Java JSTL：检查字符串是否为空
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JSTL: check if a string is empty
提问by DaanCelie
I'm trying to test in JSTL if a session attribute is empty. However the attribute is empty JSTL sees it as a non-empty attribute.
如果会话属性为空，我正在尝试在 JSTL 中进行测试。然而，该属性为空 JSTL 将其视为非空属性。
This is the hardcode I'm trying to replace with JSTL. This code is working correctly:
这是我试图用 JSTL 替换的硬代码。此代码工作正常：
<% if (request.getAttribute("error") != null) { %>
<div class="alert alert-danger">
<strong>Oh snap, something's wrong, maybe the following error could help you out?<br /></strong>
<%= request.getAttribute("error")%>
</div>
<% } %>
This is how I replaced it with JSTL. When checked, the error-attribute is not empty, however it is empty.
这就是我用 JSTL 替换它的方式。勾选后，error-attribute 不是空的，而是空的。
<c:if test="${not empty sessionScope.error}">
<div class="alert alert-danger">
<strong>Oh snap, something's wrong, maybe the following error could help you out?<br /></strong>
<c:out value="${sessionScope.error}" />
</div>
</c:if>
采纳答案by gyanu
Add the JSTL library and declare the core taglib:
添加JSTL库并声明核心标签库：
<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
The JSTL equivalent of
JSTL 相当于
<% if (request.getAttribute("error") != null) { %>
is
是
<c:if test="${not empty error}">
回答by Braj
I'm trying to test in JSTL if a session attributeis empty
如果 asession attribute为空，我正在尝试在 JSTL 中进行测试
You have to check in session scopenot in request scope.
您必须检查会话范围而不是请求范围。
It should be
它应该是
<c:if test="${not empty sessionScope.error}">
instead of
代替
<c:if test="${not empty requestScope.error}">
If you are not sure about the scope then use without scope that will look in all scope from bottom to top (page, request, session then finally in application)
如果您不确定范围，则使用不使用范围，从下到上查看所有范围（页面、请求、会话，最后在应用程序中）
<c:if test="${not empty error}">
