code: private static Runnable removeFileSystemRunnable(URI uri) {
      if (Jimfs.systemProvider == null) {
        // TODO(cgdecker): Use Runnables.doNothing() when it's out of @Beta
        return DO_NOTHING;
      }
  
      // We have to invoke the SystemJimfsFileSystemProvider.removeFileSystemRunnable(URI)
      // method reflectively since the system-loaded instance of it may be a different class
      // than the one we'd get if we tried to cast it and call it like normal here.
      try {
        Method method =
            Jimfs.systemProvider.getClass().getDeclaredMethod("removeFileSystemRunnable", URI.class);
        return (Runnable) method.invoke(null, uri);
      } catch (NoSuchMethodException | InvocationTargetException | IllegalAccessException e) {
        throw new RuntimeException(
            "Unable to get Runnable for removing the FileSystem from the cache when it is closed", e);
      }
    }
docs: Returns a {@code Runnable} that will remove the file system with the given {@code URI} from the
  system provider's cache when called.