JSTL 操作符

el表达式的取值默认顺序: 
    pageScope  requestScope  sessionScope  applicationScope 
    
结构，采用.导航，也称为存取器   ${user.name}  ${user.class.name} 
                      输出map   ${mapValue.key1} 
                     输出数组   ${strArray[1]} 
                     输出list   ${list[1].name} 
                     
el表达式的运算  
               ${1+2}=       3 
               ${10/5}=      2.0 
               ${10%3}=      1 
               ${10 div 2}=  5.0 
               ${10 mod 4}=  2 
               
el表达式对应的运算符 
                     等于  ==    eq 
                   不等于  !=    ne 
                     大于  >     gt 
                     小于  <     lt 
                 大于等于  >=    ge 
                 小于等于  <=    le 
                       与  &&    or 
                       或  ||    and 
                       非  !     not 
                       加  + 
                       减  - 
                       乘  * 
                     除以  /     div 
                     求莫  %     mod 
                     
el表达式判断是否为空 
                    request.setAttribute("value1",null)                    ${empty requestScope.value1} = true 
                    request.setAttribute("value2","")                      ${empty requestScope.value1} = true 
                    request.setAttribute("value3",new ArrayList())         ${empty requestScope.value1} = true 
                    request.setAttribute("value4","i love you")            ${empty requestScope.value1} = false 
                    request.setAttribute("value4","i love you")            ${!empty requestScope.value1} = true 
                    
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web.xml的 
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" 
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" 
id="WebApp_ID" version="2.4">  大于等于2.4 方可支持jstl 
先引入 standard.jar 和 jstl.jar 
jstl标签的用法  在页面引入  <%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>

输出标签 ----------- 
                 <c:out value="ppp" />                                              输出   ppp 
                 <c:out value="null" default="ddd"/>                                输出   ddd   可以设置默认值 
                 <c:out value="<font color="red">kkk</font>" escapeXml="false"/>    输出 红色的kkk    默认的escapeXml为true 
    赋值标签 -----------     
                 <c:set value="123" var="jj" scope="session" />       此时有个名为jj，值为123的session ，默认的scope为page 
                                                          ${jj}       在页面打印出123 
                 <c:remove var="jj" />                                将其删除,可以指定scope，默认为page       
    逻辑标签 ----------- 
                 <c:if test="?" />                                   ?：代表条件 可以用el表达式      
             ----------- 
                 <c:choose> 
                     <c:when test="?"> 
                        ......                                         相当于 if...else...   c:when 可以有多个,相当于else if 
                     </c:when> 
                     <otherwise> 
                       ...... 
                     </otherwise> 
                 </choose> 
循环控制标签 ----------- 
                 <c:forEach items="${list}" var="u" >                   ${list}是一个集合 
                     ${u.name} 
                 </c:forEach> 
                 
                 
                 <c:forEach items="${list}" var="u" varStatu="ss">                  
                     <c:choose> 
                     <c:when test="u.age % 2 == 0"> 
                        <tr bgColor="red">                                  如果集合中的数字是偶数，则当行为红色 
                     </c:when> 
                     <otherwise> 
                        <tr>    
                     </otherwise> 
                     </choose> 
                 </c:forEach> 
                 
                 
                 <c:forEach items="${list}" var="u" begin="2" end="8" step="2">                 输出下表(从0开始)为2到8的元素，每次递增2 
                     ${u.name}                                                                  将输出u[2].name  u[4].name  u[6].name  u[8].name 
                 </c:forEach> 
                                   
                 Map m = new Map(); 
                 m.put("k1","v1");         
                 m.put("k2","v2");         
                 request.setAttribute("mapValue",m); 
                 
                 <c:forEach items="mapValue" var="ma"> 
                     ${ma.key} = ${ma.value}                                 输出：  k1=v1  k2=v2   
                 </c:forEach> 
                 
   间隔符控制标签------------- 
            
                    request.setAttribute("str","1,2,3,4,5");      
                    
                    <c:forTokens items="str" delims="," var="de">             
                        ${de}                                                  输出 12345 
                    </c:forTokens>     
                    
   捕获异常标签  ---------- 
           
                   <c:catch var="info"> 
                       <% 
                         Integer.parseInt("sdfsdf");                 
                       %>        
                   </c:catch> 
                   
                   ${info}                                             输出：异常信息 
                   
   导入页面标签 ----------- 
        
                   <c:import url="http://www.baidu.com" />            相当于include 
                   
   链接标签    ------------ 
    
                   <c:url value="http://www.baidu.com" var="v"/> 
                       <c:param name="name" value="jack" /> 
                       <c:param name="age" value="20" /> 
                   </url>     
                   
                   ${v}              输出        http://www.baidu.com?name=jack&age=20 
                   
    重定向标签 ------------ 
        
                   <c:redirect context="/company" url="/index.jsp">   定位到名叫company的项目下的index.jsp页面