130 EL表达式（Expression Language)
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文章目录
1.概念
- EL使JSP写起来更简单、简洁。
- 主要用于获取作用域中的数据
2.作用
- 用于替换 作用域对象.getAttribute(“name”);
3. EL的应用1（获取基本类型、字符串）
- ${scope.name}获取具体某个作用域中的数据
- ${name}获取作用域中的数据，逐级查找(pageContext、request、session、application)，这种方式需要确定name 的值是唯一的
3.1 EL应用案例
<%
//存储在request作用域
request.setAttribute("name","tom");
request.setAttribute("age",18);
%>
${requestScope.name} <%--获取request作用域中name对应的值，找到就返回，没找到返回" "--%>
${name} <%--从最小作用域逐级查找name对应的值，找到就返回，没找到返回" " --%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>EL表达式</title>
</head>
<body>
<%
request.setAttribute("key1","value1");
session.setAttribute("key2","value2");
application.setAttribute("key3","value3");
request.setAttribute("key666","value6");
session.setAttribute("key666","value7");
application.setAttribute("key666","value8");
%>
<h1>通过作用域对象获取：</h1>
<!--通过作用域对象获取数据 -->
<h1><%=request.getAttribute("key8")%></h1>
<h1><%=session.getAttribute("key2")%></h1>
<h1><%=application.getAttribute("key3")%></h1>
<hr/>
<h1>通过EL表达式获取数据：</h1>
<!--通过EL表达式获取数据 -->
<h1>${requestScope.key666}</h1>
<h1>${sessionScope.key666}</h1>
<h1>${applicationScope.key666}</h1>
<hr/>
<h1>${key666}</h1>
<h1>${key666}</h1>
<h1>${key666}</h1>
</body>
</html>
3.2EL和JSP脚本的区别
- <%=request.getAttribute() %> 没有找到返回 null
- ${requestScope.name}没找到返回 " "(空，就是不显示任何东西)
4. EL的应用2（获取引用类型）
- 使用EL获取作用域中的对象调用属性时，只能访问对象的get方法，必须遵守命名规范定义(实体类要有get方法)
<%
Emp e = new Emp();
e.setName("gavin");
e.setAge(19);
request.setAttribute("e" ,e);
%>
${requestScope.e.name} <%--调用getName()方法--%>
<%@ page import="com.wlw.entity.User" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>EL获取对象</title>
</head>
<body>
<%
User user = new User("gavin","123456");
request.setAttribute("user",user);
%>
<%
User u = (User)request.getAttribute("user");
out.println(u.getUsername());
out.println(u.getPassword());
%>
<hr/>
${requestScope.user}<br/>
${user}<br/>
${user.username}<br/><%--调用getUsername()--%>
${user.password}<br/>
</body>
</html>
5. EL的应用3（获取数组、集合的元素）
- EL可以获取Array、List、Map中的元素，Set由于没下标，无法直接访问元素，后续可遍历
<%@ page import="java.util.List" %>
<%@ page import="java.util.ArrayList" %>
<%@ page import="java.util.HashMap" %>
<%@ page import="java.util.Map" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>数组、集合的访问</title>
</head>
<body>
<%
int[] array = new int[]{1,2,3,4,5};
request.setAttribute("array",array);
List<String> nums = new ArrayList<String>();
nums.add("A");
nums.add("B");
nums.add("C");
request.setAttribute("nums",nums);
Map<String,String> maps = new HashMap<String,String>() ;
maps.put ("CN","中国");
maps.put ("US", "美国");
maps.put ("IT", "意大利");
request.setAttribute("maps", maps) ;
%>
<!--EL访问数组 -->
${array[1]}<br/>
${array[2]}<br/>
${array[3]}<br/>
<hr/>
${nums[0]}<br/>
${nums[1]}<br/>
${nums.get(2)}
<hr/>
${requestScope.maps["CN"]}<br/>
${maps.US}<br/>
${maps["IT"]}
</body>
</html>
6. EL的运算符
操作符
| 描述
|
.
