code: @Deprecated
      public void weakAddWatcher(Listener watcher) {
          if (watcher == null) {
              throw new IllegalArgumentException("Null watcher added");
          }
  
          synchronized (mutex) {
              if (watcherExecutor.isShutdown()) {
                  throw new IllegalStateException("Adding watcher on closed FileWatcher");
              }
              watchers.add(new WeakReference<>(watcher)::get);
          }
      }
docs: Add a non-persistent file watcher.
  
  @param watcher The watcher to add.
  @deprecated Use {@link #weakAddWatcher(File, Listener)}.