1.<c:if test="${var} != null"></c:if>
2.
<c:foreach var="singleVar" items="" varStatus="states"></c:foreach>
<c:if test="${states.index%2=0}"></c:if>
<c:if test="${states.index%2=1}"></c:if>
<c:if test="${states.last}"></c:if>
3.
<c:choose>
<c:when test="${empty param.username}">
Nnknown user.
</c:when>
<c:when test="${param.username=='Tom'}">
${param.username} is manager.
</c:when>
<c:otherwise>
${param.username} is employee.
</c:otherwise>
</c:choose>