引入

在使用SpringBoot開發時，最常用的註解有@Component、@Service、@Controller、@Configuration等。當類使用這些註解標記時，類會被Spring IOC容器管理，包括創建，填充屬性和實例化。

但是Spring容器如何發現並將這些類放到容器進行管理呢？

今天這篇博客主要分析Spring如何處理@Configuration註解。

@Configuration定義

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Configuration {
	@AliasFor(annotation = Component.class)
	String value() default "";
	
	//Since: 5.2
	boolean proxyBeanMethods() default true;

	//Since: 6.0
	boolean enforceUniqueMethods() default true;
}

看Configuration註解的定義，它本質也是一個@Component註解。

所以，標記有@Configuration註解的類在Spring註冊bean的第一階段就會被註冊到容器中，但是使用@Configuration註解的類內部的其他bean並沒有被註冊到容器中。

想知道@Configuration註解被如何處理，需要看invokeBeanFactoryPostProcessors 方法。

ConfigurationClassPostProcessor處理@Configuration註解

先大致說一下invokeBeanFactoryPostProcessors 方法的基本流程，因為這個方法會處理所有BeanDefinitionRegistryPostProcessor和BeanFactoryPostProcessor介面的子類：

• 先處理方法入參：beanFactoryPostProcessors（如果只是分析AnnotationConfigApplicationContext，這個參數是空的。只有在分析SpringBoot的啟動過程時，這個參數才會有值）
• 再處理BeanDefinitionRegistryPostProcessor的子類
  • 這裡最重要的一件事情是調用ConfigurationClassPostProcessor的processConfigBeanDefinitions方法。
• 最後處理BeanFactoryPostProcessor的子類

invokeBeanFactoryPostProcessors 的部分代碼：

public static void invokeBeanFactoryPostProcessors(
		ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {
	
	Set<String> processedBeans = new HashSet<>();
	if (beanFactory instanceof BeanDefinitionRegistry) {
		//省略部分代碼，處理入參：beanFactoryPostProcessors

        //開始處理BeanDefinitionRegistryPostProcessor介面的子類
        //保存所有符合的子類，即實現BeanDefinitionRegistryPostProcessor，同樣實現PriorityOrdered
		List<BeanDefinitionRegistryPostProcessor> currentRegistryProcessors = new ArrayList<>();
		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));
				processedBeans.add(ppName);
			}
		}
		sortPostProcessors(currentRegistryProcessors, beanFactory);
		registryProcessors.addAll(currentRegistryProcessors);
        //調用currentRegistryProcessors中所有的BDRPP
		invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry, beanFactory.getApplicationStartup());
		currentRegistryProcessors.clear();

		//省略部分代碼，同樣處理BeanDefinitionRegistryPostProcessor介面的子類
	}
	else {
		//省略部分代碼
	}
	//省略部分代碼 處理BeanFactoryPostProcessor介面的子類
}

以上是invokeBeanFactoryPostProcessors的部分代碼，接下來看ConfigurationClassPostProcessor的processConfigBeanDefinitions方法，在這個方法里開始正式地處理@Configruation註解：

public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
	List<BeanDefinitionHolder> configCandidates = new ArrayList<>();
	String[] candidateNames = registry.getBeanDefinitionNames();

	//遍歷所有的候選者
	for (String beanName : candidateNames) {
		BeanDefinition beanDef = registry.getBeanDefinition(beanName);
		if (beanDef.getAttribute(ConfigurationClassUtils.CONFIGURATION_CLASS_ATTRIBUTE) != null) {
			if (logger.isDebugEnabled()) {
				logger.debug("Bean definition has already been processed as a configuration class: " + beanDef);
			}
		}
		//校驗候選者類上是否使用了@Configuration註解
		else if (ConfigurationClassUtils.checkConfigurationClassCandidate(beanDef, this.metadataReaderFactory)) {
			configCandidates.add(new BeanDefinitionHolder(beanDef, beanName));
		}
	}

