EL表达式和JSTL

1.EL的概述

EL为了使JSP写起来更加简单
语法:${EL表达式}
功能:获取数据,执行运算

2.EL获取域对象中的数据

    <%
    //pageContext.setAttribute("name", "pValue");
    //request.setAttribute("name", "rValue");
    //session.setAttribute("name", "sValue");
    application.setAttribute("name", "aValue");
%>
<%=pageContext.getAttribute("name") %> <!-- 如果没找到 返回null -->
<%=request.getAttribute("name") %>
<%=session.getAttribute("name") %>
<%=application.getAttribute("name") %>
<hr/>
${ pageScope.name } <!-- 返回的是"" -->
${ requestScope.name }
${ sessionScope.name }
${ applicationScope.name }
<hr/>

${name}就类似于findAttribute("name"),它会先从page域中查找,没找到去request域中查询,没有找到去session域中找,最后去application中找

3.EL获取数组中的数据

    <%
    String[] arrs = {"张三","李四希","王五","赵六"};
    pageContext.setAttribute("arrs", arrs);
%>
${ arrs[0] }
${ arrs[1] }
${ arrs[2] }
${ arrs[3] }

4.获取集合中的数据

list集合:

<%
    List<String> list = new ArrayList<String>();
    list.add("张三");
    list.add("李四");
    list.add("王五凤");
    pageContext.setAttribute("list", list);
%>
${ list[0] }
${ list[1] }
${ list[2] }

map集合

<%
    Map<String,String> map = new HashMap<String,String>();
    map.put("aaa","张三");
    map.put("bbb","李四");
    map.put("ccc","王五");
    pageContext.setAttribute("map", map);
%>
${ map.aaa }
${ map.bbb }
${ map.ccc }

5.获取对象的数据

<%
    User user = new User(1,"aaa","123");
    pageContext.setAttribute("user", user);
%>
${ user.id }
${ user.username }
${ user.password }

6.获取对象的集合的数据

<%
    User user1 = new User(1,"aaa","123");
    User user2 = new User(2,"bbb","123");
    User user3 = new User(3,"ccc","123");
    
    List<User> userList = new ArrayList<User>();
    userList.add(user1);
    userList.add(user2);
    userList.add(user3);
    
    pageContext.setAttribute("userList", userList);
%>

${ userList[0].id } - ${ userList[0].username } - ${ userList[0].password }<br/>
${ userList[1].id } - ${ userList[1].username } - ${ userList[1].password }<br/>
${ userList[2].id } - ${ userList[2].username } - ${ userList[2].password }<br/>

注意:
.和[]的区别:[]用于有下标的数据(数组,list集合),用于有属性的数据(map,对象),如果属性名中包含有特殊的字符,必须使用[]

7.EL执行运算

执行算数运算

<%
    pageContext.setAttribute("n1", "10");
    pageContext.setAttribute("n2", "20");
    pageContext.setAttribute("n3", "30");
    pageContext.setAttribute("n4", "40");
%>
${ n1 + n2 + n3 }

执行逻辑运算

${ n1 < n2 } - ${ n1 lt n2 } <br/>
${ n1 > n2 } - ${ n1 gt n2 } <br/>
${ n1 <= n2 } - ${ n1 le n2 }<br/>
${ n1 >= n2 } - ${ n1 ge n2 } <br/>
${ n1 == n2 } - ${ n1 eq n2 } <br/>

EL执行关系运算

${ n1<n2 && n3 < n4 } - ${ n1<n2 and n3 < n4 }<br/>
${ n1<n2 || n3 < n4 } - ${ n1<n2 or n3 < n4 }<br/>
${ !(n1 < n2) } - ${ not(n1<n2) }

执行三元运算

${ n1 < n2 ? "正确":"错误" }

emp运算

${ user == null } - ${ empty user }
${ user != null } - ${ not empty user }

8.EL常用的11个对象

<!-- 
    pageScope,requestScope,sessionScope,applicationScope - 获取JSP中域中的数据
    param,paramValues     - 接收参数.
    header,headerValues - 获取请求头信息
    initParam            - 获取全局初始化参数
    cookie                - WEB开发中cookie
    pageContext            - WEB开发中的pageContext.
 -->

9.JSTL的标签库

core(核心标签),fmt(国际化标签),xml(XML标签),sql(SQL标签),fn(JSTL提供EL函数库)

引入JSTL的相关的jar包.
在页面中引入标签库.<%@ taglib uri=”” prefix=””%>

10.EL提供的函数

${ fn:contains("Hello World","Hello") }
${ fn:length("HelloWorld") }
${ fn:toLowerCase("ABCDE") }
<c:forEach var="i" items='${ fn:split("a-b-c-d","-") }'>
    ${ i }
</c:forEach>

11.页面显示商品的部分代码

    <c:forEach var="p" items="${list }">
    <tr>
        <td>${ p.pid }</td>
        <td>${ p.pname }</td>
        <td>${ p.price }</td>
        <td>
            <c:if test="${ p.hot == 1 }">
                是
            </c:if>
            <c:if test="${ p.hot != 1 }">
                否
            </c:if>
        </td>
        <td>${ p.pdesc }</td>
    </tr>    
    </c:forEach>