一：概要理解：
（1）语法格式：
${表达式}
（2）作用：
El表达式一般支持作用域（application、session、request、pagecontext）中的属性。EL变量只支持某一作用域中的属性。
注意：EL表达式不能操作局部变量
（3）优点：
表达式内容为对象时不需要导包
可以实现变量类型的自动转换
表达式可以被Jsp注释注释掉
属性值读取不到时不会出错
（4）注意事项及特殊应用
取值顺序
对不同作用域的同名属性进行取值时，顺序是从小到大的顺序
（5）跨作用域取值
${作用域.属性名}
作用域包含：pageScope、request.Scope、sessionScope、applicationScope
注意：如果指定了作用域，就只能从特定的作用域进行取值
（6）获取请求参数
${param.参数名}
${paramValues.参数名[索引下标] }
注意：如果不使用param将从作用域中取值
（7）访问javaBean的属性
${对象名.属性名}
案例1：
FirstServlet.java:
package com.bjsxt.el.web;
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.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.bjsxt.el.bean.Addr;
import com.bjsxt.el.bean.User;
public class FirstServlet extends HttpServlet {
@Override
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
String uname=req.getParameter("uname");
String pwd=req.getParameter("pwd");
String fav[]=req.getParameterValues("fav");
//信息输出
System.out.println("前台参数信息为："+uname+"--->"+pwd+"--->"+fav[1]);
//往request作用域存值
req.setAttribute("realname", "lulu");
//引入容器 ArrayList HashMap
List<String> list=new ArrayList<String>();
list.add("aa");
list.add("bb");
list.add("cc");
list.add("dd");
req.setAttribute("list", list);
Map<String,String> map=new HashMap<String,String>();
map.put("xx", "110");
map.put("yy", "114");
map.put("zz", "119");
map.put("11", "120");
req.setAttribute("map", map);
//引入真实应用场景
//单个用户对象
req.setAttribute("user", new User("moon", "123", "月亮", new Addr("BeiJing", "HaiDian", "XiSanQi")));
//容器中多个user对象
List<User> userList=new ArrayList<User>();
userList.add(new User("sun", "123", "太阳", new Addr("BeiJing", "TongZhou", "yiZhuang1")));
userList.add(new User("moon", "123", "月亮", new Addr("BeiJing", "HaiDian", "XiSanQi")));
userList.add(new User("star", "123", "星星", new Addr("BeiJing", "HaiDian", "DongSanQi")));
userList.add(new User("earth", "123", "地球", new Addr("BeiJing", "TongZhou", "yiZhuang2")));
req.setAttribute("userList", userList);
Map<String,User> userMap=new HashMap<String,User>();
userMap.put("a", new User("sun", "123", "太阳", new Addr("BeiJing", "TongZhou", "yiZhuang1")));
userMap.put("b", new User("star", "123", "星星", new Addr("BeiJing", "HaiDian", "DongSanQi")));
userMap.put("c", new User("earth", "123", "地球", new Addr("BeiJing", "TongZhou", "yiZhuang2")));
req.setAttribute("userMap", userMap);
/*//额外的bug----没有解决
Map<Integer,String> newMap=new HashMap<Integer,String>();
newMap.put(new Integer(11), "110");
newMap.put(new Integer("22"), "114");
req.setAttribute("newMap", newMap);*/
//跳转显示
req.getRequestDispatcher("/el.jsp").forward(req, resp);
return;
}
}
el.jsp:
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%-- <%@page import="com.bjsxt.el.bean.*" %> --%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>">
<title>My JSP 'el.jsp' starting page</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
</head>
<body>
<h2>EL表达式的使用</h2>
<h5>EL:用于替换jsp中的java代码，使用时一切向美元"$"看</h5>
<h5>EL:获取请求参数</h5>
<h5>EL:获取作用域中的值</h5>
<%-- <ol>
<li><%=request.getParameter("uname") %> </li>
<li><%=request.getParameter("pwd") %> </li>
<li><%=request.getParameterValues("fav")[1] %> </li>
<li><%=(String)request.getAttribute("realname") %> </li>
<li><%=((List<String>)request.getAttribute("list")).get(1) %> </li>
<li><%=((Map<String,String>)request.getAttribute("map")).get("xx") %> </li>
<li><%=((User)request.getAttribute("user")).getAddr().getTown() %> </li>
<li><%=((List<User>)request.getAttribute("userList")).get(2).getAddr().getTown() %></li>
<li><%=((Map<String,User>)request.getAttribute("userMap")).get("c").getAddr().getTown() %></li>
</ol> --%>
<h3>EL写法</h3>
<ol>
<h5>使用EL获取请求参数</h5>
<li>${param.uname }</li>
<li>${param.pwd}</li>
<li>${paramValues.fav[1]}</li>
<h5>使用EL获取作用域的值</h5>
<li>${realname}</li>
<li>${list[1]}</li>
<li>${map["xx"]}----</li>
<h5>使用EL获取作用域中指定对象的属性值</h5>
<li>${user.addr.town} </li>
<h5>使用EL获取作用域中容器(List Map)中对象的值</h5>
<li>${userList[2].addr.town }</li>
