<c>----<choose><when><otherwise>

　

  <c:choose>
       <c:when test="${username== '1' && password== '2'}">
                 <jsp:forward page="welcome.jsp"></jsp:forward>
       </c:when>
       <c:otherwise>
                  <jsp:forward page="fail.jsp"></jsp:forward>
        </c:otherwise>
 </c:choose>            

　　when和otherwise是并列关系,相当于if和else,test是条件语句.条件语句判断只能用"&&" "||"

     <c:choose>
         <c:when test="${username== '1'}">
 3             <c:if test="${password== '2' }">
                 <jsp:forward page="welcome.jsp"></jsp:forward>
              </c:if>
         </c:when>
         <c:otherwise>
             <jsp:forward page="fail.jsp"></jsp:forward>
         </c:otherwise>
     </c:choose>

嵌套<if>之后相当于&&,但是在进入<when>后,如果没有进入<if>也同样不能进入<otherwise>,只会报错.

暂时这样有新进展再修改