private Class<?> getServerActionClass(String classname) throws AmbariException{
      Class<?> actionClass = null;
      try {
        actionClass = Class.forName(classname);
        if (actionClass == null) {
          LOG.warn(String.format("Unable to load server action class: %s from Ambari", classname));
        }
      } catch (ClassNotFoundException e) {
        LOG.error(String.format("Unable to load server action class: %s", classname), e);
      }
      return actionClass;
    }
Load server action classes defined in Ambari source code
ambari-server/src/main/java/org/apache/ambari/server/serveraction/ServerActionExecutor.java