code: public void registerListenerExtensions() {
      if (listenerExtensionsRegistered) {
        throw H2O.fail("Listeners already registered");
      }
  
      long before = System.currentTimeMillis();
      ServiceLoader<H2OListenerExtension> extensionsLoader = ServiceLoader.load(H2OListenerExtension.class);
      for (H2OListenerExtension ext : extensionsLoader) {
        ext.init();
        listenerExtensions.put(ext.getName(), ext);
      }
      listenerExtensionsRegistered = true;
      registerListenerExtensionsMillis = System.currentTimeMillis() - before;
    }
docs: Register various listener extensions
  
  Use reflection to find all classes that inherit from {@link water.api.AbstractRegister}
  and call the register() method for each.