Spring 源码学习 12：registerBeanPostProcessors

前言

前面通过 invokeBeanFactoryPostProcessors 这一步了解到了什么是 BeanFactoryPostProcessor ，以及 BeanFactoryPostProcessor 的使用及作用，并通过 invokeBeanFactoryPostProcessors 这一步源码，对 BeanFactoryPostProcessor 的加载流程有了进一步了解。

现在就一起进入下一个环节：

registerBeanPostProcessors(beanFactory);

这一步主要的作用是加载 BeanPostProcessor，从名字也可以看出，只是加载，并没有执行。

不过，在进入源码之前，依然是结合官网，先了解以下几个问题：

什么是 BeanPostProcessor？
BeanPostProcessor 是如何使用的？
BeanPostProcessor 有什么用？

什么是 BeanPostProcessor ？

如截图所示，在官网 1.8.1 Customizing Beans by Using a BeanPostProcessor 中介绍， BeanPostProcessor 接口定义回调方法，可以实现这些方法，从而在 Bean 实例化期间修改 Bean 的属性。

BeanPostProcessor 是如何使用的?

@Component
public class MyBeanPostProcessor implements BeanPostProcessor {


	@Override
	public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {

		if (bean instanceof UserComponent) {

			System.out.println("BeanPostProcessor 开始执行 初始化前..." + beanName);

			UserComponent userComponent = (UserComponent) bean;
			userComponent.setUserName("liuzhihang-postProcessBeforeInitialization");

			return userComponent;

		}

		return bean;
	}

	@Override
	public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {


		if (bean instanceof UserComponent) {

			System.out.println("BeanPostProcessor 开始执行 初始化后..." + beanName);

			UserComponent userComponent = (UserComponent) bean;
			userComponent.setUserName("liuzhihang-postProcessAfterInitialization");

			return userComponent;

		}

		return bean;
	}
}

如代码所示，只需要声明一个自己的 MyBeanPostProcessor 来实现 BeanPostProcessor 并重写其方法：

postProcessBeforeInitialization ：在 Bean 实例后调用初始化方法之前进行处理。

postProcessAfterInitialization ：在 Bean 实例化后调用初始化方法之后进行处理。

并且在测试时可以发现，BeanPostProcessor 修改的属性会覆盖 BeanFactoryPostProcessor，至于原因可以阅读下 Spring 源码学习 11：invokeBeanFactoryPostProcessors 这篇文章，相信对 BeanFactoryPostProcessor 有了一定的了解之后，一定会有自己的答案。

当然我个人的理解就是 BeanFactoryPostProcessor 是在 Bean 实例化之前，是通过修改元数据从而修改的 Bean 的元素信息。

这块也可以通过代码进行验证。

可以看出，执行顺序是

BeanFactoryPostProcessor#postProcessBeanFactory
->
BeanPostProcessor#postProcessBeforeInitialization
->
BeanPostProcessor#postProcessAfterInitialization

所以，后面修改的属性，会覆盖之前修改的属性。

至于 BeanPostProcessor 是如何修改属性的，在这里先不做介绍，继续进入本节的主角 registerBeanPostProcessors 源码分析

registerBeanPostProcessors 源码

在 Spring refresh 方法中，执行 registerBeanPostProcessors 主要作用是将 BeanPostProcessor 注册到容器中，源码如下:

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

这里调用了 PostProcessorRegistrationDelegate 类的静态方法，继续跟进：

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

    // 查找 BeanPostProcessor 类型的 Bean 的名称集合, 就是获取所有继承了 BeanPostProcessor 的类
    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.
    //  注册一个 BeanPostProcessorChecker，用来记录 bean 在 BeanPostProcessor 实例化时的信息。
    int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 + postProcessorNames.length;
    beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount));

    // Separate between BeanPostProcessors that implement PriorityOrdered,
    // Ordered, and the rest.
    // 四个集合 区分实现不同接口的 BeanPostProcessors
    List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
    List<BeanPostProcessor> internalPostProcessors = new ArrayList<>();
    List<String> orderedPostProcessorNames = new ArrayList<>();
    List<String> nonOrderedPostProcessorNames = new ArrayList<>();
    for (String ppName : postProcessorNames) {
        if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
            BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
            priorityOrderedPostProcessors.add(pp);
            if (pp instanceof MergedBeanDefinitionPostProcessor) {
                internalPostProcessors.add(pp);
            }
        } else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
            orderedPostProcessorNames.add(ppName);
        } else {
            nonOrderedPostProcessorNames.add(ppName);
        }
    }

    // First, register the BeanPostProcessors that implement PriorityOrdered.
    // 排序后执行 实现 PriorityOrdered 的 BeanPostProcessors
    sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
    registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);

    // Next, register the BeanPostProcessors that implement Ordered.
    List<BeanPostProcessor> orderedPostProcessors = new ArrayList<>(orderedPostProcessorNames.size());
    for (String ppName : orderedPostProcessorNames) {
        BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
        orderedPostProcessors.add(pp);
        if (pp instanceof MergedBeanDefinitionPostProcessor) {
            internalPostProcessors.add(pp);
        }
    }
    sortPostProcessors(orderedPostProcessors, beanFactory);
    registerBeanPostProcessors(beanFactory, orderedPostProcessors);

    // Now, register all regular BeanPostProcessors.
    List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<>(nonOrderedPostProcessorNames.size());
    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));
}