Web - EL表达式

【EL的概述】

EL:Expression Language：表达式语言
简化JSP的代码，而且减少<%%>
语法：${EL 表达式}
EL的功能：
*获取数据的：（JSP的四个域）
*执行运算
*操作web开发的常用的对象
*调用Java中的方法（不常用）

【获取数据】

<h1>EL功能：获取数据</h1>
    <%
        pageContext.setAttribute("pname", "pvalue");
        request.setAttribute("rname", "rvalue");
        session.setAttribute("sname", "svalue");
        application.setAttribute("aname", "avalue");
    %>
<%= pageContext.getAttribute("pageNmae") %><!-- 如果找不到就返回NUll -->
<hr>
${ pageScope.pname }   <!-- 如果找不到，就返回“”，空值 -->
${ requestScope.rname}
${ sessionScope.pname }
${ applicationScope.rname}

获取数组

    <%
    String[] arrs = {"tommy","allen","hans"};
    pageContext.setAttribute("arrs", arrs);
    %>

    ${ pageScope.arrs[0] }

获取List集合

<!-- 招数据，先从page-request-session-application -->

    <hr>
    <%
    String[] arrs = {"tommy","allen","hans"};
    pageContext.setAttribute("arrs", arrs);
    %>

    ${ pageScope.arrs[0] }

    <%

        List<String> list = new ArrayList<String>();
        list.add("tommy");
        list.add("allen");

        pageContext.setAttribute("list", list);
    %>

    ${ list[0]  }
    ${ list[1]  }


    <hr/>
    <%

    Map map = new HashMap();

    map.put("aaa", "tommy");
    map.put("bbb", "allen");
    pageContext.setAttribute("map", map);

    %>

    ${ map.aaa }
    ${ map.bbb }

* . 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）
如果属性名中包含特殊的字符，必须使用[]

---

    <hr/>
    <%

    Map map = new HashMap();

    map.put("aaa", "tommy");
    map.put("bbb", "allen");
    pageContext.setAttribute("map", map);

    %>

    ${ map.aaa }
    ${ map.bbb }

    ${ map["ddd.eee"] }

    <hr/>
    <!-- 获得对象的属性值 -->
    <% 
     User user = new User(1,"aaa","123");
    pageContext.setAttribute("user", user);
    %>

    ${ user.id }
    ${ user.username }
    ${ user.password }

    <h3>获取对象集合的数据</h3>

    <% 
     User user1 = new User(1,"aaa","123");
    User user2 = new User(1,"bbb","123");
    User user3 = new User(1,"ccc","123");

    List userlist = new ArrayList();
    userlist.add(user1);
    userlist.add(user2);
    userlist.add(user3);
    pageContext.setAttribute("userlist", user);
    %>

    ${ userlist[0].id } - ${ userlist[0].username } - ${ userlist[0].password }
    ${ userlist[1].id } - ${ userlist[1].username } - ${ userlist[1].password }
    ${ userlist[2].id } - ${ userlist[2].username } - ${ userlist[2].password }

【EL执行运算】
【执行数学运算】

<% pageContext.setAttribute("n1", 10);
   pageContext.setAttribute("n2", 20);
%>

${ n1+n2 }

<h1>el执行逻辑运算</h1>

${ n1 < n2 } - ${ n1 lt n2 }   <!-- lt = less than -->
${ n1 <= n2 } - ${ n1 le n2 }  <!-- le = less equal -->
${ n1 > n2 } - ${ n1 gt n2 }   <!-- great than -->
${ n1 >= n2 } - ${ n1 ge n2 }   <!-- great equal -->
${ n1 == n2 } - ${ n1 eq n2 }

<h1>el不能执行关系运算</h1>

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

<h1>el执行三元运算</h1>
${ n1 < n2 ? "true" : "false" }

<h1>empty 运算</h1>
${ user == null } - ${ empty user }
${ user != null } - ${ not empty user }

【El操作Web开发的常用对象】

<h1>操作web开发的常用的对象</h1>
<!-- pageScope,requestScope,sessionScope,applicationScope,param,paramValues -- 接受参数 -->
<!-- header,headerValues -- 获取请求体 -->
<!-- initParmm -- 获取全局初始化参数 -->
<!-- cookie -- WEB开发中cookies -->
<!-- pageContext -- Web 开发中的pageContext -->
<% 
//获得其他8个内置对象 --- 主要在编写框架，通用性很高的代码中

    request.setAttribute("id", "001");
    request.setAttribute("name", "name");

%>

<%= request.getAttribute("id") %>
<%= request.getAttribute("name") %>

<!-- 接受请求的参数 -->
<%= request.getParameter("ids") %>
<%= request.getParameter("names") %>
<%= request.getParameter("hobbies") %>

<hr>
${ requestScope.id }
${ requestScope.name }

<!-- http://localhost:8080/day12JSP/demo05-EL/ELDemo3.jsp?ids=001&names=zuochunhui -->
${ requestScope.ids }
${ requestScope.names }
${ paramValues.hobbies[0] }
${ paramValues.hobbies[1] }
<!-- 001 zuochunhui 001 name basket football  -->

<hr>
<!-- 获取请求头 -->
<%= request.getHeader("User-Agent") %>

<hr>
${ header["User-Agent"] }

<!-- Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko  -->

<h3>获取全局初始化参数</h3>
<!-- <context-param>
        <param-name>username</param-name>
        <param-value>root</param-value>
    </context-param>
   -->
${ initParam.username }
${ initParam.value }

<h3>获取cookies中的值</h3>
${ cookie.history.value }

<h3>pageContext中的对象</h3>
<!-- get Ip address -->
${ pageContext.request.remoteAddr }

<!-- 获取工程路径 -->
${ pageContext.request.contextPath }