	//如果configCandidates為空，則立即返回
	if (configCandidates.isEmpty()) {
		return;
	}

	configCandidates.sort((bd1, bd2) -> {
		int i1 = ConfigurationClassUtils.getOrder(bd1.getBeanDefinition());
		int i2 = ConfigurationClassUtils.getOrder(bd2.getBeanDefinition());
		return Integer.compare(i1, i2);
	});

	SingletonBeanRegistry sbr = null;
	if (registry instanceof SingletonBeanRegistry) {
		sbr = (SingletonBeanRegistry) registry;
		if (!this.localBeanNameGeneratorSet) {
			BeanNameGenerator generator = (BeanNameGenerator) sbr.getSingleton(
					AnnotationConfigUtils.CONFIGURATION_BEAN_NAME_GENERATOR);
			if (generator != null) {
				this.componentScanBeanNameGenerator = generator;
				this.importBeanNameGenerator = generator;
			}
		}
	}

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

	// 創建一個解析@Configuration的解析器
	ConfigurationClassParser parser = new ConfigurationClassParser(
			this.metadataReaderFactory, this.problemReporter, this.environment,
			this.resourceLoader, this.componentScanBeanNameGenerator, registry);
	Set<BeanDefinitionHolder> candidates = new LinkedHashSet<>(configCandidates);
	Set<ConfigurationClass> alreadyParsed = new HashSet<>(configCandidates.size());

	do {
		StartupStep processConfig = this.applicationStartup.start("spring.context.config-classes.parse");
		//開始解析候選類
		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());
		}

		//註冊解析後的類到容器中
		this.reader.loadBeanDefinitions(configClasses);

		alreadyParsed.addAll(configClasses);
		processConfig.tag("classCount", () -> String.valueOf(configClasses.size())).end();
		candidates.clear();
		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();
	}

}

這個方法的關鍵點：

• 找到所有要解析的類
• 創建一個解析者
• 開始解析類
  • 記住，這裡只是解析，也就是確認所有需要處理的類或者方法，這些類或者方法最終也會註冊到容器中
• 註冊解析後的類到容器中：this.reader.loadBeanDefinitions(configClasses);

解析

那麼實際上如何解析呢？

protected final SourceClass doProcessConfigurationClass(
		ConfigurationClass configClass, SourceClass sourceClass, Predicate<String> filter)
		throws IOException {
	//1. 判斷當前類是否有內部類，如果有內部類是否使用了@Configuration註解，如果使用了該註解，那麼需要先處理內部類的配置
	//存在這種可能，但是實際開發中很少使用
	if (configClass.getMetadata().isAnnotated(Component.class.getName())) {
		processMemberClasses(configClass, sourceClass, filter);
	}

	// 處理@PropertySource註解
	for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(
			sourceClass.getMetadata(), PropertySources.class,
			org.springframework.context.annotation.PropertySource.class)) {
		if (this.environment instanceof ConfigurableEnvironment) {
			processPropertySource(propertySource);
		}
		else {
			logger.info("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +
					"]. Reason: Environment must implement ConfigurableEnvironment");
		}
	}

	// 處理@ComponentScan註解
	Set<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable(
			sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class);
	if (!componentScans.isEmpty() &&
			!this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) {
		for (AnnotationAttributes componentScan : componentScans) {
			Set<BeanDefinitionHolder> scannedBeanDefinitions =
					this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
			for (BeanDefinitionHolder holder : scannedBeanDefinitions) {
				BeanDefinition bdCand = holder.getBeanDefinition().getOriginatingBeanDefinition();
				if (bdCand == null) {
					bdCand = holder.getBeanDefinition();
				}
				if (ConfigurationClassUtils.checkConfigurationClassCandidate(bdCand, this.metadataReaderFactory)) {
					parse(bdCand.getBeanClassName(), holder.getBeanName());
				}
			}
		}
	}

	// 處理@Import註解
	processImports(configClass, sourceClass, getImports(sourceClass), filter, true);

