一、使用场景

实现SmartInitializingSingleton的接口后，当所有单例 bean 都初始化完成以后， Spring的IOC容器会回调该接口的 afterSingletonsInstantiated()方法。

主要应用场合就是在所有单例 bean 创建完成之后，可以在该回调中做一些事情，例如：

import org.springframework.beans.factory.ListableBeanFactory;
import org.springframework.beans.factory.SmartInitializingSingleton;
import org.springframework.stereotype.Component;

@Component
public class MyRegister implements SmartInitializingSingleton {

	private ListableBeanFactory beanFactory;

	public MyRegister(ListableBeanFactory beanFactory) {
		this.beanFactory = beanFactory;
	}

	@Override
	public void afterSingletonsInstantiated() {
		String[] beanNames = beanFactory.getBeanNamesForType(IPerson.class);
		for (String s : beanNames) {
			System.out.println(s);
		}
	}

}

二、原理分析

在Spring容器启动时，通过SpringApplication.run()-->SpringApplication.refreshContext()-->SpringApplication.refresh() -->AbstractApplicationContext.refresh()，在这个refresh()函数过程中会调用 finishBeanFactoryInitialization(beanFactory)来提前初始化单例bean，具体方法是调用beanFactory.preInstantiateSingletons()，而这里的beanFactory实例实际为接口 ConfigurableListableBeanFactory 的实现类DefaultListableBeanFactory的实例对象。

查看到DefaultListableBeanFactory.preInstantiateSingletons()的源码如下：

	@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) {
						final 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) {
				final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
				if (System.getSecurityManager() != null) {
					AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
						smartSingleton.afterSingletonsInstantiated();
						return null;
					}, getAccessControlContext());
				}
				else {
					smartSingleton.afterSingletonsInstantiated();
				}
			}
		}
	}

从源码分析中得知，当Spring将所有单例 bean 都实例初始化完成以后，如果存在实现SmartInitializingSingleton接口的bean，那么Spring还会调用到该bean的afterSingletonsInstantiated()方法。