code: protected StringValue getVariableValue(String parameterName) {
          RequestCycle cycle = RequestCycle.get();
          WebRequest webRequest = (WebRequest) cycle.getRequest();
          StringValue value = webRequest.getRequestParameters().getParameterValue(parameterName);
          return value;
      }
docs: Reads the value of the given javascript variable from the AJAX request.
  
  @param parameterName the parameter name of the javascript expression whose value to
  read. The parameterName must have been specified earlier when
  calling {@link #addJavaScriptValue(String, String)}.
  @return the string representation of the javascript expression's value