code: public static JaxRx getInstance(final String impl) {
          final String path = Systems.getSystems().get(impl);
          if (path == null) {
              throw new JaxRxException(404, "Unknown implementation: " + impl);
          }
  
          JaxRx jaxrx = INSTANCES.get(path);
          if (jaxrx == null) {
              try {
                  jaxrx = (JaxRx)Class.forName(path).newInstance();
                  INSTANCES.put(path, jaxrx);
              } catch (final Exception ex) {
                  throw new JaxRxException(ex);
              }
          }
          return jaxrx;
      }
docs: Returns the instance for the specified implementation. If the system is
  unknown, throws an exception.
  
  @param impl
  implementation to be checked.
  @return instances