EL表达式关系运算 备案

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