code: public String getStringValue(FacesContext context) {
          String result = null;
          if (hasValueExpression()) {
              result = getStringValueFromModel(context);
          } else {
              result = (null != rawValue) ? rawValue : (String) getValue();
          }
          return result;
      }
docs: <p class="changed_added_2_0">If the value of this parameter comes
  from a <code>ValueExpression</code> return the value of the
  expression, otherwise, return the local value.</p>
  
  @since 2.0