Spring源码分析（三） bean的生命周期开篇

a、在这一步具体做什么什么事情？

完成实例化和初始化，初始化剩下的单实例（非懒加载的）

b、Spring创建对象的核心步骤又是什么？

1、AbstractApplicationContext#finishBeanFactoryInitialization()

这个方法中主要做了什么事情？
Bean的初始化，生命周期、循环依赖、AOP
其中里面有几个扩展点

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
		// Initialize conversion service for this context.
		// 为上下文初始化类型转换器
		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.
		// 如果beanFactory之前没有注册嵌入值解析器，则注册默认的嵌入值解析器，主要用于注解属性值的解析
		if (!beanFactory.hasEmbeddedValueResolver()) {
			beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
		}

		// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
		// 尽早初始化loadTimeWeaverAware bean,以便尽早注册它们的转换器
		String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
		for (String weaverAwareName : weaverAwareNames) {
			getBean(weaverAwareName);
		}

		// Stop using the temporary ClassLoader for type matching.
		// 禁止使用临时类加载器进行类型匹配
		beanFactory.setTempClassLoader(null);

		// Allow for caching all bean definition metadata, not expecting further changes.
		// 冻结所有的bean定义，说明注册的bean定义将不被修改或任何进一步的处理
		beanFactory.freezeConfiguration();

		// Instantiate all remaining (non-lazy-init) singletons.
		// 实例化剩下的单例对象
		beanFactory.preInstantiateSingletons();
	}

扩展点1、转换服务

1、类型转换器，具体的可以参考这篇文章
https://zhuanlan.zhihu.com/p/466596026?utm_id=0

2、DefaultListableBeanFactory#preInstantiateSingletons()

该方法准备要进行实例化和初始化的关键方法，大致流程如下:

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.
		// 将所有BeanDefinition的名字创建一个集合
		List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

		// Trigger initialization of all non-lazy singleton beans...
		// 触发所有非延迟加载单例bean的初始化，遍历集合的对象
		for (String beanName : beanNames) {
			// 合并父类BeanDefinition
 			RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
			// 条件判断，抽象，单例，非懒加载
			if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
				// 判断是否实现了FactoryBean接口
				if (isFactoryBean(beanName)) {
					// 根据&+beanName来获取具体的对象
					Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
					// 进行类型转换
					if (bean instanceof FactoryBean) {
						FactoryBean<?> factory = (FactoryBean<?>) bean;
						// 判断这个FactoryBean是否希望立即初始化
						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());
						}
						//  如果希望急切的初始化，则通过beanName获取bean实例
						if (isEagerInit) {
							getBean(beanName);
						}
					}
				}
				else {
					// 如果beanName对应的bean不是FactoryBean，只是普通的bean，通过beanName获取bean实例
					getBean(beanName);
				}
			}
		}

		// Trigger post-initialization callback for all applicable beans...
		// 遍历beanNames，触发所有SmartInitializingSingleton的后初始化回调
		for (String beanName : beanNames) {
			// 获取beanName对应的bean实例
			Object singletonInstance = getSingleton(beanName);
			// 判断singletonInstance是否实现了SmartInitializingSingleton接口
			if (singletonInstance instanceof SmartInitializingSingleton) {
				// 类型转换
				SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
				// 触发SmartInitializingSingleton实现类的afterSingletonsInstantiated方法
				if (System.getSecurityManager() != null) {
					AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
						smartSingleton.afterSingletonsInstantiated();
						return null;
					}, getAccessControlContext());
				}
				else {
					smartSingleton.afterSingletonsInstantiated();
				}
			}
		}
	}

2.1 其中的FactoryBean是用来做什么的？

首先都是对象工厂，用来创建对象
如果使用BeanFactory的接口，那么必须要严格遵守springbean的生命周期接口，从实例化，到初始化，到invokeAwareMethod.invokeInitMethod,before,after,此流程非常复杂且麻烦，如果需要一种更加便捷简单的方式创建，怎么办？所以有了FactoryBean这个接口，不需要遵循此创建顺序

2.2 当使用factoryBean接口来创建对象的时候，我一共创建了几个对象？