code: public static Managed registerInjector(final Application application, final Injector injector) {
          Preconditions.checkNotNull(application, "Application instance required");
          Preconditions.checkArgument(!INJECTORS.containsKey(application),
                  "Injector already registered for application %s", application.getClass().getName());
          INJECTORS.put(application, injector);
          return new Managed() {
              @Override
              public void start() throws Exception {
                  // not used
              }
  
              @Override
              public void stop() throws Exception {
                  INJECTORS.remove(application);
              }
          };
      }
docs: Used internally to register application specific injector.
  
  @param application application instance
  @param injector    injector instance
  @return managed object, which must be registered to remove injector on application stop