EL表达式

概念

Expression Language 表达式语言

作用

替换和简化jsp页面中java代码的编写

语法：

${表达式}

注意：

jsp默认支持el表达式的。如果要忽略el表达式

设置jsp中page指令中：isELIgnored="true" 忽略当前jsp页面中所有的el表达式
\${表达式} ：忽略当前这个el表达式

使用：

运算

运算符：

算数运算符： + - * /(div) %(mod)
比较运算符： > < >= <= == !=
逻辑运算符： &&(and) ||(or) !(not)

空运算符：empty

功能：用于判断字符串、集合、数组对象是否为null或者长度是否为0
${empty list}:判断字符串、集合、数组对象是否为null或者长度为0
${not empty str}:表示判断字符串、集合、数组对象是否不为null并且 长度>0

示例：

<%--
  Created by IntelliJ IDEA.
  User: Liu Fei
  Date: 2020/11/3
  Time: 16:19
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
    <h3>算术运算符</h3>
    ${3+4}<br>
    ${3/4}<br>
    ${3 div 4}<br>
    ${3 % 4}<br>
    ${3 mod 4}<br>    
    <h3>比较运算符</h3>
    ${3==4}<br>    
    <h3>逻辑运算符</h3>
    ${3>4 && 3<4}<br>
    ${3>4 and 3<4}<br>
</body>
</html>

效果展示：

获取值

el表达式只能从域对象中获取值

语法

`${域名称.键名}`

从指定域中获取指定键的值

域名称：

pageScope –> pageContext

requestScope –> request

sessionScope –> session

applicationScope –> application（ServletContext）

示例：在request域中存储了name=张三 ，使用${requestScope.name}进行获取

<%--
  Created by IntelliJ IDEA.
  User: Liu Fei
  Date: 2020/11/3
  Time: 16:19
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
   <%
        request.setAttribute("name","张三");
        session.setAttribute("age","23");
   %>

   <h3>获取值</h3>
    ${requestScope.name}
    ${sessionScope.age}
</body>
</html>

`${键名}`

表示依次从最小的域中查找是否有该键对应的值，直到找到为止。

示例：

<%--
  Created by IntelliJ IDEA.
  User: Liu Fei
  Date: 2020/11/3
  Time: 16:19
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
   <%
        request.setAttribute("name","张三");
        session.setAttribute("name","23");
   %>
   <h3>获取值</h3>
    ${name}
</body>
</html>

获取对象、List集合、Map集合的值

1.对象：${域名称.键名.属性名}

本质上会去调用对象的getter方法

示例：

创建JavaBean对象

package com.uestc.domain;
import java.text.SimpleDateFormat;
import java.util.Date;
public class User {
    private String name;
    private int age;
    private Date birthday;

    public String getName() {
        return name;
    }

    /**
     * 逻辑视图
     * @return
     */
    public String getBitStr(){
        if(birthday!=null){
            //格式化日期对象
            SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
            //返回字符串即可
            return sdf.format(birthday);
        }else{
            return "";
        }
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public Date getBirthday() {
        return birthday;
    }
    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }
}

创建el.jsp

<%--
  Created by IntelliJ IDEA.
  User: Liu Fei
  Date: 2020/11/3
  Time: 17:08
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ page import="com.uestc.domain.User" %>
<%@ page import="java.util.Date" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
    <%
        User user=new User();
        user.setName("张三");
        user.setAge(23);
        user.setBirthday(new Date());
        request.setAttribute("user",user);
    %>
    <h3>使用el获取对象中的值</h3>
    ${requestScope.user}<br>

<%-- 通过的是对象的属性来获取
    setter或getter方法，去掉set或者get,在将剩余部分首字母变为小写
    setName --> Name -->name

--%>
    ${requestScope.user.name}<br>
    ${requestScope.user.age}<br>
    ${requestScope.user.bitStr}<br>
    ${requestScope.user.birthday.month}<br>
</body>
</html>

效果展示：

2. List集合：${域名称.键名[索引]}

示例：

<%--
  Created by IntelliJ IDEA.
  User: Liu Fei
  Date: 2020/11/3
  Time: 17:08
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ page import="com.uestc.domain.User" %>
<%@ page import="java.util.Date" %>
<%@ page import="java.util.List" %>
<%@ page import="java.util.ArrayList" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
    <%
        User user=new User();
        user.setName("张三");
        user.setAge(11);
        user.setBirthday(new Date());
        List list=new ArrayList();
        list.add(1);
        list.add(2);
        list.add(3);
        list.add(user);
        request.setAttribute("list",list);
    %>
    <h3>el获取list的值</h3>
    ${list}<br>
    ${list[0]}<br>
    ${list[1]}<br>
    ${list[2]}<br>
    ${list[3]}<br>
    ${list[3].name}<br>
    ${list[3].age}<br>
    ${list[3].bitStr}<br>
</body>
</html>

显示效果:

3. Map集合：

${域名称.键名.key名称}
${域名称.键名["key名称"]}

<%--
  Created by IntelliJ IDEA.
  User: Liu Fei
  Date: 2020/11/3
  Time: 17:08
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ page import="com.uestc.domain.User" %>
<%@ page import="java.util.*" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
    <%
        User user=new User();
        user.setName("张三");
        user.setAge(11);
        user.setBirthday(new Date());
        Map map=new HashMap();
        map.put("sname","李四");
        map.put("gender","男");
        map.put("user",user);
        request.setAttribute("map",map);
    %>
    <h3>el获取map的值</h3>
    ${map.gender}<br>
    ${map["gender"]}<br>
    ${map.sname}<br>
    ${map["sname"]}<br>
    ${map.user.name}
</body>
</html>

效果展示：

隐式对象

el表达式中有11个隐式对象

pageContext：

获取jsp其他八个内置对象

1	${pageContext.request.contextPath}：动态获取虚拟目录JSTL