Spring基于@Configuration的类配置的内部源码实现

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概述

• Spring容器启动时，即执行refresh方法时，主要是通过执行ConfigurationClassPostProcessor这个BeanFactoryPostProcessor，来开启整个@Configuration注解的系列类的加载的，即开启基于@Configuration的类配置代替beans标签的容器配置的相关bean的加载。
• 而ConfigurationClassPostProcessor注册到Spring容器的BeanFactoryPostProcessor列表，主要是在AnnotationConfigUtils的registerAnnotationConfigProcessors定义的。如下：

/**
 * Register all relevant annotation post processors in the given registry.
 * @param registry the registry to operate on
 * @param source the configuration source element (already extracted)
 * that this registration was triggered from. May be {@code null}.
 * @return a Set of BeanDefinitionHolders, containing all bean definitions
 * that have actually been registered by this call
 */
public static Set<BeanDefinitionHolder> registerAnnotationConfigProcessors(
		BeanDefinitionRegistry registry, @Nullable Object source) {

    ...
    
	Set<BeanDefinitionHolder> beanDefs = new LinkedHashSet<>(8);

	if (!registry.containsBeanDefinition(CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME)) {
	
	    // 注册ConfigurationClassPostProcessor
	    // 用于处理@Configuration注解的类
	    
		RootBeanDefinition def = new RootBeanDefinition(ConfigurationClassPostProcessor.class);
		def.setSource(source);
		beanDefs.add(registerPostProcessor(registry, def, CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME));
	}

	if (!registry.containsBeanDefinition(AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME)) {
	
	    // 注册AutowiredAnnotationBeanPostProcessor
	    
		RootBeanDefinition def = new RootBeanDefinition(AutowiredAnnotationBeanPostProcessor.class);
		def.setSource(source);
		beanDefs.add(registerPostProcessor(registry, def, AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME));
	}

	// Check for JSR-250 support, and if present add the CommonAnnotationBeanPostProcessor.
	if (jsr250Present && !registry.containsBeanDefinition(COMMON_ANNOTATION_PROCESSOR_BEAN_NAME)) {
	
	    // 注册CommonAnnotationBeanPostProcessor
	    
		RootBeanDefinition def = new RootBeanDefinition(CommonAnnotationBeanPostProcessor.class);
		def.setSource(source);
		beanDefs.add(registerPostProcessor(registry, def, COMMON_ANNOTATION_PROCESSOR_BEAN_NAME));
	}
}

• 而AnnotationConfigUtils的registerAnnotationConfigProcessors的被调用，则可以通过多种方式：

  1. ComponentScanBeanDefinitionParser：对应context:component-scan，在parse方法调用；
  2. AnnotationConfigBeanDefinitionParser：对应context:annotation-config，在parse方法调用；
  3. ClassPathBeanDefinitionScanner：在scan方法调用，该类在AnnotationConfigApplicationContext中使用；
  4. AnnotatedBeanDefinitionReader：在构造函数调用，该类在AnnotationConfigApplicationContext中使用。

• 由以上四种方式可知，当在xml文件中配置了context:component-scan或者context:annotation-config；或者使用AnnotationConfigApplicationContext作为spring容器的ApplicationContext，这种方式通常在结合实现WebApplicationInitializer接口，来实现全基于Java类配置时用到；则可以自动激活@Configuration注解系列配置类，及其内部的@Bean方法对应bean的加载。

类结构设计

1. ConfigurationClassPostProcessor

• 作为BeanFactoryPostProcessor，在spring容器ApplicationContext启动调用refresh方法时，遍历并调用BeanFactoryPostProcessor时被调用，执行postProcessBeanDefinitionRegistry方法，如下：

  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);
  
              // 调用BeanFactoryPostProcessor
              
  			// Invoke factory processors registered as beans in the context.
  			invokeBeanFactoryPostProcessors(beanFactory);
  
  			// Register bean processors that intercept bean creation.
  			registerBeanPostProcessors(beanFactory);
      
      ...
      
  	}
  }
  

  invokeBeanFactoryPostProcessors方法实现：

  /**
   * Instantiate and invoke all registered BeanFactoryPostProcessor beans,
   * respecting explicit order if given.
   * <p>Must be called before singleton instantiation.
   */
  protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
  
      // 在这里会调用到
      // ConfigurationClassPostProcessor的postProcessBeanDefinitionRegistry方法
      
  	PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());
  
  	// Detect a LoadTimeWeaver and prepare for weaving, if found in the meantime
  	// (e.g. through an @Bean method registered by ConfigurationClassPostProcessor)
  	if (beanFactory.getTempClassLoader() == null && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
  		beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
  		beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
  	}
  }
  

• 在该方法内先从BeanFactory中获取使用了@Configuration注解的类对应的beanDefinitions集合，即configCandidates。然后将configCandidates交给ConfigurationClassParser处理；

• ConfigurationClassParser处理会获取这些类对应的ConfigurationClass类型的集合，然后交给ConfigurationClassBeanDefinitionReader处理，从中处理@Bean方法，将对应的bean注册到BeanFactory。

