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EL：empty的用法,JSTL <c:if test GOOD

2016-07-06 16:45 405 查看
使用到jsp设计页面时，EL表达式是少不了的，有时候需要判断一个对象或一个数组是否为空，可以使用empty。

例如：有一个list数组，判断它是否有值，可以使用<c:if test="${empty list}">数组为空</c:if> <c:if test="${!empty list}">数组不为空</c:if>

对于对象的判断也是如此。

百度了一下相关文档还说明

empty用来对一个空变量值进行判断: null、一个空String、空数组、空Map、没有条目的Collection集合

这种方式的判断就比jstl的<c:if test="${fn:length(list)==0}">数组为空</c:if> 这种好用多了，也好记。


JSTL 入门: 表达式语言https://www.ibm.com/developerworks/cn/java/j-jstl0211/

EL 擅长寻找对象及其特性，然后对它们执行简单操作；它不是编程语言，甚至不是脚本编制语言。但是，与 JSTL 标记一起使用时，它就能使用简单而又方便的符号来表示复杂的行为。EL 表达式的格式是这样的：用美元符号（$）定界，内容包括在花括号（{}）中，如清单 3 所示。

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http://stackoverflow.com/questions/2811626/evaluate-empty-or-null-jstl-c-tags

You can use the 
<c:if>
 or 
<c:choose>
 for
this.
<code style="margin: 0px; padding: 0px; border: 0px; font-size: 14px; vertical-align: baseline; font-family: Consolas, Menlo, Monaco, "Lucida Console", "Liberation Mono", "DejaVu Sans Mono", "Bitstream Vera Sans Mono", "Courier New", monospace, serif; white-space: inherit; background: rgb(238, 238, 238);"><span class="tag" style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;"><c:if</span><span class="pln" style="color: rgb(0, 0, 0); margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;"> </span><span class="atn" style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;">test</span><span class="pun" style="color: rgb(0, 0, 0); margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;">=</span><span class="atv" style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;">"${empty var1}"</span><span class="tag" style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;">></span><span class="pln" style="color: rgb(0, 0, 0); margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;">
var1 is empty or null.
</span><span class="tag" style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;"></c:if></span><span class="pln" style="color: rgb(0, 0, 0); margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;">
</span><span class="tag" style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;"><c:if</span><span class="pln" style="color: rgb(0, 0, 0); margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;"> </span><span class="atn" style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;">test</span><span class="pun" style="color: rgb(0, 0, 0); margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;">=</span><span class="atv" style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;">"${not empty var1}"</span><span class="tag" style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;">></span><span class="pln" style="color: rgb(0, 0, 0); margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;">
var1 is NOT empty or null.
</span><span class="tag" style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;"></c:if></span></code>


or
<code style="margin: 0px; padding: 0px; border: 0px; font-size: 14px; vertical-align: baseline; font-family: Consolas, Menlo, Monaco, "Lucida Console", "Liberation Mono", "DejaVu Sans Mono", "Bitstream Vera Sans Mono", "Courier New", monospace, serif; white-space: inherit; background: rgb(238, 238, 238);"><span class="tag" style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;"><c:choose></span><span class="pln" style="color: rgb(0, 0, 0); margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;">
</span><span class="tag" style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;"><c:when</span><span class="pln" style="color: rgb(0, 0, 0); margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;"> </span><span class="atn" style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;">test</span><span class="pun" style="color: rgb(0, 0, 0); margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;">=</span><span class="atv" style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;">"${empty var1}"</span><span class="tag" style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;">></span><span class="pln" style="color: rgb(0, 0, 0); margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;">
var1 is empty or null.
</span><span class="tag" style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;"></c:when></span><span class="pln" style="color: rgb(0, 0, 0); margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;">
</span><span class="tag" style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;"><c:otherwise></span><span class="pln" style="color: rgb(0, 0, 0); margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;">
var1 is NOT empty or null.
</span><span class="tag" style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;"></c:otherwise></span><span class="pln" style="color: rgb(0, 0, 0); margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;">
</span><span class="tag" style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background: transparent;"></c:choose></span></code><div class="save_code tracking-ad" data-mod="popu_249" style="position: absolute; width: 105px; height: 60px; right: 30px; top: 5px; font-size: 14px; color: rgb(255, 255, 255); cursor: pointer; z-index: 2;">
</div>


The 
${not
empty var1}
can also be done by 
${!empty
var1}
.

To learn more about those 
${}
things
(the Expression Language,
which is a separate subject fromJSTL), check
here.
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