EL表达式的使用-12月23讲课内容
EL 表达式
EL 的使用
基础使用
案例代码一：获取实体类中的属性值
<%@ page import="com.fc.bean.Student" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>EL表达式</title>
</head>
<body>
<%
// 声明一个对象
Student student = new Student(1, "易烊千玺", 20, "男", "真帅");
// 将对象作为值存入pageContext中
pageContext.setAttribute("student", student);
%>
<table align="center" border="1px">
<tr>
<th>ID</th><th>姓名</th><th>年龄</th><th>性别</th><th>信息</th>
</tr>
<tr>
<%--通过EL表达式获取对应的值--%>
<td>${student.id}</td>
<td>${student.name}</td>
<td>${student.age}</td>
<td>${student.gender}</td>
<td>${student.info}</td>
</tr>
</table>
</body>
</html>【注意】可以使用任意域对象存储数据，一般常用 pageContext
使用 List 和 Map
案例代码一：使用 List
<%@ page import="java.util.ArrayList" %>
<%@ page import="java.util.List" %>
<%@ page import="com.fc.bean.Student" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>EL表达式测试List</title>
</head>
<body>
<%
List<Student> list = new ArrayList<>();
list.add(new Student(1, "易烊千玺", 20, "男", "送你一朵小红花"));
list.add(new Student(2, "迪丽热巴", 22, "女", "迪丽热巴"));
list.add(new Student(3, "古力娜扎", 18, "女", "古力娜扎"));
pageContext.setAttribute("list", list);
%>
<table align="center" border="1px">
<tr>
<th>ID</th>
<th>Name</th>
<th>Age</th>
<th>Gender</th>
<th>Info</th>
</tr>
<tr>
<td>${list[0].id}</td>
<td>${list[0].name}</td>
<td>${list[0].age}</td>
<td>${list[0].gender}</td>
<td>${list[0].info}</td>
</tr>
<tr>
<td>${list[1].id}</td>
<td>${list[1].name}</td>
<td>${list[1].age}</td>
<td>${list[1].gender}</td>
<td>${list[1].info}</td>
</tr>
<tr>
<td>${list.get(2).id}</td>
<td>${list.get(2).name}</td>
<td>${list.get(2).age}</td>
<td>${list.get(2).gender}</td>
<td>${list.get(2).info}</td>
</tr>
</table>
</body>
</html>案例代码二：使用 Map
<%@ page import="java.util.HashMap" %>
<%@ page import="java.util.Map" %>
<%@ page import="com.fc.bean.Student" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>EL表达式测试Map</title>
</head>
<body>
<%
Map<String, Object> map = new HashMap<>();
map.put("student1", new Student(1, "易烊千玺", 20, "男", "送你一朵小红花"));
map.put("student2", new Student(2, "迪丽热巴", 21, "女", "古力娜扎"));
map.put("student3", new Student(3, "欧阳娜娜", 19, "女", "缝纫机乐队"));
pageContext.setAttribute("map", map);
%>
<table align="center" border="1px">
<caption>测试Map</caption>
<tr><th>编号</th><th>姓名</th><th>年龄</th><th>性别</th><th>信息</th></tr>
<tr>
<td>${map.student1.id}</td>
<td>${map.student1.name}</td>
<td>${map.student1.age}</td>
<td>${map.student1.gender}</td>
<td>${map.student1.info}</td>
</tr>
<tr>
<td>${map.student2.getId()}</td>
<td>${map.student2.getName()}</td>
<td>${map.student2.getAge()}</td>
<td>${map.student2.getGender()}</td>
<td>${map.student2.getInfo()}</td>
</tr>
<tr>
<td>${map["student3"].id}</td>
<td>${map["student3"].name}</td>
<td>${map["student3"].age}</td>
<td>${map["student3"].gender}</td>
<td>${map["student3"].info}</td>
</tr>
</table>
</body>
</html>使用 EL 表达式中的 empty 关键字
empty 和 not empty 可用于判断是否为null，同时可以判断集合形式数据中是否存在元素，用于条件解析
<%@ page import="com.fc.bean.Student" %>
<%@ page import="java.util.ArrayList" %>
<%@ page import="java.util.List" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>测试EL中的empty</title>
</head>
<body>
<%
String str1 = null;
String str2 = "";
String str3 = new String();
Student student = null;
List<Student> list = new ArrayList<>();
pageContext.setAttribute("str1", str1);
pageContext.setAttribute("str2", str2);
pageContext.setAttribute("str3", str3);
pageContext.setAttribute("student", student);
pageContext.setAttribute("list", list);
%>
${empty str1}
${empty str2}
${empty str3}
${empty student}
${empty list}
</body>
</html>从 Servlet 获取参数【重点】
案例代码一
@WebServlet("/getParams")
public class TransmitDataServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// 设置请求编码集
req.setCharacterEncoding("UTF8");
// 设置属性键值对
req.setAttribute("username", "易烊千玺");
req.setAttribute("password", "四个字");
// 转发
req.getRequestDispatcher("jsp/getParam.jsp").forward(req, resp);
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
doGet(req, resp);
}
}前端页面
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>获取后端传递的参数</title>
</head>
<body>
${username}
${password}
</body>
</html>