Web - EL表達式

【EL的概述】

EL:Expression Language：表達式語言
簡化JSP的代碼，而且減少<%%>
語法：${EL 表達式}
EL的功能：
*獲取數據的：（JSP的四個域）
*執行運算
*操作web開發的常用的對象
*調用Java中的方法（不常用）

【獲取數據】

<h1>EL功能：獲取數據</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}

獲取數組

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

    ${ pageScope.arrs[0] }

獲取List集合

<!-- 招數據，先從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[] 的區別
[] 用於有下標的數據（數組，List集合），用於有屬性的數據（map，對象）
如果屬性名中包含特殊的字符，必須使用[]

---

    <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/>
    <!-- 獲得對象的屬性值 -->
    <% 
     User user = new User(1,"aaa","123");
    pageContext.setAttribute("user", user);
    %>

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

    <h3>獲取對象集合的數據</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執行運算】
【執行數學運算】

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

${ n1+n2 }

<h1>el執行邏輯運算</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不能執行關係運算</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執行三元運算</h1>
${ n1 < n2 ? "true" : "false" }

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

【El操作Web開發的常用對象】

<h1>操作web開發的常用的對象</h1>
<!-- pageScope,requestScope,sessionScope,applicationScope,param,paramValues -- 接受參數 -->
<!-- header,headerValues -- 獲取請求體 -->
<!-- initParmm -- 獲取全局初始化參數 -->
<!-- cookie -- WEB開發中cookies -->
<!-- pageContext -- Web 開發中的pageContext -->
<% 
//獲得其他8個內置對象 --- 主要在編寫框架，通用性很高的代碼中

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

%>

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

<!-- 接受請求的參數 -->
<%= 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>
<!-- 獲取請求頭 -->
<%= 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>獲取全局初始化參數</h3>
<!-- <context-param>
        <param-name>username</param-name>
        <param-value>root</param-value>
    </context-param>
   -->
${ initParam.username }
${ initParam.value }

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

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

<!-- 獲取工程路徑 -->
${ pageContext.request.contextPath }