code: @Override
      public void run() {
          if (state.compareAndSet(State.STARTED, State.STOPPING)) {
              for (final Runnable hook : hooks) {
                  try {
                      hook.run();
                  } catch (final Throwable t) {
                      LOGGER.error(SHUTDOWN_HOOK_MARKER, "Caught exception executing shutdown hook {}", hook, t);
                  }
              }
              state.set(State.STOPPED);
          }
      }
docs: Executes the registered shutdown callbacks.