为什么输出是null???

Question

String un=getInitParameter("usename");

Answer 1

试着改成：String un=sc.getInitParameter("usename");

Answer 2

"String un=getInitParameter("usename");"改成：String un=sc.getInitParameter("usename");web.xml如下：
<web-app>
           <context-param>
           <param-name>uesname</param-name>
           <param-value>mary</param-value>
           </context-param>
            </web-app>

Answer 3

String un=sc.getInitParameter("usename");

Answer 4

倒啊，还是null
怎么解决呢？