maven專案中匯入  jstl.jar 和 standard.jar 依賴包

 <!-- jstl.jar 和 standard.jar 【前端判斷所需：<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>】-->
<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>jstl</artifactId>
    <version>1.2</version>
</dependency>
<dependency>
    <groupId>taglibs</groupId>
    <artifactId>standard</artifactId>
    <version>1.1.2</version>
</dependency>
 

後端session存值

JSP頁面中引入c標籤

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>

使用：

if 

<c:if test="${ sessionScope.avatar==null }">
    <a href="/login.html">登入</a>
</c:if>
<c:if test="${ sessionScope.avatar!=null }">
    <img src="${avatar}" alt="頭像">
</c:if>
 

if-else

 <c:choose>
      <c:when test="${ sessionScope.avatar==null }">
          <a href="/login.html"></a>
      </c:when>
      <c:otherwise>
          <img src="${avatar}" alt="頭像">
      </c:otherwise>
</c:choose>