JSTL-地图中key值为数字的处理方式

JSTL-map中key值为数字的处理方式

最好显示转化成Long型：

boardMap.put((Long)b.getId(), bclist);

如果写成boardMap.put(b.getId()+"", bclist);

页面上写成

<c:set var="boardKey" value="${board.id}"/>

 <c:forEach items="${boardMap[boardKey]}" var="boardItem">
   						<img id="img${boardItem.id}" src="<c:url value='images/node.gif'/>" alt="" align="absmiddle"/><a href="<c:out value='/forum/${board.id}'/>">${boardItem.boardName}</a>
   				</c:forEach>

如果boardKey等于1

循环中是不会有输出的，因为JSTL会将作为1处理，而不是"1".

参考http://stackoverflow.com/questions/924451/jstl-access-a-map-value-by-key

    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. Thats it in a nutshell.

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>

JSTL-地图中key值为数字的处理方式

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