1 EL表达式
1.1 EL的概述
- EL，全名为Expression Language。
- 主要作用：
- ①EL表达式主要用于替换jsp页面中的脚本表达式，以便于从各种类型的web域中检索java对象（某个web域中的对象），获取数据（访问javabean的属性、访问List集合、访问Map集合、访问数组）。
- ②执行运算。
- ③利用EL表达式可以在jsp页面中执行一些基本的关系运算、逻辑运算和算数运算，以便于在jsp页面完成一些简单的逻辑运算。
- ④获取web开发常用的对象。
- ⑤EL表达式定义了一些隐式的对象，利用这些隐式对象，web开发人员可以很轻松的获取web常用对象的引用，从而获取这些对象中的数据。
- ⑥调用案卷方法。
- ⑦EL表达式允许用户开发自定义的EL函数，以便于在jsp页面通过EL表达式调用java类的方法。
1.2 EL的基本语法
- ${标识符}
- EL表达式在执行的时候，会调用pageContext.findAttribute("")方法，用标识符作为关键字。分别从page、request、session、application四个域对象中查找对应的对象，如果找到了，就返回对应的对象；否则返回""。（不是null）
- Scriptlet vs EL
<% ???List<User> users = (List<User>) request.getSession().getAttribute("users"); ???out.write(users.get(0).getUsername());%>${users[0].name}1.3 获取数据
- 示例：EL获取对象
package com.domain;import java.io.Serializable;/** * 2017/11/6 * 说明： */public class Student implements Serializable { ???private String name; ???private int age; ???private String gender; ???public String getName() { ???????return name; ???} ???public void setName(String name) { ???????this.name = name; ???} ???public int getAge() { ???????return age; ???} ???public void setAge(int age) { ???????this.age = age; ???} ???public String getGender() { ???????return gender; ???} ???public void setGender(String gender) { ???????this.gender = gender; ???}}<%@ page import="com.domain.Student" %><%@ page contentType="text/html;charset=UTF-8" language="java" %><html><head> ???<title>EL表达式</title></head><body> ???<% ???????Student student = new Student(); ???????student.setName("刘谦"); ???????student.setAge(29); ???????student.setGender("男"); ???????pageContext.setAttribute("student",student); ???%> ???${student} ???<%-- ???????${student} ?相等于 ???????Student student = (Student)pageContext.findAttribute("student"); ???????out.write(student) ???--%></body></html>- 示例：EL获取javaBean对象的属性
<%@ page import="com.domain.Student" %><%@ page contentType="text/html;charset=UTF-8" language="java" %><html><head> ???<title>EL表达式</title></head><body> ???<% ???????Student student = new Student(); ???????student.setName("刘谦"); ???????student.setAge(29); ???????student.setGender("男"); ???????pageContext.setAttribute("student",student); ???%> ???${student.name} ???<%-- ???????${student} ?相等于 ???????Student student = (Student)pageContext.findAttribute("student"); ???????out.write(student.getName()) ???--%></body></html>- 示例：获取javaBean关联对象的属性
package com.domain;/** * 2017/11/6 * 说明： */public class Address { ???private String province; ???private String city; ???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; ???}}package com.domain;import java.io.Serializable;/** * 2017/11/6 * 说明： */public class Student implements Serializable { ???private String name; ???private int age; ???private String gender; ???private Address address = new Address(); ???public Address getAddress() { ???????return address; ???} ???public void setAddress(Address address) { ???????this.address = address; ???} ???public String getName() { ???????return name; ???} ???public void setName(String name) { ???????this.name = name; ???} ???public int getAge() { ???????return age; ???} ???public void setAge(int age) { ???????this.age = age; ???} ???public String getGender() { ???????return gender; ???} ???public void setGender(String gender) { ???????this.gender = gender; ???}}<%@ page import="com.domain.Student" %><%@ page import="com.domain.Address" %><%@ page contentType="text/html;charset=UTF-8" language="java" %><html><head> ???<title>EL表达式</title></head><body> ???<% ???????Address address = new Address(); ???????address.setProvince("江苏省"); ???????address.setCity("泰州市"); ???????Student student = new Student(); ???????student.setName("刘谦"); ???????student.setAge(29); ???????student.setGender("男"); ???????student.setAddress(address); ???????pageContext.setAttribute("student",student); ???%> ???${student.address.province}</body></html>- 示例：获取数组中的元素
<%@ page import="com.domain.Student" %><%@ page import="com.domain.Address" %><%@ page contentType="text/html;charset=UTF-8" language="java" %><html><head> ???<title>EL表达式</title></head><body> ???<% ???????String[] strs = {"aa","bb"}; ???????pageContext.setAttribute("strs",strs); ???%> ???${strs[0]}</body></html>- 示例：获取Map集合中的元素
<%@ page import="com.domain.Student" %><%@ page import="com.domain.Address" %><%@ page import="java.util.HashMap" %><%@ page import="java.util.Map" %><%@ page contentType="text/html;charset=UTF-8" language="java" %><html><head> ???<title>EL表达式</title></head><body> ???<% ???????Map<String,String> map = new HashMap<>(); ???????map.put("aa","bb"); ???????map.put("cc","dd"); ???????pageContext.setAttribute("map",map); ???%> ???${map.aa}</body></html>- 示例：获取List集合中的元素
