code: @Override
      protected ApplicationContext loadParentContext(ServletContext servletContext) {
          String name = servletContext.getInitParameter("embeddedWebContext");
          if (name == null) {
              throw new IllegalStateException("context param [embeddedWebContext] not specified for ["
                      + servletContext.getServletContextName() + "]");
          }
  
          ApplicationContext appContext = appContextMap.get(name);
          if (appContext == null) {
              throw new IllegalStateException("application context not registered for [" + name + "]");
          }
  
          return appContext;
      }
docs: Loads the parent {@link ApplicationContext} for the {@link ServletContext} by retrieving the previously
  {@link #registerAppContext(String, ApplicationContext) registered} one for the embedded web context.