blocks|key|343779|text|初步答复(EL+2.1,2009年5月)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|343780|正如在this+java+forum+thread中提到的|offset|length|343781|343782|基本上是自动装箱将一个整数对象放入映射中。即：|blockquote|343783|343784|343785|map.put(new+Integer(0),+"myValue")|code-block|syntax|javascript|343786|EL+(表达式语言)将0计算为长整型，因此在映射中查找长整型作为键。例如，它会评估：|343787|map.get(new+Long(0))|343788|由于Long永远不等于Integer对象，因此它不会在映射中找到相应的条目。|style|CODE|343789|简而言之，就是这样。|343790|343791|自2009年5月以来更新(EL+2.2)|343792|带few+differences+compared+to+EL+2.1的Dec+2009+saw+the+introduction+of+EL+2.2+with+JSP+2.2+/+Java+EE+6。|343793|看起来("EL+Expression+parsing+integer+as+long")：|343794|对于EL+2.2+intValue中的Long+|BOLD|343795|+对象本身，您可以在EL+2.2中调用方法：|343796|343797|343798|<c:out+value="${map[(1).intValue()]}"/>|343799|这可能是一个很好的解决办法(下面也会在Tobias+Liefke的answer中提到)。|343800|343801|原始答案：|343802|EL使用以下包装器：|343803|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|343804|JSP页面演示了这一点：|343805|+<%25@+taglib+prefix="c"+uri="http://java.sun.com/jsp/jstl/core"%25>

+<%25@+page+import="java.util.*"+%25>

+<h2>+Server+Info</h2>
Server+info+=+<%25=+application.getServerInfo()+%25>+ 
Servlet+engine+version+=+<%25=++application.getMajorVersion()+%25>.<%25=+application.getMinorVersion()+%25> 
Java+version+=+<%25=+System.getProperty("java.vm.version")+%25> 
<%25
++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);++
%25>
++<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="${myMap['2']}"/> 
++++Evaluating:+${"${myMap[2]}"}+++=+<c:out+value="${myMap[2]}"/> ++++
++++Evaluating:+${"${myMap[42]}"}+++=+<c:out+value="${myMap[42]}"/> ++++

++++
++++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
++++

++++lifeInteger+=+<%25=+lifeInteger+%25> 
++++lifeLong+=+<%25=+lifeLong+%25> 
++++lifeInteger.equals(lifeLong)+:+<%25=+lifeInteger.equals(lifeLong)+%25>+ |343806|entityMap|0|LINK|mutability|MUTABLE|url|http://forums.sun.com/thread.jspa?messageID=9702932#9702932|1|https://stackoverflow.com/a/7209200/6309|2|https://stackoverflow.com/a/4812883/6309|3|https://stackoverflow.com/a/18269163/6309|4|https://stackoverflow.com/users/4371659/tobias-liefke|5|https://stackoverflow.com/a/27533414/6309^0|0|3|M|0|0|0|0|0|0|0|0|0|2|4|B|7|0|0|0|0|1|Y|1|10|1S|2|0|5|11|3|0|9|8|J|4|9|8|J|4|0|1|F|0|0|0|0|J|D|4|X|6|5|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|2B|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|2C|8|@]|9|@$D|2D|E|2E|1|2F]]|A|$]]|$1|F|3|-4|5|6|7|2G|8|@]|9|@]|A|$]]|$1|G|3|H|5|I|7|2H|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|2I|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|2J|8|@]|9|@]|A|$]]|$1|L|3|M|5|N|7|2K|8|@]|9|@]|A|$O|P]]|$1|Q|3|R|5|6|7|2L|8|@]|9|@]|A|$]]|$1|S|3|T|5|N|7|2M|8|@]|9|@]|A|$O|P]]|$1|U|3|V|5|6|7|2N|8|@$D|2O|E|2P|W|X]|$D|2Q|E|2R|W|X]]|9|@]|A|$]]|$1|Y|3|Z|5|6|7|2S|8|@]|9|@]|A|$]]|$1|10|3|-4|5|6|7|2T|8|@]|9|@]|A|$]]|$1|11|3|12|5|6|7|2U|8|@]|9|@]|A|$]]|$1|13|3|14|5|6|7|2V|8|@]|9|@$D|2W|E|2X|1|2Y]|$D|2Z|E|30|1|31]]|A|$]]|$1|15|3|16|5|6|7|32|8|@]|9|@$D|33|E|34|1|35]]|A|$]]|$1|17|3|18|5|6|7|36|8|@$D|37|E|38|W|19]|$D|39|E|3A|W|19]|$D|3B|E|3C|W|X]|$D|3D|E|3E|W|X]]|9|@]|A|$]]|$1|1A|3|1B|5|I|7|3F|8|@$D|3G|E|3H|W|19]]|9|@]|A|$]]|$1|1C|3|-4|5|6|7|3I|8|@]|9|@]|A|$]]|$1|1D|3|-4|5|6|7|3J|8|@]|9|@]|A|$]]|$1|1E|3|1F|5|N|7|3K|8|@]|9|@]|A|$O|P]]|$1|1G|3|1H|5|6|7|3L|8|@]|9|@$D|3M|E|3N|1|3O]|$D|3P|E|3Q|1|3R]]|A|$]]|$1|1I|3|-4|5|6|7|3S|8|@]|9|@]|A|$]]|$1|1J|3|1K|5|6|7|3T|8|@]|9|@]|A|$]]|$1|1L|3|1M|5|6|7|3U|8|@]|9|@]|A|$]]|$1|1N|3|1O|5|N|7|3V|8|@]|9|@]|A|$O|P]]|$1|1P|3|1Q|5|6|7|3W|8|@]|9|@]|A|$]]|$1|1R|3|1S|5|N|7|3X|8|@]|9|@]|A|$O|P]]|$1|1T|3|-4|5|6|7|3Y|8|@]|9|@]|A|$]]]|1U|$1V|$5|1W|1X|1Y|A|$1Z|20]]|21|$5|1W|1X|1Y|A|$1Z|22]]|23|$5|1W|1X|1Y|A|$1Z|24]]|25|$5|1W|1X|1Y|A|$1Z|26]]|27|$5|1W|1X|1Y|A|$1Z|28]]|29|$5|1W|1X|1Y|A|$1Z|2A]]]]

