Refer to API, genericServlet has method called getInitParameter(), but what does it refer to? You know, there are two types of init parameter. One from <context-param> and one from <init-param> inside <servlet>.
If you read the javadoc carefully (listed below), you should find that it refers to the servlet init parameter, i.e. ServletConfig. public java.lang.String getInitParameter(java.lang.String name) Returns a String containing the value of the named initialization parameter, or null if the parameter does not exist. See ServletConfig.getInitParameter(java.lang.String). This method is supplied for convenience. It gets the value of the named parameter from the servlet's ServletConfig object.