Java 使用<；c:jsp中的url

在我的web应用程序中，我有一个具有

public class Item {
    private Long item_id;
    private String name;
...
}

...
<servlet-mapping>
  <servlet-name>edititemservlet</servlet-name>
  <url-pattern>/edititem</url-pattern>
 </servlet-mapping>
...

我试着打印 ${editItemurl}并显示一个空字符串

你知道我该怎么纠正吗

谢谢

标记

p、 s: 这是堆栈跟踪

java.lang.NumberFormatException: For input string: ""
    at java.lang.NumberFormatException.forInputString(NumberFormatException.java:48)
    at java.lang.Long.parseLong(Long.java:424)
    at java.lang.Long.parseLong(Long.java:461)
    at myshop.servlets.ItemEditServlet.doGet(Unknown Source)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:470)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:857)
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
    at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
    at java.lang.Thread.run(Thread.java:619)

必须进入

内部才能访问

${item}

---

但是，我无法解释为什么会抛出NFE。为此，我需要知道整个stacktrace。

我不确定异常的原因。你还有更多的stacktrace吗？还请提及您正在使用的JSTL版本。不管怎样，你实际上应该把

放在

里面，这样才能访问

${item}

@BalusC。很抱歉这个愚蠢的错误..我应该自己解决的..非常感谢它能解决问题吗？我将转载这篇评论作为回答。然而，我仍然想知道为什么会抛出NFE。我不希望它被抛出到这个构造中。你能包含整个stacktrace吗？抱歉..我昨晚不在..我想这是因为传递了item_id的空字符串，导致了长.parseLong（）的NFE…我看到你在问题中包含了stacktrace。stacktrace只指示您自己的servlet代码中的bug，而不是

实现中的bug。此异常仅在提交表单时发生，而不是在显示表单时发生。您的servlet盲目地期望一个数字作为请求参数，而没有首先验证它。因此，在

实现中根本没有问题。

<c:url value="editItem" var="editItemurl">
    <c:param name="itemId" value="${item.item_id}" />
</c:url>
<c:forEach var="item" items="${items}" varStatus="itemslistCount">
<tr><td>${itemslistCount.count} </td>
<td><a href="${editItemurl}" >${item.name}</a></td>

java.lang.NumberFormatException: For input string: ""
    java.lang.NumberFormatException.forInputString(NumberFormatException.java:48)
    java.lang.Long.parseLong(Long.java:424)

java.lang.NumberFormatException: For input string: ""
    at java.lang.NumberFormatException.forInputString(NumberFormatException.java:48)
    at java.lang.Long.parseLong(Long.java:424)
    at java.lang.Long.parseLong(Long.java:461)
    at myshop.servlets.ItemEditServlet.doGet(Unknown Source)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:470)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:857)
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
    at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
    at java.lang.Thread.run(Thread.java:619)