Spring5源碼解析5-ConfigurationClassPostProcessor (上)

接上回，我們講到了refresh()方法中的invokeBeanFactoryPostProcessors(beanFactory)方法主要在執行BeanFactoryPostProcessor和其子接口BeanDefinitionRegistryPostProcessor的方法。

在創建AnnotationConfigApplicationContext對象時Spring就添加了一個非常重要的BeanFactoryPostProcessor接口實現類：ConfigurationClassPostProcessor。注意，這里說的添加只是添加到容器的beanDefinitionMap中，還沒有創建真正的實例Bean。

簡單回顧一下ConfigurationClassPostProcessor是在什么時候被添加到容器中的：在AnnotationConfigApplicationContext的無參構造器中創建AnnotatedBeanDefinitionReader對象時會向傳入的BeanDefinitionRegistry中注冊解析注解配置類相關的processors的BeanDefinition，ConfigurationClassPostProcessor就是在此處被添加到容器中的。

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ConfigurationClassPostProcessor

先看一些ConfigurationClassPostProcessor的繼承體系：

ConfigurationClassPostProcessor實現了BeanDefinitionRegistryPostProcessor接口，也就擁有了在Spring容器啟動時，往容器中注冊BeanDefinition的能力。

我們知道，ConfigurationClassPostProcessor#postProcessBeanDefinitionRegistry方法是在refresh();方法中的invokeBeanFactoryPostProcessors(beanFactory);中被執行的，下面我們就一起來看一下該方法。

ConfigurationClassPostProcessor#postProcessBeanDefinitionRegistry

@Override
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);
}

主要的邏輯在processConfigBeanDefinitions(registry);中，點開源碼：

public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
	List<BeanDefinitionHolder> configCandidates = new ArrayList<>();
	//獲取所有的BeanDefinitionName
	String[] candidateNames = registry.getBeanDefinitionNames();

	for (String beanName : candidateNames) {
		BeanDefinition beanDef = registry.getBeanDefinition(beanName);

		// https://docs.spring.io/spring/docs/5.1.8.RELEASE/spring-framework-reference/core.html#beans-java-basic-concepts
		// Full @Configuration vs “lite” @Bean mode
		if (ConfigurationClassUtils.isFullConfigurationClass(beanDef) ||
				ConfigurationClassUtils.isLiteConfigurationClass(beanDef)) {
			if (logger.isDebugEnabled()) {
				logger.debug("Bean definition has already been processed as a configuration class: " + beanDef);
			}
		}

		// 校驗是否為配置類
		// 配置類分為兩種 Full @Configuration vs “lite” @Bean mode
		// 校驗之后在 BeanDefinition 中添加標志屬性
		// 如果滿足條件則加入到configCandidates
		else if (ConfigurationClassUtils.checkConfigurationClassCandidate(beanDef, this.metadataReaderFactory)) {
			// 如果是配置類,就放到 configCandidates 變量中
			configCandidates.add(new BeanDefinitionHolder(beanDef, beanName));
		}
	}

	// Return immediately if no @Configuration classes were found
	if (configCandidates.isEmpty()) {
		return;
	}

	// Sort by previously determined @Order value, if applicable
	configCandidates.sort((bd1, bd2) -> {
		int i1 = ConfigurationClassUtils.getOrder(bd1.getBeanDefinition());
		int i2 = ConfigurationClassUtils.getOrder(bd2.getBeanDefinition());
		return Integer.compare(i1, i2);
	});

	// Detect any custom bean name generation strategy supplied through the enclosing application context
	SingletonBeanRegistry sbr = null;
	// 傳入的 registry 是 DefaultListableBeanFactory
	if (registry instanceof SingletonBeanRegistry) {
		sbr = (SingletonBeanRegistry) registry;
		if (!this.localBeanNameGeneratorSet) {
			//獲取自定義BeanNameGenerator,一般情況下為空
			BeanNameGenerator generator = (BeanNameGenerator) sbr.getSingleton(CONFIGURATION_BEAN_NAME_GENERATOR);
			if (generator != null) {
				this.componentScanBeanNameGenerator = generator;
				this.importBeanNameGenerator = generator;
			}
		}
	}

	if (this.environment == null) {
		this.environment = new StandardEnvironment();
	}

	// Parse each @Configuration class
	// new ConfigurationClassParser,用來解析 @Configuration 類
	ConfigurationClassParser parser = new ConfigurationClassParser(
			this.metadataReaderFactory, this.problemReporter, this.environment,
			this.resourceLoader, this.componentScanBeanNameGenerator, registry);

	// 將 configCandidates 轉成 set  candidates , 去重
	Set<BeanDefinitionHolder> candidates = new LinkedHashSet<>(configCandidates);
	Set<ConfigurationClass> alreadyParsed = new HashSet<>(configCandidates.size());
	do {
		// 解析配置類
		parser.parse(candidates);
		parser.validate();

		Set<ConfigurationClass> configClasses = new LinkedHashSet<>(parser.getConfigurationClasses());
		configClasses.removeAll(alreadyParsed);

