How to use commonly used el expressions

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8" isELIgnored="false"%>

Generally add this isELIgnored="false" before it can be used${}

##1.Constant

There must be no spaces between $ and {}, and there can be spaces in {}, such as: ${user.username }

Boolean type: ${true}

## Integer type: ${10}

## Floating point type: ${10.5}

String type: ${"username"}

Empty type: ${null} page does not display

## 2. Four arithmetic operations

EL arithmetic operators: +, -, *,/or p, % or mod

EL relational operator: == or eq,! =or ne, or gt, =or ge

# # EL logical operators: && or and, || or or,! Or not

## Other operations: ${ A == a ? B : C}

empty (unary operator): Used to determine whether the value is null (empty), return true, otherwise return false.

## ${empty A} ${not empty.A}

ELWhen operating on the variables in the expression, it Use pageContext.findAttribute("") to find variables

Search from the page, request, session, and application domains in sequence

If the value is not found in any of these four fields. Just return null, or you can specify to search for

in a specific field. Format:

${pageScope.param}

##${requestScope.param}

${sessionScope.param}

${applicationScope.param}

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