code: public static void closeAtShutdown(Closeable closeable) {
      if (SHUTDOWN_HOOK.addCloseable(closeable)) {
        try {
          // Spark uses SHUTDOWN_HOOK_PRIORITY + 30; this tries to execute earlier
          ShutdownHookManager.get().addShutdownHook(SHUTDOWN_HOOK, FileSystem.SHUTDOWN_HOOK_PRIORITY + 40);
        } catch (IllegalStateException ise) {
          log.warn("Can't close {} at shutdown since shutdown is in progress", closeable);
        }
      }
    }
docs: Adds a shutdown hook that tries to call {@link Closeable#close()} on the given argument
  at JVM shutdown. This integrates with Hadoop's {@link ShutdownHookManager} in order to
  better interact with Spark's usage of the same.
  
  @param closeable thing to close