code: public int doStartTag()
              throws JspException {
          Tag parent = getParent();
          if(parent == null || !(parent instanceof AbstractCallMethod)) {
              String msg = Bundle.getErrorString("Tags_MethodParameter_invalidParent");
              registerTagError(msg, null);
              reportErrors();
              return SKIP_BODY;
          }
  
          if(!_isNull && _value == null) {
              String msg = Bundle.getErrorString("Tags_MethodParameter_undefinedValue");
              registerTagError(msg, null);
              reportErrors();
              return SKIP_BODY;
          }
  
          return SKIP_BODY;
      }
docs: Start this tag's lifecycle.  Verify that this tag is nested within
  a {@link AbstractCallMethod} tag and that one of the "null" and "value"
  attributes are set.
  
  @return {@link #SKIP_BODY}
  @throws JspException if an error occurs getting the parameter