		// Read the model and create bean definitions based on its content
		if (this.reader == null) {
			this.reader = new ConfigurationClassBeanDefinitionReader(
					registry, this.sourceExtractor, this.resourceLoader, this.environment,
					this.importBeanNameGenerator, parser.getImportRegistry());
		}
		// Import類,@Bean,@ImportResource 轉化為 BeanDefinition
		this.reader.loadBeanDefinitions(configClasses);
		alreadyParsed.addAll(configClasses);

		candidates.clear();
		// 再獲取一下容器中BeanDefinition的數據,如果發現數量增加了,說明有新的BeanDefinition被注冊了
		if (registry.getBeanDefinitionCount() > candidateNames.length) {
			String[] newCandidateNames = registry.getBeanDefinitionNames();
			Set<String> oldCandidateNames = new HashSet<>(Arrays.asList(candidateNames));
			Set<String> alreadyParsedClasses = new HashSet<>();
			for (ConfigurationClass configurationClass : alreadyParsed) {
				alreadyParsedClasses.add(configurationClass.getMetadata().getClassName());
			}
			for (String candidateName : newCandidateNames) {
				if (!oldCandidateNames.contains(candidateName)) {
					BeanDefinition bd = registry.getBeanDefinition(candidateName);
					if (ConfigurationClassUtils.checkConfigurationClassCandidate(bd, this.metadataReaderFactory) &&
							!alreadyParsedClasses.contains(bd.getBeanClassName())) {
						candidates.add(new BeanDefinitionHolder(bd, candidateName));
					}
				}
			}
			candidateNames = newCandidateNames;
		}
	}
	while (!candidates.isEmpty());

	// Register the ImportRegistry as a bean in order to support ImportAware @Configuration classes
	if (sbr != null && !sbr.containsSingleton(IMPORT_REGISTRY_BEAN_NAME)) {
		sbr.registerSingleton(IMPORT_REGISTRY_BEAN_NAME, parser.getImportRegistry());
	}

	if (this.metadataReaderFactory instanceof CachingMetadataReaderFactory) {
		// Clear cache in externally provided MetadataReaderFactory; this is a no-op
		// for a shared cache since it'll be cleared by the ApplicationContext.
		((CachingMetadataReaderFactory) this.metadataReaderFactory).clearCache();
	}
}

獲取所有的BeanDefinitionNames，然后循環這個數組，判斷其是否為配置類。

前5個是Spring注冊的內置processor，最后一個是傳入給AnnotationConfigApplicationContext的配置類AppConfig.class。

在Spring中存在兩種ConfigurationClass，一種是FullConfigurationClass另一種是LiteConfigurationClass。關於這兩者的區別，可以參看筆者文章之前關於Full @Configuration 和 lite @Bean mode的文章。

ConfigurationClassUtils#checkConfigurationClassCandidate方法內部就是在判斷屬於哪種配置類，並在BeanDefinition中標記判斷結果。其具體的判斷邏輯如下：

public static boolean isFullConfigurationCandidate(AnnotationMetadata metadata) {
	return metadata.isAnnotated(Configuration.class.getName());
}
private static final Set<String> candidateIndicators = new HashSet<>(8);

static {
	candidateIndicators.add(Component.class.getName());
	candidateIndicators.add(ComponentScan.class.getName());
	candidateIndicators.add(Import.class.getName());
	candidateIndicators.add(ImportResource.class.getName());
}

public static boolean isLiteConfigurationCandidate(AnnotationMetadata metadata) {
	// Do not consider an interface or an annotation...
	if (metadata.isInterface()) {
		return false;
	}

	// Any of the typical annotations found?
	for (String indicator : candidateIndicators) {
		if (metadata.isAnnotated(indicator)) {
			return true;
		}
	}

	// Finally, let's look for @Bean methods...
	try {
		return metadata.hasAnnotatedMethods(Bean.class.getName());
	} catch (Throwable ex) {
		if (logger.isDebugEnabled()) {
			logger.debug("Failed to introspect @Bean methods on class [" + metadata.getClassName() + "]: " + ex);
		}
		return false;
	}
}

判斷configCandidates變量中存放的

配置類是否為空，如果不為空，則對其進行排序。

創建ConfigurationClassParser對象，用於解析@Configuration類，完成包的掃描、BeanDefinition的注冊。主要通過執行parser.parse(candidates);方法來完成。

執行parser.parse(candidates)方法前 ：

執行parser.parse(candidates)方法后 ：

解析完配置類之后，緊接着又執行了this.reader.loadBeanDefinitions(configClasses);方法。這個方法主要是用來處理Import類、@Bean和@ImportResource注解。關於這兩個方法的具體細節，我們下次再講。

最后又加了入了對ImportAware接口支持所需要的Bean。

// Register the ImportRegistry as a bean in order to support ImportAware @Configuration classes
if (sbr != null && !sbr.containsSingleton(IMPORT_REGISTRY_BEAN_NAME)) {
	sbr.registerSingleton(IMPORT_REGISTRY_BEAN_NAME, parser.getImportRegistry());
}

關於對ImportAware接口的使用，我們也下次再講。

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未完待續......

源碼學習筆記：https://github.com/shenjianeng/spring-code-study

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