EL表达式判断字符串为空(条件连接时用and &&、or ||、not ! ):
几种情况如下:
=null true
="" true
String username.. ---> ${username==''}..
=null false
="" true
String username.. ---> ${username==null}..
=null true
="" false
String username.. ---> ${username=='null'}..
=null false
="" false