EL 通过 Integer 键访问映射值

我有一个由整数键控的映射。使用 EL，我如何通过它的键访问一个值？

Map<Integer, String> map = new HashMap<Integer, String>();
map.put(1, "One");
map.put(2, "Two");
map.put(3, "Three");

我原以为这会有用，但实际并非如此(map 已经在请求的属性中) :

<c:out value="${map[1]}"/>

跟进: 我找到了这个问题。显然，${name[1]}使用数字 Long进行地图查找。当我将 HashMap改为 TreeMap并接收到错误时，我明白了这一点:

java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.Long

如果我把地图改成:

Map<Long, String> map = new HashMap<Long, String>();
map.put(1L, "One");

然后 ${name[1]}返回“1”。这是怎么回事？为什么 <c:out>把一个数字当作一个长的。对我来说似乎有违直觉(因为 int 比 long 更常用)。

因此，我的新问题是，是否存在一个 EL 符号来通过 Integer值访问映射？

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最佳答案

Initial answer (EL 2.1, May 2009)

As mentioned in this java forum thread:

Basically autoboxing puts an Integer object into the Map. ie:

map.put(new Integer(0), "myValue")

EL (Expressions Languages) evaluates 0 as a Long and thus goes looking for a Long as the key in the map. ie it evaluates: 

map.get(new Long(0))

As a Long is never equal to an Integer object, it does not find the entry in the map.
That's it in a nutshell.

Update since May 2009 (EL 2.2)

Dec 2009 saw the introduction of EL 2.2 with JSP 2.2 / Java EE 6, with a few differences compared to EL 2.1.
It seems ("EL Expression parsing integer as long") that:

you can call the method intValue on the Long object self inside EL 2.2:

<c:out value="${map[(1).intValue()]}"/>

That could be a good workaround here (also mentioned below in Tobias Liefke's answer)

Original answer:

EL uses the following wrappers:

Terms                  Description               Type
null                   null value.               -
123                    int value.                java.lang.Long
123.00                 real value.               java.lang.Double
"string" ou 'string'   string.                   java.lang.String
true or false          boolean.                  java.lang.Boolean

JSP page demonstrating this:

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


<%@ page import="java.util.*" %>


<h2> Server Info</h2>
Server info = <%= application.getServerInfo() %> <br>
Servlet engine version = <%=  application.getMajorVersion() %>.<%= application.getMinorVersion() %><br>
Java version = <%= System.getProperty("java.vm.version") %><br>
<%
Map map = new LinkedHashMap();
map.put("2", "String(2)");
map.put(new Integer(2), "Integer(2)");
map.put(new Long(2), "Long(2)");
map.put(42, "AutoBoxedNumber");


pageContext.setAttribute("myMap", map);
Integer lifeInteger = new Integer(42);
Long lifeLong = new Long(42);
%>
<h3>Looking up map in JSTL - integer vs long </h3>


This page demonstrates how JSTL maps interact with different types used for keys in a map.
Specifically the issue relates to autoboxing by java using map.put(1, "MyValue") and attempting to display it as ${myMap[1]}
The map "myMap" consists of four entries with different keys: A String, an Integer, a Long and an entry put there by AutoBoxing Java 5 feature.


<table border="1">
<tr><th>Key</th><th>value</th><th>Key Class</th></tr>
<c:forEach var="entry" items="${myMap}" varStatus="status">
<tr>
<td>${entry.key}</td>
<td>${entry.value}</td>
<td>${entry.key.class}</td>
</tr>
</c:forEach>
</table>


<h4> Accessing the map</h4>
Evaluating: ${"${myMap['2']}"} = <c:out value="${myMap['2']}"/><br>
Evaluating: ${"${myMap[2]}"}   = <c:out value="${myMap[2]}"/><br>
Evaluating: ${"${myMap[42]}"}   = <c:out value="${myMap[42]}"/><br>


<p>
As you can see, the EL Expression for the literal number retrieves the value against the java.lang.Long entry in the map.
Attempting to access the entry created by autoboxing fails because a Long is never equal to an Integer
<p>


lifeInteger = <%= lifeInteger %><br/>
lifeLong = <%= lifeLong %><br/>
lifeInteger.equals(lifeLong) : <%= lifeInteger.equals(lifeLong) %> <br>
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Just another helpful hint in addition to the above comment would be when you have a string value contained in some variable such as a request parameter. In this case, passing this in will also result in JSTL keying the value of say "1" as a sting and as such no match being found in a Map hashmap.

One way to get around this is to do something like this.

<c:set var="longKey" value="${param.selectedIndex + 0}"/>

This will now be treated as a Long object and then has a chance to match an object when it is contained withing the map Map or whatever.

Then, continue as usual with something like 

${map[longKey]}
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Based on the above post i tried this and this worked fine I wanted to use the value of Map B as keys for Map A:

<c:if test="${not empty activityCodeMap and not empty activityDescMap}">
<c:forEach var="valueMap" items="${auditMap}">
<tr>
<td class="activity_white"><c:out value="${activityCodeMap[valueMap.value.activityCode]}"/></td>
<td class="activity_white"><c:out value="${activityDescMap[valueMap.value.activityDescCode]}"/></td>
<td class="activity_white">${valueMap.value.dateTime}</td>
</tr>
</c:forEach>
</c:if>
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If you just happen to have a Map with Integer keys you cannot change, you could write a custom EL function to convert a Long to Integer. This would allow you to do something like:

<c:out value="${map[myLib:longToInteger(1)]}"/>
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You can use all functions from Long, if you put the number into "(" ")". That way you can cast the long to an int:

<c:out value="${map[(1).intValue()]}"/>

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