Spring扩展点系列-SmartInitializingSingleton

简介

SmartInitializingSingleton是spring的扩展点之一，主要用于在Spring容器启动完成时进行扩展操作，其只有一个方法afterSingletonsInstantiated()。
触发的关键点是在单例预实例化阶段结束时调用，保证所有常规单例 bean 都已创建。接口的bean的作用域必须是单例才会触发。

源码分析

方法 preInstantiateSingletons实例化非懒加载的单例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);
			// 如果实例实现了SmartInitializingSingleton，执行afterSingletonsInstantiated方法。
			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 {
					// 所有的非懒加载单例bean都创建完成之后调用
					smartSingleton.afterSingletonsInstantiated();
				}
				smartInitialize.end();
			}
		}
	}

应用场景

SmartInitializingSingleton 提供了一种在所有单例 Bean 实例化后但在应用完全启动之前执行自定义逻辑的机制。它主要用于那些需要在所有 Bean 完全实例化后进行全局初始化、处理依赖关系、配置外部库或确保初始化顺序的场景。

示例代码

@Slf4j
@Configuration
public class ExtendSmartInitializingSingleton implements SmartInitializingSingleton {

    private ListableBeanFactory beanFactory;

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

    @Override
    public void afterSingletonsInstantiated() {
        String[] beanNames = beanFactory.getBeanNamesForType(TestAutowired.class);
        for (String s : beanNames) {
            log.info("=================================");
            log.info(s);
        }
    }
}

运行示例