Spring 之 BeanPostProcessor

一、相关概念

BeanPostProcessor - Bean 的后置处理器，用于Bean在初始化前后对Bean Definetion进行动态操作。

public interface BeanPostProcessor {
   
    //该方法在Bean初始化操作之前调用
	@Nullable
	default Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
   
		return bean;
	}
    //该方法在Bean初始化完成后调用
	@Nullable
	default Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
   
		return bean;
	}
}

简单来说，Spring容器内开始是以BeanDefinition形式存储POJO对象，第一次调用getBean()获取Bean对象时，会根据BeanDefinition内的信息对Bean进行初始化操作，若此时容器中存在BeanPostProcessor类型的Bean，则会在Bean初始化前调用所有BeanPostProcessor对象的postProcessBeforeInitialization方法，在Bean初始化完成后，调用所有BeanPostProcessor对象的postProcessAfterInitialization方法。

设置Bean的初始化方式有三种：

• 先执行 @PostConstruct
• 再执行 InitializingBean#afterPropertiesSet()
• 最后执行 `@Bean(initMethod =“”)

ps：这里有几个问题，初始化顺序为什么是这样？这个与BeanPostProcessor有关联吗？

二、源码探究

上面我们谈到了BeanPostProcessor内的方法是在Bean初始化前后调用的，以及简单介绍了Bean的三种初始化方式以及调用顺序，那么接下来我们就去看一下Bean初始化的相关源码。

众所周知，Spring容器的启动过程是从 AbstractApplicationContext类中的refresh() 方法开始的(纳尼，你还不知道，那你现在立刻记住)。refresh()方法中调用了finishBeanFactoryInitialization(beanFactory);该方法就是用来初始化容器中剩余的还未被初始化的Bean。我们就从这个方法开始看起

	protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
   
		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));
		}
		if (!beanFactory.hasEmbeddedValueResolver()) {
   
			beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
		}
        //获取 LoadTimeWeaverAware 类型的所有BeanName，此时获取的都是为初始化的Bean的名称
		String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
		for (String weaverAwareName : weaverAwareNames) {
   
            //该方法就是获取Bean，若是第一次获取，则对Bean进行初始化
			getBean(weaverAwareName);
		}
		beanFactory.setTempClassLoader(null);
		beanFactory.freezeConfiguration();
        //实例化所有剩余（非惰性初始化）单例。该方法是接口定义的方法，最终由子类 DefaultListableBeanFactory 实现
		beanFactory.preInstantiateSingletons();
	}

	// DefaultListableBeanFactory 类，预实例化单例
	@Override
	public void preInstantiateSingletons() throws BeansException {
   
		// 获取所有尚未初始的beanNames，
		List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

		//触发所有非惰性单例 bean 的初始化...
		for (String beanName : beanNames) {
   
			RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
            //这里判断Bean是不抽象的、单例的、非惰性加载的
			if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
   
                //判断是不是factoryBean
				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 {
   
                    //这里进行初始化
					getBean(beanName);
				}
			}
		}

		// 触发所有适用 bean 的初始化后回调...
		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