java JSP EL 中 LENGTH[...] 和 fn:length(...) 的区别
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Difference between LENGTH[...] and fn:length(...) in JSP EL
提问by Sammy
What is the difference between the LENGTH[...]and the JSTL function fn:length(...)?
LENGTH[...]和 JSTL 函数有fn:length(...)什么区别？
I tried to search to difference but I did not see any example that uses the first one.
我试图寻找差异，但我没有看到任何使用第一个的例子。
Here is an example:
下面是一个例子：
<c:when test="${object.field ne null || LENGTH[object.field] > 0}">
<td valign="top">
.....print something
</td>
</c:when>
回答by BalusC
Since there is no such function like LENGTH[...]in standard JSP/JSTL/EL, it's impossible to tell about the differences. The fn:length()is the only way to obtain the length of a String, an Object[]or Collection.
由于没有像LENGTH[...]标准 JSP/JSTL/EL 中那样的功能，因此无法说明差异。这fn:length()是获取 a String、 anObject[]或长度的唯一方法Collection。
${fn:length(someCollection)}
Updateas per your (fixed) example:
根据您的（固定）示例更新：
<c:when test="${object.field ne null || LENGTH[object.field] > 0}">
I've never seen this before. It look like that your webapp/servletcontainer is using a custom EL resolver. If this is true, you should see it been declared in webapp's web.xmlfile.
我以前从未见过这个。看起来您的 webapp/servletcontainer 正在使用自定义 EL 解析器。如果这是真的，您应该会看到它已在 webapp 的web.xml文件中声明。
Regardless, you'd rather like to use the EL emptykeyword here. It not only checks for null, but also for the length of the String, Object[]or Collection.
无论如何，您更愿意在empty此处使用 EL关键字。它不仅检查null, 还检查String, Object[]or的长度Collection。
<c:when test="${not empty object.field}">
No need for fn:length()here.
fn:length()这里不需要。
The brace notation []is in turn by the way often used to access properties by dynamic keys. E.g.
大括号表示法[]通常用于通过动态键访问属性。例如
${bean[propertyname]}
If propertynameresolves to "foo", then the above does effectively the same as ${bean.foo}. It's also often used on Mapobjects in the scope.
如果propertyname解析为“foo”，则上述内容与${bean.foo}. 它也经常用于Map范围内的对象。
