重读Spring之ConfigurationClassPostProcessor-改正错误

　　如果一个spring项目是使用xml方式配置的，那就不用关注ConfigurationClassPostProcessor因为不写@Configuration注解，ConfigurationClassPostProcessor是不会生效的。

　　但是，在spring boot里，这个类太重要了。可以说是最重要的，自动装配就靠它实现的。

　　本文就浅析一下ConfigurationClassPostProcessor。

一 概述

　　

　　 ConfigurationClassPostProcessor 是实现了 BeanDefinitionRegistryPostProcessor 接口的

　　同时实现了它的接口方法  public void processConfigBeanDefinitions(BeanDefinitionRegistry registry)  

　　

二 调用地点

　　先抛开SpringBoot回到Spring，AbstractApplicationContext中最重要的方法refresh()

public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            prepareRefresh();

            // Tell the subclass to refresh the internal bean factory.
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // Prepare the bean factory for use in this context.
            prepareBeanFactory(beanFactory);

            try {
                // Allows post-processing of the bean factory in context subclasses.
                postProcessBeanFactory(beanFactory);

                // Invoke factory processors registered as beans in the context.
                invokeBeanFactoryPostProcessors(beanFactory);

　　中间的一些环节省略，继续看  PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors 

public static void invokeBeanFactoryPostProcessors(
            ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {

        // Invoke BeanDefinitionRegistryPostProcessors first, if any.
        Set<String> processedBeans = new HashSet<>();

        if (beanFactory instanceof BeanDefinitionRegistry) {
            BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
            List<BeanFactoryPostProcessor> regularPostProcessors = new LinkedList<>();
            List<BeanDefinitionRegistryPostProcessor> registryProcessors = new LinkedList<>();

            for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
                if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
                    BeanDefinitionRegistryPostProcessor registryProcessor =
                            (BeanDefinitionRegistryPostProcessor) postProcessor;
                    registryProcessor.postProcessBeanDefinitionRegistry(registry);
                    registryProcessors.add(registryProcessor);
                }

在这里 ConfigurationClassPostProcessor 就会被调用，我们继续看

public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
        int registryId = System.identityHashCode(registry);
        if (this.registriesPostProcessed.contains(registryId)) {
            throw new IllegalStateException(
                    "postProcessBeanDefinitionRegistry already called on this post-processor against " + registry);
        }
        if (this.factoriesPostProcessed.contains(registryId)) {
            throw new IllegalStateException(
                    "postProcessBeanFactory already called on this post-processor against " + registry);
        }
        this.registriesPostProcessed.add(registryId);

        processConfigBeanDefinitions(registry);
    }

　　开始正式解析

public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
        List<BeanDefinitionHolder> configCandidates = new ArrayList<>();
        String[] candidateNames = registry.getBeanDefinitionNames();//去拿当前所有的BeanDefinition，看下面的截图

        for (String beanName : candidateNames) {
            BeanDefinition beanDef = registry.getBeanDefinition(beanName);
            if (ConfigurationClassUtils.isFullConfigurationClass(beanDef) ||
                    ConfigurationClassUtils.isLiteConfigurationClass(beanDef)) {
                if (logger.isDebugEnabled()) {
                    logger.debug("Bean definition has already been processed as a configuration class: " + beanDef);
                }
            }
            else if (ConfigurationClassUtils.checkConfigurationClassCandidate(beanDef, this.metadataReaderFactory)) {
                configCandidates.add(new BeanDefinitionHolder(beanDef, beanName));
            }
        }

        // Return immediately if no @Configuration classes were found 在这里只会过滤出来注解@Configuration 

　　　　if (configCandidates.isEmpty()) { return; }

　　

　　前几个就不看了，主要看6号，这个就是我们SpringBoot的启动类，这个启动类会在Spring框架启动前就把自己注册进去，代码在

　　 SpringApplication.prepareContext 

private void prepareContext(ConfigurableApplicationContext context,
            ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
            ApplicationArguments applicationArguments, Banner printedBanner) {
        context.setEnvironment(environment);
        postProcessApplicationContext(context);
        applyInitializers(context);
        listeners.contextPrepared(context);
        if (this.logStartupInfo) {
            logStartupInfo(context.getParent() == null);
            logStartupProfileInfo(context);
        }

        // Add boot specific singleton beans
        context.getBeanFactory().registerSingleton("springApplicationArguments",
                applicationArguments);
        if (printedBanner != null) {
            context.getBeanFactory().registerSingleton("springBootBanner", printedBanner);
        }

        // Load the sources
        Set<Object> sources = getAllSources();
        Assert.notEmpty(sources, "Sources must not be empty");
        load(context, sources.toArray(new Object[0]));/在这里把启动类作为一个BeanDefinition注册进去
        listeners.contextLoaded(context);
    }

　　好了，准备工作都做完了，@Configuration也找到了，接下来就是正式的解析了。解析我有空之后会详细的来解析

三 总结

　　ConfigurationClassPostProcessor 有两个最重要的方法

　　1  postProcessBeanDefinitionRegistry （接口BeanDefinitionRegistryPostProcessor的方法）

　　2  postProcessBeanFactory　　　　    （接口BeanFactoryPostProcessor的方法）

　　而这两个方法都是通过调用 processConfigBeanDefinitions来完成的

===================== 2020-12-24 ===========================================

又跟了一次代码，发现上面的说法有误

　　SpringApplication类中applyInitializers(context);，它会将三个默认的内部类加入到 spring 容器DefaultListableBeanFactory中，如下：

ConfigurationWarningsApplicationContextInitializer$ConfigurationWarningsPostProcessor
SharedMetadataReaderFactoryContextInitializer$CachingMetadataReaderFactoryPostProcessor
ConfigFileApplicationListener$PropertySourceOrderingPostProcessor

　　而在方法 

PostProcessorRegistrationDelegate.public static void invokeBeanFactoryPostProcessors(
            ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) 

　　上面的  beanFactoryPostProcessors 就是上面的三个默认内部类

　　也就是说容器启动的时候其实是走不到 

if (beanFactory instanceof BeanDefinitionRegistry) {
            BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
            List<BeanFactoryPostProcessor> regularPostProcessors = new LinkedList<>();
            List<BeanDefinitionRegistryPostProcessor> registryProcessors = new LinkedList<>();

            for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {//这里面就没有ConfigurationClassPostProcessor
                if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
                    BeanDefinitionRegistryPostProcessor registryProcessor =
                            (BeanDefinitionRegistryPostProcessor) postProcessor;
                    registryProcessor.postProcessBeanDefinitionRegistry(registry);
                    registryProcessors.add(registryProcessor);
                }
                else {
                    regularPostProcessors.add(postProcessor);
                }
            }

　　而是要往下走 

String[] postProcessorNames =
                    beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
            for (String ppName : postProcessorNames) {
                if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
                    currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));//在这里实例化ConfigurationClassPostProcessor
                    processedBeans.add(ppName);
                }
            }
            sortPostProcessors(currentRegistryProcessors, beanFactory);
            registryProcessors.addAll(currentRegistryProcessors);
            invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);//调用
            currentRegistryProcessors.clear();