Initial answer (EL 2.1, May 2009) 

As mentioned in <a href="http://forums.sun.com/thread.jspa?messageID=9702932#9702932" rel="noreferrer">this java forum thread</a>:

<blockquote>
 Basically autoboxing puts an Integer object into the Map.
 ie:
</blockquote>

<pre><code>map.put(new Integer(0), "myValue")
</code></pre>

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

<pre><code>map.get(new Long(0))
</code></pre>

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

<hr>

<h2>Update since May 2009 (EL 2.2)</h2>

<a href="https://stackoverflow.com/a/4812883/6309">Dec 2009 saw the introduction of EL 2.2 with JSP 2.2 / Java EE 6</a>, with a <a href="https://stackoverflow.com/a/7209200/6309">few differences compared to EL 2.1</a>. 
It seems ("<a href="https://stackoverflow.com/a/18269163/6309">EL Expression parsing integer as long</a>") that:

<blockquote>
 you can call the method <code>intValue</code> on the <code>Long</code> object self inside EL 2.2:
</blockquote>

<pre><code>&lt;c:out value="${map[(1).intValue()]}"/&gt;
</code></pre>

That could be a good workaround here (also mentioned below in <a href="https://stackoverflow.com/users/4371659/tobias-liefke">Tobias Liefke</a>'s <a href="https://stackoverflow.com/a/27533414/6309">answer</a>)

<hr>

Original answer:

EL uses the following wrappers:

<pre><code>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
</code></pre>

JSP page demonstrating this:

<pre><code> &lt;%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%&gt;

 &lt;%@ page import="java.util.*" %&gt;

 &lt;h2&gt; Server Info&lt;/h2&gt;
Server info = &lt;%= application.getServerInfo() %&gt; &lt;br&gt;
Servlet engine version = &lt;%= application.getMajorVersion() %&gt;.&lt;%= application.getMinorVersion() %&gt;&lt;br&gt;
Java version = &lt;%= System.getProperty("java.vm.version") %&gt;&lt;br&gt;
&lt;%
 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); 
%&gt;
 &lt;h3&gt;Looking up map in JSTL - integer vs long &lt;/h3&gt;

 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. 

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

 &lt;h4&gt; Accessing the map&lt;/h4&gt; 
 Evaluating: ${"${myMap['2']}"} = &lt;c:out value="${myMap['2']}"/&gt;&lt;br&gt;
 Evaluating: ${"${myMap[2]}"} = &lt;c:out value="${myMap[2]}"/&gt;&lt;br&gt; 
 Evaluating: ${"${myMap[42]}"} = &lt;c:out value="${myMap[42]}"/&gt;&lt;br&gt; 

 &lt;p&gt;
 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
 &lt;p&gt;

 lifeInteger = &lt;%= lifeInteger %&gt;&lt;br/&gt;
 lifeLong = &lt;%= lifeLong %&gt;&lt;br/&gt;
 lifeInteger.equals(lifeLong) : &lt;%= lifeInteger.equals(lifeLong) %&gt; &lt;br&gt;
</code></pre>

blocks|key|3323475|text|除了上面的注释之外，另一个有用的提示是当您在某些变量中包含字符串值时，例如请求参数。在这种情况下，传入此参数还会导致JSTL将值"1“作为一个字符串，因此在Map+hashmap中找不到匹配项。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3323476|解决这个问题的一种方法就是做这样的事情。|3323477|<c:set+var="longKey"+value="${param.selectedIndex+%2B+0}"/>|code-block|syntax|javascript|3323478|现在，这将被视为一个长对象，然后有机会匹配一个对象，当它包含在map+Map或其他对象中时。|3323479|然后，像往常一样继续执行下面的操作|3323480|${map[longKey]}|3323481|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|T|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|U|8|@]|9|@]|A|$]]|$1|M|3|N|5|F|7|V|8|@]|9|@]|A|$G|H]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

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.

