day6_EL

最新推荐文章于 2022-09-05 19:39:08 发布

原创最新推荐文章于 2022-09-05 19:39:08 发布 · 119 阅读

0 ·

CC 4.0 BY-SA版权

文章标签：

#El

前端专栏收录该内容

10 篇文章

订阅专栏

博客介绍了EL标签，它可用于取数据和输出数据，能简化JSP代码。阐述了EL表达式语法，如${输出数据或者取对象} ，还提及JSP传值的三种方法。介绍了EL隐含对象查找值的范围，指出EL不能做循环，需引入jstl，同时给出了jstl表达式控制标签示例。

EL:通过EL标签取数据和输出数据，简化代码，尽量在JSP中不要使用Java代码。
语法：${输出数据或者取对象} 可不能用它来搞for循环
一．el中的表达式

    a)算术运算符
       +、-、*、/和 %（或 mod）
    b)关系运算符
	 ==（或 eq）、!=（或 ne）、<（或 lt）、
	>（或 gt）、<=（或 le）和 >=（或 ge）
    c)逻辑运算符
	 &&（或 and）、||（或 or）和 !（或 not） 
    d)判空运算符
	**empty**
	      <%String name="";
	      request.setAttribute("name",name);%>
	  如：${empty name}
 测试：

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
	<%
	String path = request.getContextPath();
	String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
	%>
	<base href="<%=basePath%>" />
	
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>邮箱登录</title>
<style type="text/css">
	body{
		font-size: 20px;
		margin: 0px;
		padding: 0px;
		background: url("logo.gif");
		background-repeat: no-repeat;
		background-position: right,bottom;
		background-color: #7DDC87;
	}
	input {
		border: 1px solid yellow;
		border-radius:20px;
		line-height: 25px;
	}
	.btn:LINK;.btn:VISITED {
		background-color: #F0F0F0;
    }
    .btn:HOVER {
		background-color: #148D08;

	}
</style>
</head>
<body>
<%=basePath %>
<%
String error = request.getParameter("error");
if(error!=null&&error.equals("")){
	out.println(error);
                         }
%>
	<center>
	<!-- String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";已经到了项目下边  -->
		<form action="form/loginCL.jsp">
			<table>
				<tr><td>用户名:</td>
				<td><input type="text" name="name"></td>
					<td>@163.com</td></tr>
				<tr><td>密&nbsp;&nbsp;码:</td>
				<td><input type="password" name="pwd"></td>
					<td><a href="">忘记密码</a></td></tr>	
			</table>
				<input type="submit" value="登录" class="btn">
					&nbsp;<a href="register.html">注册邮箱</a>
		</form>
	</center>
</body>
</html>

loginCL.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>逻辑处理</title>
</head>
<body>
<%
//在jsp中这些都是可以直接用的 不需要再new出来
    String name1  = request.getParameter("name");
     String pwd  = request.getParameter("pwd"); 
   if("briup".equals(name1)&&"briup".equals(pwd)){
	   request.setAttribute("name","briup");
	   /* request.getDispatcherType("success").forward(request.response); */
	   %>
	   <!-- 动态引入 -->
	  <jsp:include  page = "success.jsp"></jsp:include> 
	  <%-- <jsp:param value="briup" name="name"/> --%>

	   
	   
	   <%
	/*    response.sendRedirect("success.jsp"); */
   }else{
	   /* request.getRequestDispatcher("login.jsp").forward( request, response); */
	   %>
	   <jsp:forward page="login.jsp">
	      <jsp:param  value="用户名密码不正确" name="error"/>
	   </jsp:forward>
	   <% 
	
   }
%>
</body>
</html>

success.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>登录成功</title>
</head>

    <body>

<%-- 恭喜<%=request.getParameter("name") %>登陆成功 --%>
恭喜${name}, 登陆成功 
</body>
</html>

恭喜${name}, 登陆成功中采用了el 表达式；
jsp中传值有三种方法：
1.jsp的动态传入

<jsp:include  page = "success.jsp">
<jsp:param value="briup" name="name"/>
</jsp:include>

2.java代码传值动态跳转

<% request.setAttribute("name","briup");%>
 <jsp:include  page = "success.jsp"></jsp:include>

3.纯java传值和跳转

<%
request.setAttribute("name","briup");
 request.getRequestDispatcher("success.jsp").forward(request,response);
%>

二：表达式语言(EL)中可以使用的隐含对象:

1) pageContext: jsp页面的上下文，它提供了访问以下对象的方法，唯一一个既是jsp的隐含对象，又是el的隐含对象。通过.操作符来得到数据。

        a) servletContext 
			${pageContext.servletContext}
			等于
			out.println(pageContext.getServletContext())
		b) session
			${pageContext.session.id}
			等于
			out.println(pageContext.getSession().getId())
		c) request
			${pageContext.request}
			等于
			out.println(pageContext.getRequest())
		d) response
			${pageContext.response}
			等于
			out.println(pageContext.getResponse())

2) param: 把请求中的参数和单个值进行映射
          ${param.name}或者${param["name"]}或者${param['name']}
  		 等于
  		out.println(request.getParameter("name"))

