// The SynchronizedObject subclasses don't need a writeObject method since
    // they don't contain any non-transient member variables, while the
    // following writeObject() handles the SynchronizedObject members.

    @GwtIncompatible // java.io.ObjectOutputStream
    @J2ktIncompatible
    private void writeObject(ObjectOutputStream stream) throws IOException {
      synchronized (mutex) {
        stream.defaultWriteObject();