<pre><code>&lt;c:set var="longKey" value="${param.selectedIndex + 0}"/&gt;
</code></pre>

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 

<pre><code>${map[longKey]}
</code></pre>

blocks|key|3323507|text|基于上面的帖子，我尝试了一下，这个方法效果很好，我想使用Map+B的值作为Map+A的键：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3323508|<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>|code-block|syntax|javascript|3323509|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

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:

<pre><code>&lt;c:if test="${not empty activityCodeMap and not empty activityDescMap}"&gt;
&lt;c:forEach var="valueMap" items="${auditMap}"&gt;
&lt;tr&gt;
&lt;td class="activity_white"&gt;&lt;c:out value="${activityCodeMap[valueMap.value.activityCode]}"/&gt;&lt;/td&gt;
&lt;td class="activity_white"&gt;&lt;c:out value="${activityDescMap[valueMap.value.activityDescCode]}"/&gt;&lt;/td&gt;
&lt;td class="activity_white"&gt;${valueMap.value.dateTime}&lt;/td&gt;
&lt;/tr&gt;
&lt;/c:forEach&gt;
&lt;/c:if&gt;
</code></pre>

blocks|key|340347|text|如果您恰好有一个带有Integer密钥的Map，那么您可以编写一个custom+EL+function将Long转换为Integer。这将允许您执行以下操作：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|340348|<c:out+value="${map[myLib:longToInteger(1)]}"/>|code-block|syntax|javascript|340349|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/7079978/how-to-create-a-custom-el-function^0|A|7|K|3|1G|4|1N|7|X|I|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@$9|11|A|12|1|13]]|E|$]]|$1|F|3|G|5|H|7|14|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|15|8|@]|D|@]|E|$]]]|L|$M|$5|N|O|P|E|$Q|R]]]]

If you just happen to have a <code>Map</code> with <code>Integer</code> keys you cannot change, you could write a <a href="https://stackoverflow.com/questions/7079978/how-to-create-a-custom-el-function">custom EL function</a> to convert a <code>Long</code> to <code>Integer</code>. This would allow you to do something like:

<pre><code>&lt;c:out value="${map[myLib:longToInteger(1)]}"/&gt;
</code></pre>

blocks|key|3323545|text|如果您将数字放入"(“”)中，则可以使用Long中的所有函数。这样就可以将long类型转换为int类型：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3323546|<c:out+value="${map[(1).intValue()]}"/>|code-block|syntax|javascript|3323547|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

You can use all functions from Long, if you put the number into "(" ")". That way you can cast the long to an int:

<pre><code>&lt;c:out value="${map[(1).intValue()]}"/&gt;
</code></pre>

I have a Map keyed by Integer. Using EL, how can I access a value by its key?

<pre><code>Map&lt;Integer, String&gt; map = new HashMap&lt;Integer, String&gt;();
map.put(1, "One");
map.put(2, "Two");
map.put(3, "Three");
</code></pre>

I thought this would work but it doesn't (where map is already in the request's attributes):

<pre><code>&lt;c:out value="${map[1]}"/&gt;
</code></pre>

Follow up: I tracked down the problem. Apparently <code>${name[1]}</code> does a map lookup with the number as a <code>Long</code>. I figured this out when I changed <code>HashMap</code> to <code>TreeMap</code> and received the error:

<pre><code>java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.Long
</code></pre>

If I change my map to be:

<pre><code>Map&lt;Long, String&gt; map = new HashMap&lt;Long, String&gt;();
map.put(1L, "One");
</code></pre>

then <code>${name[1]}</code> returns "One". What's with that? Why does <code>&lt;c:out&gt;</code> treat a number as a long. Seems counterintuitive to me (as int is more commonly used than long).

So my new question is, is there a EL notation to access a map by an <code>Integer</code> value?

EL access a map value by Integer key

Java

我有一个以整数为关键字的地图。使用EL，我如何通过键访问值？Map<Integer, String> map = new HashMap<Integer, String>();map.put(1, "One");map.put(2, "Two");map.put(3, "Three");我认为这会起作用，但它不是( m...

问EL通过整数键访问映射值
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