code: static synchronized List<Watcher> get(MavenSession session) {
          List<Watcher> watchers = (List<Watcher>) session.getExecutionProperties().get(WATCHERS_KEY);
          if (watchers == null) {
              watchers = new ArrayList<>();
              session.getExecutionProperties().put(WATCHERS_KEY, watchers);
          }
          return watchers;
      }
docs: Gets the list of watchers from the given MavenSession.
  
  @param session the Maven session
  @return the list of watcher, empty if none. Modifying the resulting list, updates the stored list.