<li>${userMap.c.addr.town}---${userMap["c"].addr.town}</li>
</ol>
</body>
</html>
index.jsp:
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>">
<title>My JSP 'index.jsp' starting page</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
<!--
<link rel="stylesheet" type="text/css" href="styles.css">
-->
</head>
<body>
<h2>EL表达式的学习</h2>
<ol>
<li><a href="first?uname=momo&pwd=666&fav=1&fav=2&fav=3">El表达式1</a> </li>
<li><a href="second">El表达式2</a> </li>
</ol>
</body>
</html>
案例2：
Addr.java:
package com.bjsxt.el.bean;
public class Addr {
private String province;
private String city;
private String town;
public String getProvince() {
return province;
}
public void setProvince(String province) {
this.province = province;
}
public String getCity() {
return city;
}
public void setCity(String city) {
this.city = city;
}
public String getTown() {
return town;
}
public void setTown(String town) {
this.town = town;
}
@Override
public String toString() {
return "Addr [province=" + province + ", city=" + city + ", town=" + town + "]";
}
public Addr(String province, String city, String town) {
super();
this.province = province;
this.city = city;
this.town = town;
}
public Addr() {
super();
}
}
User.java:
package com.bjsxt.el.bean;
public class User {
private String uname;
private String pwd;
private String realName;
private Addr addr;
public String getUname() {
return uname;
}
public void setUname(String uname) {
this.uname = uname;
}
public String getPwd() {
return pwd;
}
public void setPwd(String pwd) {
this.pwd = pwd;
}
public String getRealName() {
return realName;
}
public void setRealName(String realName) {
this.realName = realName;
}
public Addr getAddr() {
return addr;
}
public void setAddr(Addr addr) {
this.addr = addr;
}
@Override
public String toString() {
return "User [uname=" + uname + ", pwd=" + pwd + ", realName=" + realName + ", addr=" + addr + "]";
}
public User(String uname, String pwd, String realName, Addr addr) {
super();
this.uname = uname;
this.pwd = pwd;
this.realName = realName;
this.addr = addr;
}
public User() {
super();
}
}
SecondServlet.java:
package com.bjsxt.el.web;
import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.bjsxt.el.bean.Addr;
import com.bjsxt.el.bean.User;
public class SecondServlet extends HttpServlet {
@Override
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//往作用域中存值
req.setAttribute("hello", "request");
req.getSession().setAttribute("hello", "session");
this.getServletContext().setAttribute("hello", "application");
req.setAttribute("user", new User("moon", "123", "月亮", new Addr("BeiJing", "HaiDian", "XiSanQi")));
req.setAttribute("field", "realName");
req.setAttribute("e1", " ");
req.setAttribute("e2", null);
req.setAttribute("e3", new ArrayList());
req.setAttribute("e4", new HashMap());
//跳转显示
req.getRequestDispatcher("el2.jsp").forward(req, resp);
return;
}
}
el2.jsp:
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
<%
pageContext.setAttribute("hello", "pageContext");
String game="cf";
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>">
<title>My JSP 'el2.jsp' starting page</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
</head>
<body>
<h2>El表达式从作用域中获取值的顺序？--->从小到大</h2>
${hello}-----pageContext----request---session---application
<h2>El表达式如何实现跨作用域中取值？---->加入指定的作用域名即可，如requestScope</h2>
${pageScope.hello}-- ${requestScope.hello} ---${sessionScope.hello}---${applicationScope.hello}
<h2>El表达式能否操作局部变量？---->el不能操作</h2>
++${game} ------<%=game %>
<h2>空的判断</h2>
${empty e1}
${empty e2}
${empty e3}
${empty e4}
<h2>"."方式和"[]"方式的区别？--->"."的方式简单方便，"[]"的功能较强大，但是还是推荐用“.”的方式</h2>
${user.realName}------${user["realName"] }----${field}----${user[field]}
<h2>逻辑运算,注意，+不能用于字符串拼接，只能用于算术运算且注意，勿写为<%-- ${4+"a"} --%></h2>
${3+8}-----${3*9}-----${9/3} ---${12-6}----${1/0}----${3+"10"}---${"20"+23}---${2==3?"YES":"NO"}
<h2>当作用域中没有改值的时候，用el获取值会不会报错？--->不会报错，也不会有所显示</h2>
${lol}========
</body>
</html>