java.lang.NumberFormatException：对于输入字符串：“id”

Question

I am using SPRING MVC to develop a project to display a list of users in a JSP file. My Controller file has:

Map<String, Object> model = new HashMap<String, Object>();
model.put("user", userService.getUser()); //userService.getUser() returns a List

The JSP file has:

<c:if test="${!empty user}">  
  <table>
    <tr>  
      <td>User Id</td>  
      <td>First Name</td>  
      <td>Last Name</td>  
      <td>Gender</td>  
      <td>City</td>  
   </tr>  
   <c:forEach items="${user}" var="user">
   <tr>  
     <td><c:out value="${user.id}"/></td>  
     <td><c:out value="${user.firstName}"/></td>  
     <td><c:out value="${user.lastName}"/></td>  
     <td><c:out value="${user.gender}"/></td>  
     <td><c:out value="${user.city}"/></td>  
   </tr>  
   </c:forEach>  
  </table>  
</c:if>

When displaying the above JSP file, java.lang.NumberFormatException: For input string: "id" is shown. Can anybody please help to find out the solution?

Thank you very much.

Answer 1

NumberFormatException happens during an failed attempt to convert a string to a number. Check if any of your input data that should be a number is actually receiving something different (ex: an empty string, which can't be converted to a number, is a possible cause).

Answer 2

First good rule: variables containing lists you schould naming plural for example, list of users put name "users":

<c:forEach items="${users}" var="user">

Now you know what variable contain, "users" have list of users but "user" is one item of users list.

Second rule: if you want check, whether a variable is a number, do not catch exceptions, it's slow. Better and faster is regex:

if (anyString.matches("\\d+")){
    isANumber();
}
else{
    notNumber();
}

Answer 3

In addition to heldrarocha and Damien's answers, I think your problem is that you are not checking ID before adding it into the User object.

Ensure that the id variable in User is an integer (or long or whatever). This way if you fill it from the DB, your rowmapper will add numeric values.

If it HAS to be a String, ensure that whatever goes in is numeric.

An easy way to check if a String is numeric is to user Apache Commons StringUtils isNumeric method:

http://commons.apache.org/proper/commons-lang/javadocs/api-2.6/org/apache/commons/lang/StringUtils.html#isNumeric(java.lang.String)

Answer 4

I'm not sure your problem is, but I remember to put below code at the beginning of the jsp page

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"pageEncoding="ISO-8859-1"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>

Answer 5

Since you made sure that userService.getUser() returns a list that can be iterated over using forEach in jsp, the only change i would think of is

model.put("users", userService.getUser())

and in jsp file as follows:

<c:forEach items="${users}" var="user">

It may eliminate ambiguity and may actually solve your problem.

I came accross same problem today and later realised that I was iterating over the object itself when I intend to iterate over a list of such objects.