关于EL表达式随笔记录

时间：2023-03-08 19:03:21
关于EL表达式随笔记录 
<%@ page language="java" contentType="text/html; charset=UTF-8"

    pageEncoding="UTF-8"%>

<%@ page import="java.util.*"%>

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

<html>

<head>

<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

<title>Insert title here</title>

</head>

<body>

    <%

        List<String> list1 = null;

        List<String> list2 = new ArrayList<String>();

        list2.add("aaaaa");

        List<String> list3 = new ArrayList<String>();

        // int a; // 当没有给数组变量赋值的时候会报错(本地变量没有被初始化)

        int[] a = null;

        int[] b = new int[3];

        int[] c = {};

        request.setAttribute("list1", list1);

        request.setAttribute("list2", list2);

        request.setAttribute("list3", list3);

        request.setAttribute("array1", a);

        request.setAttribute("array2", b);

        request.setAttribute("array3", c);

    %>

    list1: ${empty list1 }<br> <!-- true -->

    list2: ${empty list2 }<br> <!-- false -->

    list3: ${empty list3 }<br> <!-- 返回true, 只new一个集合对象不调用add方法对EL表达式来说此集合是空的 -->

    array1: ${empty array1 }<br> <!-- true -->

    array2: ${empty array2 }<br> <!-- false -->

    array3: ${empty array3 }<br> <!-- 返回false, 声明一个空数组对象对EL表达式来说是有值的 -->

</body>

</html>

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