<%@ page import="com.domain.Student" %><%@ page import="com.domain.Address" %><%@ page import="java.util.HashMap" %><%@ page import="java.util.Map" %><%@ page import="java.util.ArrayList" %><%@ page import="java.util.List" %><%@ page contentType="text/html;charset=UTF-8" language="java" %><html><head> ???<title>EL表达式</title></head><body> ???<% ???????List<String> list = new ArrayList<>(); ??????list.add("aa"); ??????list.add("bb"); ???????pageContext.setAttribute("list",list); ???%> ???${list[0]}</body></html>1.4 EL的运算
- 算数
- 逻辑
- 关系
- empty运算符：判断一个字符串是否为""或其他对象是否为null，同时还能判断是集合中是否含有元素。
<%@ page import="com.domain.Student" %><%@ page import="com.domain.Address" %><%@ page import="java.util.HashMap" %><%@ page import="java.util.Map" %><%@ page import="java.util.ArrayList" %><%@ page import="java.util.List" %><%@ page contentType="text/html;charset=UTF-8" language="java" %><html><head> ???<title>EL表达式</title></head><body> ???<% ???????List<String> list = new ArrayList<>(); ???????pageContext.setAttribute("s",""); ???????pageContext.setAttribute("s1",null); ???????pageContext.setAttribute("s2",list); ???%> ???${empty s1}<%--true--%> ???${empty s1}<%--true--%> ???${empty s2}<%--true--%></body></html>- 三元运算符：
<%@ page import="com.domain.Student" %><%@ page import="com.domain.Address" %><%@ page import="java.util.HashMap" %><%@ page import="java.util.Map" %><%@ page import="java.util.ArrayList" %><%@ page import="java.util.List" %><%@ page contentType="text/html;charset=UTF-8" language="java" %><html><head> ???<title>EL表达式</title></head><body> ???<% ???????pageContext.setAttribute("gender","男"); ???%> ???<input type="radio" name="gender" value="男" ${gender == "男"?"checked=‘true‘":""}>男<br/> ???<input type="radio" name="gender" value="女" ${gender == "女"?"checked=‘true‘":""}>女<br/></body></html>1.5 EL的隐式对象（11个）
| EL隐式对象的名称 | 表示的类型 | JSP的隐式对象 | 备注 |
| pageContext | javax.servlet.jsp.PageContext | pageContext | 一样 |
| pageScope | java.utl.Map | 没有 | 页面域范围的数据 |
| sessionScope | java.utl.Map | 没有 | reqeust域范围的数据 |
| requestScope | java.utl.Map | 没有 | session域范围的数据 |
| aplicattionScope | java.utl.Map | 没有 | application域范围的数据 |
| header | java.utl.Map | 没有 | key：请求消息头 value：消息头对应的值 |
| headerValues | java.utl.Map | 没有 | key：请求消息头 value：消息头对应的值数组 |
| param | java.utl.Map | 没有 | key：请求参数名，value：请求参数值 |
| paramValues | java.utl.Map | 没有 | key：请求参数名，value：请求参数值数组 |
| cookie | java.utl.Map | 没有 | key：Cookie对象的name ,value是Cookie对象本身 |
| initParam | java.utl.Map | 没有 | key：全局参数的name,value是全局参数的值 |
- 示例：
<%@ page contentType="text/html;charset=UTF-8" language="java" %><html><head> ???<title>EL表达式</title></head><body> ???<% ???????pageContext.setAttribute("p","p"); ???????request.setAttribute("p","rp"); ???????session.setAttribute("p","sp"); ???????application.setAttribute("p","ap"); ???%> ???${p} <%=pageContext.findAttribute("p")%> <br/> ???${pageScope.p} <%=pageContext.getAttribute("p")%> <br/> ???${requestScope.p} <%=request.getAttribute("p")%> <br/> ???${sessionScope.p} <%=session.getAttribute("p")%> <br/> ???${applicationScope.p} <%=application.getAttribute("p")%> <br/></body></html>- 示例：
<%@ page contentType="text/html;charset=UTF-8" language="java" %><html><head> ???<title>EL表达式</title></head><body> ???${header["Accept-Encoding"]}<br/> ???<%=request.getHeader("Accept-Encoding")%></body></html>1.6 定义和使用EL函数
- EL不支持字符串操作的
1.6.1 定义EL函数的开发步骤
- ①定义一个普通类，提供实现功能的静态方法（EL函数只能调用静态方法）。
package com;/** * 2017/11/6 * 说明： */public class ELFunction { ???????public static String toUpperCase(String str) { ???????return str.toUpperCase(); ???}}- ②EL函数的配置：在WEB-INF目录下建立一个扩展名为*.tld的xml文件，此时新建的*.tld的名称为elfunction.tld
<?xml version="1.0" encoding="UTF-8"?><taglib xmlns="http://java.sun.com/xml/ns/javaee" ???????xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" ???????xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-jsptaglibrary_2_1.xsd" ???????version="2.1"> ???<tlib-version>1.0</tlib-version> ???<short-name>elfunction</short-name> ???<uri>http://www.xuweiwei.com/jsp/functions</uri> ???<function> ???????<name>toUpperCase</name> ???????<function-class>com.ELFunction</function-class> ???????<function-signature>java.lang.String toUpperCase(java.lang.String)</function-signature> ???</function></taglib>
- ③在jsp中引入tld文件
<%@ page contentType="text/html;charset=UTF-8" language="java" %><%@ taglib prefix="elfunction" uri="http://www.xuweiwei.com/jsp/functions" %><html><head> ???<title>EL表达式</title></head><body> ???<% ???????pageContext.setAttribute("s","abcde"); ???%> ???${elfunction:toUpperCase(s)}</body></html>EL表达式和JSTL标签库
原文地址：http://www.cnblogs.com/xuweiweiwoaini/p/7792413.html