3) paramValues: 把请求中的参数和一个array值进行映射,比如获取多选框的值。
	${paramValues.hobby}
	或者${paramValues["hobby"]}
	或者${paramValues['hobby']}
   	等于
	String[] array = 
	request.getParameterValues("hobby")
    	out.println(array);
	${paramValues.hobby[0]}
4) cookie: 把请求中的Cookie和单个值进行映射
	Cookie cookie = new Cookie("height","100");
	Cookie cookie2 = new Cookie("width","200");
	response.addCookie(cookie);
	response.addCookie(cookie2);

     在服务器端获得从客户端发送过来的cookie:
	${cookie.height}: 输出一个Cookie的对象
	${cookie.height.name}=${cookie.height.value}
		分别输出Cookie的名称和值(height=100)
		${cookie.width}: 同上
	${cookie.width.name}=${cookie.width.value}: 同上

5) initParam: 把Web应用上下文的初始化参数和单个值进行映射
	${initParam.name}
	等于
	String value = getServletContext().getInitParameter("name");
	out.println(value);

6) pageScope: 把page范围中的key和value进行映射
	pageContext.setAttribute("name","jack");		
	${pageScope.name}
	等于
	out.println(pageContext.getAttribute("name"));
7) requestScope: 把request范围中的key和value进行映射
	request.setAttribute("name","jack");
	${requestScope.name}
	等于
	out.println(request.getAttribute("name"));
8) sessionScope: 把session范围中的key和value进行映射
	session.setAttribute("name","jack");
	${sessionScope.name}
	等于
	out.println(session.getAttribute("name"));
9) applicationScope: 把application范围中的key和value进行映射
	getServletContext().setAttribute("name","jack");
	${applicationScope.name}
	等于
	out.println(getServletContext().getAttribute("name"));

注意: 如果没有指明任何的范围根据key来查找对应的value，默认从page、request、session和application从小到大的范围开始查找，若找到就不往更大的
范围去查找。
例如: ${name} ,分别从page request session 和 application中去查找name的值
(scope.getAttribute(“name”)),scope为上面四种范围。
测试超链接后谁能拿到值：
el1.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>el表达式</title>
</head>
<body>
${4+5}
<br>
${6>4 }
<br>
<%
request.setAttribute("name", "rr");
pageContext.setAttribute("name1","pp");
session.setAttribute("name","ss");
application.setAttribute("name","aa");

%>
${ empty name} <br>
${name }
<br>
	${pageContext.session.id}
	<%out.println(pageContext.getSession().getId()); %>
<br>
page:${pageScope.name }
<br>
session:${sessionScope.name }
<br>
app:${applicationScope.name }
<br>
request:${requestScope.name }
<br>
name:${name }
<br>
<a href = "el2.jsp">跳转查传过去的能不能取到</a>
</body>
</html>

el2.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
</head>
<body>
page:${pageScope.name }
<br>
session:${sessionScope.name }
<br>
app:${applicationScope.name }
<br>
request:${requestScope.name }
</body>
</html>

在这里插入图片描述发现只是剩下了application和session
同时我们也发现，el是不能做循环的，所以就引入了jstl这是第三方的得引入架包，但是刚有架包还不行，还得在jsp页面中引进来

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
 <%@ taglib prefix = "c" uri="http://java.sun.com/jsp/jstl/core" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>jstl</title>
</head>
<body>
<h1>c:out</h1>
<c:out value="briup"></c:out>
</body>
</html>

如果出现啥xml解析错误啥的，看一下你的引用中是不是有空格，。！等
关于jsti:
第一类：表达式控制标签
1、<c:out> 输出变量到JspWriter输出流中

语法1：<c:out value=“result”
[escapeXml]="{true|false}"
[default=“defaultValue”]/>
注意：a)如果result为null，则输出default中的值

<c:out value="${name }" default="tom" ></c:out>

       b)escapeXml：设定是否转换特殊字符（如&lt;、&gt;等一些转
  义字符),在默认值为true的情况下直接是输出&lt;的，如果改为 false将会进行转义输出“<”等。
  2) 语法2：<c:out value="result" [escapeXml]="{true|false}">default value</c:out>

例如: value可以使用el表达式
<c:out value="${student.address.country}"/>
实例：
Login.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>login页面</title>
<%
	String path = request.getContextPath();
	String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
	%>
	<base href="<%=basePath%>" />
</head>

<body>
<form action="login">
账号：<input type="text" name = "name"/><br>
密码：<input type="text" name="password"/><br>
<input type="submit" value="提交">
</form>
</body>
</html>

servlet

package com.briup.servlet;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import javax.websocket.Session;

import com.briup.bean.User;

public class UserLoginServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		 request.setCharacterEncoding("utf-8");
   	    response.setCharacterEncoding("utf-8");
   	    response.setContentType("text/html;charset=utf-8");
		String name = (String)request.getParameter("name");
		String password = (String)request.getParameter("password");
		System.out.println(name);
		User u = new User();
		   u.setName(name);
           u.setPassword(password);	
           
              HttpSession session = request.getSession();
              session.setAttribute("u",u);
           
           request.getRequestDispatcher("success.jsp").forward(request,	 response);
	}
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doGet(request, response);
	}

}

success.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
 <%@taglib prefix="c"  uri="http://java.sun.com/jsp/jstl/core" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>登录成功</title>
</head>
<body>
账号：<c:out  value="${u.name}" default="null"/><br>
密码：<c:out  value="${u.password}" default="null" />
</body>
</html>

在这里插入图片描述