• ConfigurationClassPostProcessor的processConfigBeanDefinitions相关代码如下：其中刚开始能从BeanFactory获取@Configuration的类对应beanDefinition，是因为@Configuration是@Component的一个子类，所以通常在配置component-scan时，需要能加载到@Configuration注解的类所在的包。

  /**
   * Build and validate a configuration model based on the registry of
   * {@link Configuration} classes.
   */
  public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
  	List<BeanDefinitionHolder> configCandidates = new ArrayList<>();
  	String[] candidateNames = registry.getBeanDefinitionNames();
      
      // 从BeanFactory中获取使用@Configuration注解的类
      
  	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));
  		}
  	}
  
      // 没有则直接返回
      
  	// 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);
  	});
  
  	...
  	
  	// 使用ConfigurationClassParser处理这些类上面的注解，
  	// 如@ComponentScan，@Import，@PropertySource等
  	// 并从这些类生成一个ConfigurationClass类型的列表
  
  	// Parse each @Configuration class
  	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 {
  	
  	    // 执行解析
  	    
  		parser.parse(candidates);
  		parser.validate();
  
  		Set<ConfigurationClass> configClasses = new LinkedHashSet<>(parser.getConfigurationClasses());
  		configClasses.removeAll(alreadyParsed);
  
          // 交给ConfigurationClassBeanDefinitionReader
          // 完成@Configuration注解的类内部注解的处理
          // 如@Bean，嵌套@Configuration等。
          
  		// 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());
  		}
  		
  		// 注册@Bean方法对应的bean到BeanFactory
  		
  		this.reader.loadBeanDefinitions(configClasses);
  	
  	...
  	
  }
  

2. ConfigurationClassParser

• 遍历并处理configCandidates集合，针对集合中的每个@Configuration注解的类时，会处理该类上同时配置的其他注解，如@ComponentScan，扫描指定的包并注册有相关注解的类到BeanFactory中；还包括其他注解的处理，如@Import，@PropertySource等。
• 同时会创建该类对应的一个类型为ConfigurationClass的对象，将该类中@Bean注解的方式生成BeanMethod，放到类型为ConfigurationClass的对象内部的一个集合beanMethods（类型为LinkedHashSet）里面。并将该类型为ConfigurationClass的对象，放到ConfigurationClassParser的一个map（类型为LinkedHashMap），其中key和value，均为该类型为ConfigurationClass的对象。
• 处理方法源码如下：

  /**
   * Apply processing and build a complete {@link ConfigurationClass} by reading the
   * annotations, members and methods from the source class. This method can be called
   * multiple times as relevant sources are discovered.
   * @param configClass the configuration class being build
   * @param sourceClass a source class
   * @return the superclass, or {@code null} if none found or previously processed
   */
  @Nullable
  protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)
  		throws IOException {
  	if (configClass.getMetadata().isAnnotated(Component.class.getName())) {
  		// Recursively process any member (nested) classes first
  		processMemberClasses(configClass, sourceClass);
  	}
  
  	// Process any @PropertySource annotations
  	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");
  		}
  	}
  
  	// Process any @ComponentScan annotations
  	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) {
  			// The config class is annotated with @ComponentScan -> perform the scan immediately
  			Set<BeanDefinitionHolder> scannedBeanDefinitions =
  					this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
  			// Check the set of scanned definitions for any further config classes and parse recursively if needed
  			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());
  				}
  			}
  		}
  	}
  
  	// Process any @Import annotations
  	processImports(configClass, sourceClass, getImports(sourceClass), true);
  
  	// Process any @ImportResource annotations
  	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);
  		}
  	}
  
  	// Process individual @Bean methods
  	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;
  }
  

3. ConfigurationClassBeanDefinitionReader

• 调用ConfigurationClassParser的getConfigurationClasses方法，获取以上所说的map的keySet，即configCandidates对应的类型为ConfigurationClass的列表。
• 然后遍历这个ConfigurationClass列表，针对每个ConfigurationClass，使用ConfigurationClassBeanDefinitionReader的loadBeanDefinitions方法，从该ConfigurationClass中获取其内部的@Bean注解的方法列表，即BeanMethod列表，或者嵌套@Configuration等包含的BeanMethod，然后将生成的bean，注册到BeanFactory中。
• 实现源码如下：

  /**
   * Read a particular {@link ConfigurationClass}, registering bean definitions
   * for the class itself and all of its {@link Bean} methods.
   */
  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);
  	}
  
  	// 将@Bean注解的方法注册到BeanFactory中
  	
  	for (BeanMethod beanMethod : configClass.getBeanMethods()) {
  		loadBeanDefinitionsForBeanMethod(beanMethod);
  	}
  
  	loadBeanDefinitionsFromImportedResources(configClass.getImportedResources());
  	loadBeanDefinitionsFromRegistrars(configClass.getImportBeanDefinitionRegistrars());
  }