docs: protected final Object getEventSessionMutex(HttpSession session) {
          synchronized (WebUtils.getSessionMutex(session)) {
              SerializableObject mutex =
                      (SerializableObject) session.getAttribute(EVENT_SESSION_MUTEX);
              if (mutex == null) {
                  mutex = new SerializableObject();
                  session.setAttribute(EVENT_SESSION_MUTEX, mutex);
              }
  
              return mutex;
          }
      }