	// 處理@ImportResource註解
	AnnotationAttributes importResource =
			AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);
	if (importResource != null) {
		String[] resources = importResource.getStringArray("locations");
		Class<? extends BeanDefinitionReader> readerClass = importResource.getClass("reader");
		for (String resource : resources) {
			String resolvedResource = this.environment.resolveRequiredPlaceholders(resource);
			configClass.addImportedResource(resolvedResource, readerClass);
		}
	}

	// 處理使用了@Bean的方法
	Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);
	for (MethodMetadata methodMetadata : beanMethods) {
		configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
	}
	// Process default methods on interfaces
	processInterfaces(configClass, sourceClass);
	// Process superclass, if any
	if (sourceClass.getMetadata().hasSuperClass()) {
		String superclass = sourceClass.getMetadata().getSuperClassName();
		if (superclass != null && !superclass.startsWith("java") &&
				!this.knownSuperclasses.containsKey(superclass)) {
			this.knownSuperclasses.put(superclass, configClass);
			// Superclass found, return its annotation metadata and recurse
			return sourceClass.getSuperClass();
		}
	}
	// No superclass -> processing is complete
	return null;
}

• 確認當前類是否有內部類，內部類是否也使用了@Configuration註解
• 處理@PropertySource註解
• 處理@ComponentScan註解
• 處理@Import註解
• 處理@ImportResource註解
• 處理@Bean註解

這裡以@Import和@Bean為例進行分析：

@Configuration
@Import(MyBeanFactoryPostProcessor.class)
public class JavaConfig {
    @Bean
    public Country country() {
        return new Country();
    }
}

JavaConfig類上使用了@Configuration註解和@Import註解，在@Import註解引入了MyBeanFactoryPostProcessor類：

public class MyBeanFactoryPostProcessor implements ImportBeanDefinitionRegistrar {
    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry, BeanNameGenerator importBeanNameGenerator) {
        ImportBeanDefinitionRegistrar.super.registerBeanDefinitions(importingClassMetadata, registry, importBeanNameGenerator);
        RootBeanDefinition rootBeanDefinition = new RootBeanDefinition();
        rootBeanDefinition.setBeanClass(Person.class);

        registry.registerBeanDefinition("person", rootBeanDefinition);
    }

    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
        ImportBeanDefinitionRegistrar.super.registerBeanDefinitions(importingClassMetadata, registry);
    }
}

MyBeanFactoryPostProcessor實現了ImportBeanDefinitionRegistrar介面。

下麵分析一下如何處理@Import註解：

private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
		Collection<SourceClass> importCandidates, Predicate<String> exclusionFilter,
		boolean checkForCircularImports) {
	if (importCandidates.isEmpty()) {
		return;
	}
	if (checkForCircularImports && isChainedImportOnStack(configClass)) {
		this.problemReporter.error(new CircularImportProblem(configClass, this.importStack));
	}
	else {
		this.importStack.push(configClass);
		try {

			//@Import註解的參數可能不止一個類，可以有多個
			for (SourceClass candidate : importCandidates) {
				//candidate繼承了ImportSelector
				if (candidate.isAssignable(ImportSelector.class)) {
					// Candidate class is an ImportSelector -> delegate to it to determine imports
					Class<?> candidateClass = candidate.loadClass();
					ImportSelector selector = ParserStrategyUtils.instantiateClass(candidateClass, ImportSelector.class,
							this.environment, this.resourceLoader, this.registry);
					Predicate<String> selectorFilter = selector.getExclusionFilter();
					if (selectorFilter != null) {
						exclusionFilter = exclusionFilter.or(selectorFilter);
					}
					if (selector instanceof DeferredImportSelector) {
						this.deferredImportSelectorHandler.handle(configClass, (DeferredImportSelector) selector);
					}
					else {
						String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
						Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames, exclusionFilter);
						processImports(configClass, currentSourceClass, importSourceClasses, exclusionFilter, false);
					}
				}
				//candidate繼承了ImportBeanDefinitionRegistrar
				else if (candidate.isAssignable(ImportBeanDefinitionRegistrar.class)) {
					// Candidate class is an ImportBeanDefinitionRegistrar ->
					// delegate to it to register additional bean definitions
					Class<?> candidateClass = candidate.loadClass();
					ImportBeanDefinitionRegistrar registrar =
							ParserStrategyUtils.instantiateClass(candidateClass, ImportBeanDefinitionRegistrar.class,
									this.environment, this.resourceLoader, this.registry);

