【理解Spring】Spring初始化所有非懒加载单例Bean

@Override  
public void preInstantiateSingletons() throws BeansException {  
   if (logger.isTraceEnabled()) {  
      logger.trace("Pre-instantiating singletons in " + this);  
   }  
  
   // Iterate over a copy to allow for init methods which in turn register new bean definitions.  
   // While this may not be part of the regular factory bootstrap, it does otherwise work fine.   
   List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);  
  
   // Trigger initialization of all non-lazy singleton beans...  
   for (String beanName : beanNames) {  
      RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);  
      if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {  
         if (isFactoryBean(beanName)) {  
            Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);  
            if (bean instanceof FactoryBean) {  
               FactoryBean<?> factory = (FactoryBean<?>) bean;  
               boolean isEagerInit;  
               if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {  
                  isEagerInit = AccessController.doPrivileged(  
                        (PrivilegedAction<Boolean>) ((SmartFactoryBean<?>) factory)::isEagerInit,  
                        getAccessControlContext());  
               }  
               else {  
                  isEagerInit = (factory instanceof SmartFactoryBean &&  
                        ((SmartFactoryBean<?>) factory).isEagerInit());  
               }  
               if (isEagerInit) {  
                  getBean(beanName);  
               }  
            }  
         }  
         else {  
            getBean(beanName);  
         }  
      }  
   }  
  
   // Trigger post-initialization callback for all applicable beans...  
   for (String beanName : beanNames) {  
      Object singletonInstance = getSingleton(beanName);  
      if (singletonInstance instanceof SmartInitializingSingleton) {  
         StartupStep smartInitialize = this.getApplicationStartup().start("spring.beans.smart-initialize")  
               .tag("beanName", beanName);  
         SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;  
         if (System.getSecurityManager() != null) {  
            AccessController.doPrivileged((PrivilegedAction<Object>) () -> {  
               smartSingleton.afterSingletonsInstantiated();  
               return null;  
            }, getAccessControlContext());  
         }  
         else {  
            smartSingleton.afterSingletonsInstantiated();  
         }  
         smartInitialize.end();  
      }  
   }  
}