引用:http://www.85java.com/thread-2863-1-1.html

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在jsp页面用到jstl的if或when标签判断字符串是否为空的时候，有时候经常这样判断：

Java代码
<c:when test="${allowedUploadImageExtension != ""}">??
<c:if test="${allowedUploadImageExtension != ""}">?

<c:when test="${allowedUploadImageExtension != ""}">
<c:if test="${allowedUploadImageExtension != ""}">
这样判断会报错，提示有未结束的标签，正确的书写格式是这样的：

Java代码
<c:when test="${not empty allowedUploadImageExtension}">??
或者??
<c:when test="${allowedUploadImageExtension != ''}"?

<c:when test="${not empty allowedUploadImageExtension}">
或者
<c:when test="${allowedUploadImageExtension != ''}"