Spring基于@Configuration的类配置的内部源码实现

概述

Spring容器启动时，即ApplicationContext接口实现类的对象实例执行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AutowiredAnnotationBeanPostProcessorRootBeanDefinition 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CommonAnnotationBeanPostProcessorRootBeanDefinition 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 foundif (configCandidates.isEmpty()) {return;}// Sort by previously determined @Order value, if applicableconfigCandidates.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 classConfigurationClassParser 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 contentif (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BeanFactorythis.reader.loadBeanDefinitions(configClasses);...}

2. ConfigurationClassParser

遍历并处理configCandidates集合：
1. 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。
2. 同时会创建该@Configuration注解的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 firstprocessMemberClasses(configClass, sourceClass);}// Process any @PropertySource annotationsfor (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 annotationsSet<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 immediatelySet<BeanDefinitionHolder> scannedBeanDefinitions =this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());// Check the set of scanned definitions for any further config classes and parse recursively if neededfor (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 annotationsprocessImports(configClass, sourceClass, getImports(sourceClass), true);// Process any @ImportResource annotationsAnnotationAttributes 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 methodsSet<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);for (MethodMetadata methodMetadata : beanMethods) {configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));}// Process default methods on interfacesprocessInterfaces(configClass, sourceClass);// Process superclass, if anyif (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 recursereturn sourceClass.getSuperClass();}}// No superclass -> processing is completereturn 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());
}