Spring源码分析（四） AnnotationConfigBeanDefinitionParser

之前我们说了XmlBeanDefinitionReader是怎么读取xml配置文件的，那么其实我们都知道,spring的bean配置有2种方式，一种是xml文件配置,一种是注解配置,那么spring是怎么解析注解并注册到ioc容器中的呢,这篇文章我们就来看看spring是怎么做的。
这里我们就要介绍一个类AnnotationConfigBeanDefinitionParser,这个类实现了BeanDefinitionParser接口,我们看一下parse方法是怎么实现的

@Override
@Nullable
public BeanDefinition parse(Element element, ParserContext parserContext) {
    Object source = parserContext.extractSource(element);

    // Obtain bean definitions for all relevant BeanPostProcessors.
    Set processorDefinitions =
            AnnotationConfigUtils.registerAnnotationConfigProcessors(parserContext.getRegistry(), source);

    // Register component for the surrounding  element.
    CompositeComponentDefinition compDefinition = new CompositeComponentDefinition(element.getTagName(), source);
    parserContext.pushContainingComponent(compDefinition);

    // Nest the concrete beans in the surrounding component.
    for (BeanDefinitionHolder processorDefinition : processorDefinitions) {
        parserContext.registerComponent(new BeanComponentDefinition(processorDefinition));
    }

    // Finally register the composite component.
    parserContext.popAndRegisterContainingComponent();

    return null;
}

这里调用了AnnotationConfigUtils的registerAnnotationConfigProcessors方法,注册了一个ConfigurationClassPostProcessor到ioc容器中,我们再看一下这个类的postProcessBeanDefinitionRegistry方法

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);
}

再到processConfigBeanDefinitions方法

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

    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
    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;
    if (registry instanceof SingletonBeanRegistry) {
        sbr = (SingletonBeanRegistry) registry;
        if (!this.localBeanNameGeneratorSet) {
            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
    ConfigurationClassParser parser = new ConfigurationClassParser(
            this.metadataReaderFactory, this.problemReporter, this.environment,
            this.resourceLoader, this.componentScanBeanNameGenerator, registry);

    Set candidates = new LinkedHashSet<>(configCandidates);
    Set alreadyParsed = new HashSet<>(configCandidates.size());
    do {
        parser.parse(candidates);
        parser.validate();

        Set 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());
        }
        this.reader.loadBeanDefinitions(configClasses);
        alreadyParsed.addAll(configClasses);

        candidates.clear();
        if (registry.getBeanDefinitionCount() > candidateNames.length) {
            String[] newCandidateNames = registry.getBeanDefinitionNames();
            Set oldCandidateNames = new HashSet<>(Arrays.asList(candidateNames));
            Set 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();
    }
}

简单来说,就是把读取注解的属性值放到BeanDefinition里,然后再注册到ioc容器中。
至此,注解方式的bean配置就完成了。
AnnotationConfigBeanDefinitionParser的分析就到这里了。