EL表达式从哪里取值？一般情况下，从作用域对象中取值
${a}
EL表达式优先从哪个作用域对象中取值？默认情况下从最小作用域取值
${scope}
EL表达式从指定作用域对象中取值：pageScope、requestScope、sessionScope、applicationScope
${pageScope.scope} --- ${requestScope.scope} --- ${sessionScope.scope} --- ${applicationScope.scope}
EL表达式可以解析什么类型的数据？基本数据类型、对象类型、复合对象类型、Map、List
${list[2]} --- ${map.address} --- ${user.realname} --- ${user.addr.town}
EL也可以取请求参数的数据：param、paramValues
${param.name} --- ${paramValues.fav[2]}
EL表达式的[]内也可以做EL表达式的解析
${user.realname} --- ${user["realname"]} --- ${user[param.type]}
EL表达式可以做为空判断：空字符串、null、空map、空list这几个值为true
${empty aaa} --- ${empty bbb} --- ${empty ccc} --- ${empty ddd} --- ${empty scope}
EL表达式可以做基本的逻辑运算，和数学运算
EL表达式没有字符串拼接的概念，也没有字符的概念
${5+4} --- ${5-4} --- ${5*4} --- ${5/4} --- ${5%4} --- ${5+"4"} ---
${5>4} --- ${5 < 4} --- ${5>=4} --- ${5 le 4} --- ${5 eq 4}
EL表达式中pageContext可以获取pageContext作用域对象本身
${pageContext}
