Web

【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 }