| 访问一个Bean属性或者一个映射条目
|
[]
| 访问一个数组或者链表的元素
|
+
| 加
|
-
| 减或负
|
*
| 乘
|
/ or div
| 除
|
% or mod
| 取模
|
==or eq
| 测试是否相等
|
!=or ne
| 测试是否不等
|
< or lt
| 测试是否小于
|
>or gt
| 测试是否大于
|
<=or le
| 测试是否小于等于
|
>=or ge
| 测试是否大于等于
|
&&orand
| 测试逻辑与
|
|| or or
| 测试逻辑或
|
! or not
| 测试取反
|
empty
| 测试是否空值
|
6.1 EL表达式执行运算
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>EL表达式运算符</title>
</head>
<body>
<%
request.setAttribute("nums",1234);
request.setAttribute("ss","b");
%>
<h1>empty运算符</h1>
<h1>${empty ss}</h1><!--测试ss 是否为空 ， null和" " 都是空-->
<hr/>
<h1>算术运算符</h1>
<h1>${nums + 5 } </h1>
<h1>${nums - 5 } </h1>
<h1>${nums * 5 } </h1>
<h1>${nums div 5 } </h1>
<h1>${nums mod 5 } </h1>
<hr/>
<h1>关系运算符</h1>
<h1>${nums eq 1234}</h1>
<h1>${nums ne 1234}</h1>
<h1>${nums gt 1234}</h1>
<h1>${nums lt 1234}</h1>
<h1>${nums ge 1234}</h1>
<h1>${nums le 1234}</h1>
<h1>逻辑运算符</h1>
<h1>${nums > 1000 and nums !=1200}</h1>
<h1>${nums > 1000 or nums == 2000}</h1>
<h1>${not(num > 1234)}</h1>
</body>
</html>
6.2 empty 关键字
<%
String s1=" ";
pageContext.setAttribute("s1",s1);
String s2=null;
pageContext.setAttribute("s2",s2);
String s3="122222";
pageContext.setAttribute("s3",s3);
List listl =new ArrayList();
pageContext.setAttribute( "list1", list1);
%>
<!-- empty关键只要内容是空true -->
${empty s1}<br>
${empty s2}<br>
${empty s3}<br>
${empty listl}<br>
7.隐式对象
隐含对象
| 描述
|
pageScope
| page作用域
|
requestScope
| request作用域
|
sessionScope
| session作用域
|
applicationScope
| application 作用域
|
param
| Request对象的参数，字符串
|
paramValues
| Request对象的参数，字符串集合
|
header
| HTTP信息头，字符串
|
headerValues
| HTTP信息头，字符串集合
|
initParam
| 上下文初始化参数
|
cookie
| Cookie 值
|
pageContext
| 当前页面的pageContext
|
7.1获得应用上下文
<%=request.getContextPath() %>
${pageContext.request.contextPath}
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>EL内置对象</title>
<link href="${pageContext.request.contextPath}/css/xxx.css">
</head>
<body>
<%
String path = request.getContextPath();
%>
<%=path%>
<br/>
<a href="<%=request.getContextPath()%>/manager/safe/xxxController">Click me</a><br/>
<a href="${pageContext.request.contextPath}/manager/safe/xxxController">Click target</a>
</body>
</html>
7.2获取Cookie对象
${cookie.username}//获取名为 username 的 cookie 对象
${cookie.password} //获取名为 password 的cookie 对象
${cookie.password.value}//获取password 的cookie 的value值package com.wlw.servlet;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
@WebServlet(name = "CookieServlet",value = "/cookie")
public class CookieServlet extends HttpServlet {
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
Cookie cookie = new Cookie("username","tom");
Cookie cookie1 = new Cookie("password","123456");
response.addCookie(cookie);
response.addCookie(cookie1);
}
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
doPost(request,response);
}
}
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>获取Cookie的EL</title>
</head>
<body>
<!--之前的做法-->
<%
Cookie[] cookies = request.getCookies();
String username = "";
String password = "";
if (cookies != null) {
for (Cookie cookie : cookies) {
if (cookie.getName().equals("username")) {
username = cookie.getValue();
}
if(cookie.getName().equals("password")){
password = cookie.getValue();
}
}
}
%>
<%=username%> <br/>
<%=password%>
<hr/>
<!--EL-->
<h1>${cookie.username.value}</h1>
<h1>${cookie.password.value}</h1>
<!--自动登录-->
<input type="text" name="username" value="<%=username%>"><br/>
<input type="text" name="password" value="<%=password%>"><br/>
<input type="text" name="username" value="${cookie.username.value}"><br/>
<input type="text" name="username" value="${cookie.password.value}"><br/>
</body>
</html>
