docs: public static Object tryGetSingleton(Class<?> invokerClass, App app) {
          Object singleton = app.singleton(invokerClass);
          if (null == singleton) {
              if (isGlobalOrStateless(invokerClass, new HashSet<Class>())) {
                  singleton = app.getInstance(invokerClass);
              }
          }
          if (null != singleton) {
              app.registerSingleton(singleton);
          }
          return singleton;
      }