					//將candidate放到configClass中，後續進行處理
					configClass.addImportBeanDefinitionRegistrar(registrar, currentSourceClass.getMetadata());
				}
				//candidate沒有繼承上述兩個類，按照@Configuration處理
				else {
					// Candidate class not an ImportSelector or ImportBeanDefinitionRegistrar ->
					// process it as an @Configuration class
					this.importStack.registerImport(
							currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());
					processConfigurationClass(candidate.asConfigClass(configClass), exclusionFilter);
				}
			}
		}
		catch (BeanDefinitionStoreException ex) {
			throw ex;
		}
		catch (Throwable ex) {
			throw new BeanDefinitionStoreException(
					"Failed to process import candidates for configuration class [" +
					configClass.getMetadata().getClassName() + "]", ex);
		}
		finally {
			this.importStack.pop();
		}
	}
}

processImports方法主要處理兩種情況：

• 實現ImportSelector介面的類
• 實現ImportBeanDefinitionRegistrar介面的類

但是需要註意，這個方法只是把這些類放到了集合中，但是還沒有開始處理。

下麵開始看如何處理使用@Bean的方法

private Set<MethodMetadata> retrieveBeanMethodMetadata(SourceClass sourceClass) {
		AnnotationMetadata original = sourceClass.getMetadata();
		//找到所有使用@Bean的方法
		Set<MethodMetadata> beanMethods = original.getAnnotatedMethods(Bean.class.getName());
		if (beanMethods.size() > 1 && original instanceof StandardAnnotationMetadata) {
			try {
				AnnotationMetadata asm =
						this.metadataReaderFactory.getMetadataReader(original.getClassName()).getAnnotationMetadata();
				Set<MethodMetadata> asmMethods = asm.getAnnotatedMethods(Bean.class.getName());
				if (asmMethods.size() >= beanMethods.size()) {
					Set<MethodMetadata> selectedMethods = new LinkedHashSet<>(asmMethods.size());
					for (MethodMetadata asmMethod : asmMethods) {
						for (MethodMetadata beanMethod : beanMethods) {
							if (beanMethod.getMethodName().equals(asmMethod.getMethodName())) {
								selectedMethods.add(beanMethod);
								break;
							}
						}
					}
					if (selectedMethods.size() == beanMethods.size()) {
						// All reflection-detected methods found in ASM method set -> proceed
						beanMethods = selectedMethods;
					}
				}
			}
			catch (IOException ex) {
				logger.debug("Failed to read class file via ASM for determining @Bean method order", ex);
				// No worries, let's continue with the reflection metadata we started with...
			}
		}
		return beanMethods;
	}

最後返回所有需要處理的方法。

註冊

private void loadBeanDefinitionsForConfigurationClass(
			ConfigurationClass configClass, TrackedConditionEvaluator trackedConditionEvaluator) {

		if (trackedConditionEvaluator.shouldSkip(configClass)) {
			String beanName = configClass.getBeanName();
			if (StringUtils.hasLength(beanName) && this.registry.containsBeanDefinition(beanName)) {
				this.registry.removeBeanDefinition(beanName);
			}
			this.importRegistry.removeImportingClass(configClass.getMetadata().getClassName());
			return;
		}

		if (configClass.isImported()) {
			registerBeanDefinitionForImportedConfigurationClass(configClass);
		}

		//註冊使用@Bean的方法
		for (BeanMethod beanMethod : configClass.getBeanMethods()) {
			loadBeanDefinitionsForBeanMethod(beanMethod);
		}

		loadBeanDefinitionsFromImportedResources(configClass.getImportedResources());

		//註冊實現ImportBeanDefinitionRegistrar介面的類
		loadBeanDefinitionsFromRegistrars(configClass.getImportBeanDefinitionRegistrars());
	}