code: protected final Object convert(Class targetType, String value) {
          Converter converter = Stapler.lookupConverter(targetType);
          if (converter==null)
              throw new IllegalArgumentException("Unable to convert to "+targetType);
  
          return converter.convert(targetType,value);
      }
docs: Helper method for {@link #parse(StaplerRequest, Annotation, Class, String)} to convert to the right type
  from String.