EL表达式
表达式语言（Expression Language），或称EL表达式，简称EL，是Java中的一种特殊的通用编程语言，借鉴于JavaScript和XPath。主要作用是在Java Web应用程序嵌入到网页（如JSP）中，用以访问页面的上下文以及不同作用域中的对象 ，取得对象属性的值，或执行简单的运算或判断操作。EL在得到某个数据时，会自动进行数据类型的转换。
表达式语法
${变量名}
- 使用时需要导入外部包，在tomcat服务器上找到此包，并导入。
apache-tomcat-8.5.34/lib/servlet-api.jar - 案例从1.jsp 页面提交，通过servlet跳转到showEL.jsp页面完成赋值的输出
- 1.jsp
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>表单提交</title>
</head>
<body>
<form action="<%=request.getContextPath() %>/ELServlet" method="post">
用户名：<input type="text" name="username"><br>
年龄： <input type="text" name="age"><br>
<input type="submit" value="提交">
</form>
</body>
</html>
- ELServlet
package com.alan.controller;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet("/ELServlet")
public class ELServlet extends HttpServlet {
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
//1、首先获取username和age的属性值
String username = request.getParameter("username");
String age = request.getParameter("age");
//2、将其保存到request域中
request.setAttribute("username", username);
request.setAttribute("age", age);
//3、跳转到showEL.jsp页面，我们通过EL表达式取出request域中的值
request.getRequestDispatcher("/showEL.jsp").forward(request, response);
}
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
doGet(request, response);
}
}
- showEL.jsp
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>提交后页面</title>
</head>
<body>
用户名：${username} <br>
年龄：${age}
</body>
</html>
JSTL
JSP标准标签库（JSP Standard Tag Library）是Java EE网络应用程序开发平台的组成部分。它在JSP规范的基础上，扩充了一个JSP的标签库来完成一些通用任务，比如XML数据处理、条件执行、数据库访问、循环和国际化。
- 需要导入此jar包
- jsp页面导入<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
JSTL标签介绍
- 通用标签 set out remove
- 条件标签 if choose
- 迭代标签 forEach
- 案例1
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
<!-- set、out、remove标签 -->
<!-- set标签主要往指定的域中存放数据 -->
<c:set var="user" value="张三" scope="request"></c:set>
<!-- 将数据打印输出 -->
<c:out value="${user}"></c:out>
<!-- remove标签 -->
<c:remove var="user" scope="request" />
<c:out value="${user}"></c:out>
<!--
if标签
test:按判断的条件,如果条件为true,执行标签体中的内容
-->
<c:set var="age" value="12" scope="request"></c:set>
<c:if test="${age==11}">
您的年龄为12岁
</c:if>
<hr>
<!-- choose标签 -->
<c:set var="age1" value="13" scope="request"></c:set>
<c:choose>
<c:when test="${age1==12}">
您的年龄为12岁
</c:when>
<c:otherwise>
您的年龄不是12岁
</c:otherwise>
</c:choose>
</body>
</html>
- 案例2 foreach
- servelet
package com.alan.controller;
import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet("/JSTLELServlet")
public class JSTLELServlet extends HttpServlet {
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
//1、将输入存入List中
Map<String,Object> dataMap1 = new HashMap<>();
dataMap1.put("shopName", "联想笔记本");
dataMap1.put("address", "北京");
dataMap1.put("price", "4999");
Map<String,Object> dataMap2 = new HashMap<>();
dataMap2.put("shopName", "神州笔记本");
dataMap2.put("address", "南京");
dataMap2.put("price", "3999");
Map<String,Object> dataMap3 = new HashMap<>();
dataMap3.put("shopName", "小米笔记本");
dataMap3.put("address", "深圳");
dataMap3.put("price", "5999");
List<Map<String,Object>> dataList = new ArrayList<>();
dataList.add(dataMap1);
dataList.add(dataMap2);
dataList.add(dataMap3);
//2、将List赋值到request的作用域中
request.setAttribute("list", dataList);
//3、在jsp页面取出List
request.getRequestDispatcher("/2.jsp").forward(request, response);
}
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
doGet(request, response);
}
}
- jsp
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>通过JSTL+EL表达式迭代List集合</title>
</head>
<body>
<table border="1" cellspacing="0" >
<tr>
<td>商品名称</td>
<td>产地</td>
<td>价格</td>
</tr>
<c:forEach items="${list}" var="map">
<tr>
<td>${map.shopName }</td>
<td>${map.address }</td>
<td>${map.price}</td>
</tr>
</c:forEach>
</table>
</body>
</html>
