code: static <T> T loadService(Class<T> ofType) {
          List<String> implClasses = loadServicesImplementations(ofType);
          for (String implClass : implClasses) {
              T result = attemptLoad(ofType, implClass);
              if (result != null) {
                  if (LOG.isLoggable(Level.FINEST)) {
                      LOG.finest("Selected " + ofType.getSimpleName()
                              + " implementation: "
                              + result.getClass().getName());
                  }
                  return result;
              }
          }
          throw(new IllegalStateException(
                  "Could not load " + ofType.getName() + " implementation"));
      }
docs: Probe for and select an implementation of the specified service
  type by using the a modified Jar SPI mechanism.  Modified in that
  the system properties will be checked to see if there is a value
  set for the naem of the class to be loaded.  If so, that value is
  treated as the class name of the first implementation class to be
  attempted to be loaded.  This provides a (unsupported) mechanism
  to insert other implementations.  Note that the supported mechanism
  is by properly ordering the classpath.
  
  @return service instance
  @throws IllegalStateException is no service implementations could be
  instantiated