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问题:

how to do this simple check in JSTL(without extending any java classes and additional JSP functions). I need it like this:

<c:if test="${fn:isNumeric()}">
 ... do something ...
</c:if>

Thanks in advance.

回答1:

If your environment supports the new EL 2.2 feature of invoking non-getter methods on EL objects (which is available in all Servlet 3.0 compatible containers, such as Tomcat 7, Glassfish 3, etc), then you could just use the String#matches() method directly in EL.

<c:set var="numberAsString">${someExpressionToTestForNumber}</c:set>
<c:if test="${numberAsString.matches('[0-9]+')}">
    It's a number!
</c:if>

(I'll leave the minus - and the thousands and fraction separators , and . outside consideration as possible characters which may appear in a technically valid number)

Note that the <c:set> with the expression in its body implicitly converts any type to String using String#valueOf(). Otherwise the matches() call in <c:if> would fail for non-String types.

回答2:

If you really insist on

(without extending any java classes and additional JSP functions)

then you could use the following hack.

<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>
<c:set var="x" value="8" />
<c:catch var="isNumber">
   <c:set var="x" value="${x * 1}" />
</c:catch>
<c:if test="${isNumber == null}">
    ... do something ...
</c:if>
<c:if test="${isNumber != null}">
      ... do not do anything...
</c:if>

回答3:

You can create a custom function as explained in the following tutorials:

Creating Custom functions in JSP using JSTL
How to create a custom Function for JSTL
Another tutorial

Steps to create a function from the above links:

Create a .tld file under /WEB-INF:

<?xml version="1.0" encoding="UTF-8"?>
<taglib version="2.1" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-jsptaglibrary_2_1.xsd">
     <tlib-version>1.0</tlib-version>
     <short-name>functionalTlds</short-name>
     <uri>http://www.rasabihari.com/functionalTlds</uri>
     <function>
         <name>isNumeric</name>
         <function-class>com.expressions.Functions</function-class>
         <function-signature>boolean isNumeric(java.lang.String)</function-signature>
     </function>
</taglib>

Create a class with the method (the logic of the method is taken from here, it uses Regular Expression)

package com.expressions;

/**
  *
  * @author rasabihari
  */
public class Functions {
    public static boolean isNumeric(String number) {  
        boolean isValid = false;  
        /*
        Explaination: 
        [-+]?: Can have an optional - or + sign at the beginning. 
        [0-9]*: Can have any numbers of digits between 0 and 9 
        \\.? : the digits may have an optional decimal point. 
        [0-9]+$: The string must have a digit at the end. 
        If you want to consider x. as a valid number change  
        the expression as follows. (but I treat this as an invalid number.). 
        String expression = "[-+]?[0-9]*\\.?[0-9\\.]+$"; 
        */  

        String expression = "[-+]?[0-9]*\\.?[0-9]+$";  
        CharSequence inputStr = number;  
        Pattern pattern = Pattern.compile(expression);  
        Matcher matcher = pattern.matcher(inputStr);  

        if(matcher.matches()){  
            isValid = true;  
        }

        return isValid;  
    }
}

And then use it in the JSP as:

<%@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">
<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@taglib uri="http://www.rasabihari.com/functionalTlds" prefix="ftld" %>
<html>
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>JSP Page</title>
</head>
<body>
    <c:if test="${ftld:isNumeric('123')}">
       <!-- ... do something ...
           This block will run
        -->
    </c:if>
</body>
</html>