Spring4.1新特性 SmartInitializingSingleton在Ribbon中应用

SmartInitializingSingleton是spring 4.1中引入的新特效，与InitializingBean的功能类似，都是bean实例化后执行自定义初始化，都是属于spring bean生命周期的增强。但是，SmartInitializingSingleton的定义及触发方式方式上有些区别，它的定义不在当前的bean中（a bean's local construction phase），它是回调接口（针对非lazy单例Bean），回调的操作是由spring事件ContextRefreshedEvent触发。

SmartInitializingSingleton是spring bean上的另一个进步，实例化bean和bean自定义初始化可以不在一个类中或一个包中！

Callback interface triggered at the end of the singleton pre-instantiation phase  during {@link BeanFactory} bootstrap. This interface can be implemented by singleton beans in order to perform some initialization after the regular singleton instantiation algorithm, avoiding side effects with accidental early initialization (e.g. from {@link ListableBeanFactory#getBeansOfType} calls). In that sense, it is an alternative to {@link InitializingBean} which gets triggered right at the end of a bean's local construction phase.

This callback variant is somewhat similar to  {@link org.springframework.context.event.ContextRefreshedEvent} but doesn't require an implementation of {@link org.springframework.context.ApplicationListener}, with no need to filter context references across a context hierarchy etc. It also implies a more minimal dependency on just the {@code beans} package and is being honored by standalone {@link ListableBeanFactory} implementations, not just in an {@link org.springframework.context.ApplicationContext} environment.

ContextRefreshedEvent触发以及回调的理解有两层意思：

1. 事件触发，通过ContextRefreshedEvent事件，实际执行AbstractApplicationContext.refresh
2. 回调指的是所有bean实例化之后，再调用SmartInitializingSingleton进行初始化

看源码吧

public abstract class AbstractApplicationContext extends DefaultResourceLoader implements ConfigurableApplicationContext {
	@Override
	public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
				...
				//看这里吧
				// Instantiate all remaining (non-lazy-init) singletons.
				finishBeanFactoryInitialization(beanFactory);
				// Last step: publish corresponding event.
				finishRefresh();
			}
			catch (BeansException ex) {
				if (logger.isWarnEnabled()) {
					logger.warn("Exception encountered during context initialization - " +"cancelling refresh attempt: " + ex);
				}
				// Destroy already created singletons to avoid dangling resources.
				destroyBeans();
				// Reset 'active' flag.
				cancelRefresh(ex);
				// Propagate exception to caller.
				throw ex;
			}
			finally {
				// Reset common introspection caches in Spring's core, since we
				// might not ever need metadata for singleton beans anymore...
				resetCommonCaches();
			}
		}
	}
	protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
		// Initialize conversion service for this context.
		if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
				beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
			beanFactory.setConversionService(
					beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
		}
		// Register a default embedded value resolver if no bean post-processor
		// (such as a PropertyPlaceholderConfigurer bean) registered any before:
		// at this point, primarily for resolution in annotation attribute values.
		if (!beanFactory.hasEmbeddedValueResolver()) {
			beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
		}
		// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
		String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
		for (String weaverAwareName : weaverAwareNames) {
			getBean(weaverAwareName);
		}
		// Stop using the temporary ClassLoader for type matching.
		beanFactory.setTempClassLoader(null);
		// Allow for caching all bean definition metadata, not expecting further changes.
		beanFactory.freezeConfiguration();
		//其实执行的是：DefaultListableBeanFactory.preInstantiateSingletons
		// Instantiate all remaining (non-lazy-init) singletons.
		beanFactory.preInstantiateSingletons();
	}
}
public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory
		implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {
	@Override
	public void preInstantiateSingletons() throws BeansException {
		if (logger.isDebugEnabled()) {
			logger.debug("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 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) {
						final FactoryBean factory = (FactoryBean) bean;
						boolean isEagerInit;
						if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
							isEagerInit = AccessController.doPrivileged((PrivilegedAction)
											((SmartFactoryBean) factory)::isEagerInit,
									getAccessControlContext());
						}
						else {
							isEagerInit = (factory instanceof SmartFactoryBean &&
									((SmartFactoryBean) factory).isEagerInit());
						}
						if (isEagerInit) {
							getBean(beanName);
						}
					}
				}
				else {
					getBean(beanName);
				}
			}
		}
		//触发SmartInitializingSingleton的afterSingletonsInstantiated
		// Trigger post-initialization callback for all applicable beans...
		for (String beanName : beanNames) {
			Object singletonInstance = getSingleton(beanName);
			if (singletonInstance instanceof SmartInitializingSingleton) {
				final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
				if (System.getSecurityManager() != null) {
					AccessController.doPrivileged((PrivilegedAction