el表达式的取值默认顺序:
pageScope  requestScope  sessionScope  applicationScope

结构，采用.导航，也称为存取器   ${user.name}  ${user.class.name}
输出map   ${mapValue.key1}
输出数组   ${strArray[1]}
输出list   ${list[1].name}

el表达式的运算
${1+2}=       3
${10/5}=      2.0
${10%3}=      1
${10 div 2}=  5.0
${10 mod 4}=  2

el表达式对应的 运算符
等于  ==    eq
不等于  !=    ne
大于  >     gt
小于  <     lt
大于等于  >=    ge
小于等于  <=    le
与  &&    or
或  ||    and
非  !     not
加  +
减  -
乘  *
除以  /     div
求莫  %     mod

el表达式判断是否为空
request.setAttribute("value1",null)                    ${empty requestScope.value1} = true
request.setAttribute("value2","")                      ${empty requestScope.value1} = true
request.setAttribute("value3",new ArrayList())         ${empty requestScope.value1} = true
request.setAttribute("value4","i love you")            ${empty requestScope.value1} = false
request.setAttribute("value4","i love you")            ${!empty requestScope.value1} = true