Spring5 源码阅读笔记（1.3）registerBeanPostProcessors(beanFactory) 注册Bean后置处理器

上一节 Spring5 源码阅读笔记（1.2） invokeBeanFactoryPostProcessors(beanFactory) 调用Bean工厂后置处理器 介绍了BeanFactoryPostProcessor，这一节介绍BeanPostProcessor。

如果上一节理解清楚了，那么这一节就很容易理解了。因此，本小节篇幅较短。

首先回顾一下 registerBeanPostProcessors 这个方法的作用：
源码中的注释是：Register bean processors that intercept bean creation.
翻译过来就是：注册拦截 bean 创建的 bean 处理器。

注意：注释里是没有 post 的。而且这个 BeanPostProcessor 实际上也不是对 bean 进行后置处理。它可以在 bean 实例化前后都进行操作。所以把 BeanPostProcessor 理解成 bean 的后置处理器并不准确。但是既然源码里它就叫 BeanPostProcessor，所以我们通常也叫它 bean 后置处理器。

当然，如果理解成是对 BeanDefinition 做后置处理，就没有问题了。

跟源码：
类 AbstractApplicationContext：

protected void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory) {
	PostProcessorRegistrationDelegate.registerBeanPostProcessors(beanFactory, this);
}

跟 registerBeanPostProcessors：
类 PostProcessorRegistrationDelegate

public static void registerBeanPostProcessors(
		ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {

	//拿到工程里面所有实现了BeanPostProcessor接口的类，获取到BeanDefinition的名称
	String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);

	// Register BeanPostProcessorChecker that logs an info message when
	// a bean is created during BeanPostProcessor instantiation, i.e. when
	// a bean is not eligible for getting processed by all BeanPostProcessors.
	int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 + postProcessorNames.length;
	beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount));

	//这几个容器是为了区分实现了PriorityOrdered的、实现了Ordered的、和其余的BeanPostProcessor
	//处理逻辑和上一节对BeanFactoryPostProcessor的处理逻辑大同小异，就不详细介绍了。
	List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
	List<BeanPostProcessor> internalPostProcessors = new ArrayList<>();
	List<String> orderedPostProcessorNames = new ArrayList<>();
	List<String> nonOrderedPostProcessorNames = new ArrayList<>();

	//装填容器，实例化、排序、注册 实现了PriorityOrdered接口的BeanPostProcessor
	for (String ppName : postProcessorNames) {
		if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
			//这里就实例化了
			BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
			priorityOrderedPostProcessors.add(pp);

			//判断类型是否是MergedBeanDefinitionPostProcessor，如果是则代码是内部使用的
			if (pp instanceof MergedBeanDefinitionPostProcessor) {
				internalPostProcessors.add(pp);
			}
		}
		else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
			orderedPostProcessorNames.add(ppName);
		}
		else {
			nonOrderedPostProcessorNames.add(ppName);
		}
	}

	sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
	registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);

	//实例化、排序、注册 实现了Ordered接口的BeanPostProcessor
	List<BeanPostProcessor> orderedPostProcessors = new ArrayList<>();
	for (String ppName : orderedPostProcessorNames) {
		BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
		orderedPostProcessors.add(pp);
		if (pp instanceof MergedBeanDefinitionPostProcessor) {
			internalPostProcessors.add(pp);
		}
	}

	sortPostProcessors(orderedPostProcessors, beanFactory);
	//这里要看一下，与1.4.2节有联系，看到那里可以回过来看看
	registerBeanPostProcessors(beanFactory, orderedPostProcessors);

	// 实例化、排序、注册 其它的的BeanPostProcessor
	List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<>();
	for (String ppName : nonOrderedPostProcessorNames) {
		BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
		nonOrderedPostProcessors.add(pp);
		if (pp instanceof MergedBeanDefinitionPostProcessor) {
			internalPostProcessors.add(pp);
		}
	}
	registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors);

	// Finally, re-register all internal BeanPostProcessors.
	sortPostProcessors(internalPostProcessors, beanFactory);
	registerBeanPostProcessors(beanFactory, internalPostProcessors);

	// Re-register post-processor for detecting inner beans as ApplicationListeners,
	// moving it to the end of the processor chain (for picking up proxies etc).
	beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(applicationContext));
}

和上一节的 invokeBeanFactoryPostProcessors 方法相比：
相同点：都进行了排序，都进行了实例化（在其他 bean 之前实例化）
不同点：对 BeanFactoryPostProcessor 进行了调用，但对 BeanPostProcessor 仅仅是注册。

为什么会有这个不同点呢？
BeanFactoryPostProcessor 是对 BeanDefinition 进行增修，所以可以直接调用。
而 BeanPostProcessor 是为了去操作后面 bean 实例化过程的，那个时候再调用。

跟 registerBeanPostProcessors：

跟 addBeanPostProcessor：
类 ConfigurableBeanFactory

类 AbstractBeanFactory：

@Override
public void addBeanPostProcessor(BeanPostProcessor beanPostProcessor) {
	Assert.notNull(beanPostProcessor, "BeanPostProcessor must not be null");
	// Remove from old position, if any
	this.beanPostProcessors.remove(beanPostProcessor);
	// Track whether it is instantiation/destruction aware
	//与1.4.2.1节有联系，看到那里可以回过来看看。
	//在这里，hasInstantiationAwareBeanPostProcessors置为true。
	//至于为什么会有这样的BeanPostProcessor，在1.1.2末尾有答案。
	if (beanPostProcessor instanceof InstantiationAwareBeanPostProcessor) {
		this.hasInstantiationAwareBeanPostProcessors = true;
	}
	if (beanPostProcessor instanceof DestructionAwareBeanPostProcessor) {
		this.hasDestructionAwareBeanPostProcessors = true;
	}
	// Add to end of list
	this.beanPostProcessors.add(beanPostProcessor);
}