code: @Deprecated
      public void addWatcher(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(() -> watcher);
          }
      }
docs: Add a file watcher that is persistent. If the reference from the
  file watcher to the watcher itself should not prevent garbage collection,
  use the {@link #weakAddWatcher(Listener)} method.
  
  @param watcher The watcher to add.
  @deprecated Use {@link #addWatcher(File, Listener)}