code: private void instantiateSingletons() {
          if (log.isDebugEnabled()) {
              log.debug("Initializing singletons in " + this);
          }
  
          Activity activity = context.getDefaultActivity();
          for (BeanRule beanRule : beanRuleRegistry.getIdBasedBeanRules()) {
              instantiateSingleton(beanRule, activity);
          }
          for (Set<BeanRule> beanRuleSet : beanRuleRegistry.getTypeBasedBeanRules()) {
              for (BeanRule beanRule : beanRuleSet) {
                  instantiateSingleton(beanRule, activity);
              }
          }
          for (BeanRule beanRule : beanRuleRegistry.getConfigurableBeanRules()) {
              instantiateSingleton(beanRule, activity);
          }
      }
docs: Instantiate all singletons(non-lazy-init).