主要用来解决，条件，判断，循环
使用jstl需要在jsp页面中写<%@ taglibprefix="c"uri="http://java.sun.com/jsp/jstl/core"%>
request.setAttribute("age",25);
${sessionScope.user}<c:if test="${age >= 18}">
<h2>成年人
</c:if>
<%--相当于有if。。。。else的感觉 当when写多个 就是if .... else if ... else if ..... else--%>
<c:choose>
<c:when test="${age >= 18}">
<h2>成年人
</c:when>
<c:otherwise>
<h2>未成年人
</c:otherwise>
</c:choose>
<c:forEach items="${user_list}" var="user" varStatus="i">
<li>${i.count}----${user.name}----${user.age}
</c:forEach>
List lst =new ArrayList<>();
User user1 =new User();
user1.setName("zhangsan");
user1.setAge(15);
lst.add(user1);
User user2 =new User();
user2.setName("lisi");
user2.setAge(35);
lst.add(user2);
request.setAttribute("user_list",lst);
request.getRequestDispatcher("user.jsp").forward(request,response);
Map map =new HashMap<>();
User user1 =new User();
user1.setName("zhangsan");
user1.setAge(15);
map.put("zhangsan",user1);
User user2 =new User();
user2.setName("lisi");
user2.setAge(35);
map.put("lisi",user2);
request.setAttribute("user_map",map);
request.getRequestDispatcher("user.jsp").forward(request,response);
<%-- var表示map中的每一个元素相对于键值对--%>
<c:forEach items="${user_map}" var="user">
<li>${user.key}---${user.value.name}----${user.value.age}
</c:forEach>
</ul>
