的所有说明进行操作.请帮忙.谢谢.

I've read this JLPT already and followed all the instructions there. Please help. Thank you.

编辑...

我按照建议解决了该错误，但是JSTL(如果不是其他逻辑)仍然无法正常工作.请帮助！！！！

I resolved the error as suggested, but still the JSTL (if else logic) is not working. HELP PLEASE!!!!

答

如果invenItem作为请求属性已经存在，则无需使用<jsp:useBean>来检索它，您可以直接在<jsp:useBean>中使用它.您的JSP，因此只需完全删除<useBean>行.

If invenItem already exists as a request attribute, then you don't need to use <jsp:useBean> to retrieve it, you can just use it directly in your JSP, so just remove the <useBean> line completely.

如果invenItem 不存在作为请求属性，则<jsp:useBean>将为您创建一个，但是您在class属性中输入的内容必须完全合格具有公共默认构造函数的类的名称，否则您将看到所看到的异常.

If invenItem doesn't already exist as a request attribute, then <jsp:useBean> will create one for you, but whatever you put in the class attribute must be the fully-qualified name of a class with a public, default constructor, else you'll get the exception you saw.

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