Spring Boot 启动过程中的后置处理

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在前一篇博文 Spring Boot 启动流程分析的调用 beanFactory 的后置处理小节中提到了 beanFactory 的后置处理。这部分是 Spring IoC 的重点，但是因为内容很长，所以在这里单独开一篇博文来分析。

调用 beanFactoryPostProcessors

话不多说，继续从 AbstractApplicationContext#refresh 调用 invokeBeanFactoryPostProcessors 方法这里开始。

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protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
    // 重点
    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 (!NativeDetector.inNativeImage() && beanFactory.getTempClassLoader() == null &&
            beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
        beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
        beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
    }
}

// PostProcessorRegistrationDelegate
public static void invokeBeanFactoryPostProcessors(
        ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {

    // WARNING: Although it may appear that the body of this method can be easily
    // refactored to avoid the use of multiple loops and multiple lists, the use
    // of multiple lists and multiple passes over the names of processors is
    // intentional. We must ensure that we honor the contracts for PriorityOrdered
    // and Ordered processors. Specifically, we must NOT cause processors to be
    // instantiated (via getBean() invocations) or registered in the ApplicationContext
    // in the wrong order.
    //
    // Before submitting a pull request (PR) to change this method, please review the
    // list of all declined PRs involving changes to PostProcessorRegistrationDelegate
    // to ensure that your proposal does not result in a breaking change:
    // https://github.com/spring-projects/spring-framework/issues?q=PostProcessorRegistrationDelegate+is%3Aclosed+label%3A%22status%3A+declined%22

    // Invoke BeanDefinitionRegistryPostProcessors first, if any.
    Set<String> processedBeans = new HashSet<>();

    if (beanFactory instanceof BeanDefinitionRegistry registry) {
        List<BeanFactoryPostProcessor> regularPostProcessors = new ArrayList<>();
        List<BeanDefinitionRegistryPostProcessor> registryProcessors = new ArrayList<>();

        for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
            if (postProcessor instanceof BeanDefinitionRegistryPostProcessor registryProcessor) {
                registryProcessor.postProcessBeanDefinitionRegistry(registry);
                registryProcessors.add(registryProcessor);
            }
            else {
                regularPostProcessors.add(postProcessor);
            }
        }

        // Do not initialize FactoryBeans here: We need to leave all regular beans
        // uninitialized to let the bean factory post-processors apply to them!
        // Separate between BeanDefinitionRegistryPostProcessors that implement
        // PriorityOrdered, Ordered, and the rest.
        List<BeanDefinitionRegistryPostProcessor> currentRegistryProcessors = new ArrayList<>();

        // First, invoke the BeanDefinitionRegistryPostProcessors that implement PriorityOrdered.
        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);
        // 这里是重点
        invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry, beanFactory.getApplicationStartup());
        currentRegistryProcessors.clear();

        // Next, invoke the BeanDefinitionRegistryPostProcessors that implement Ordered.
        postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
        for (String ppName : postProcessorNames) {
            if (!processedBeans.contains(ppName) && beanFactory.isTypeMatch(ppName, Ordered.class)) {
                currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
                processedBeans.add(ppName);
            }
        }
        sortPostProcessors(currentRegistryProcessors, beanFactory);
        registryProcessors.addAll(currentRegistryProcessors);
        invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry, beanFactory.getApplicationStartup());
        currentRegistryProcessors.clear();

        // Finally, invoke all other BeanDefinitionRegistryPostProcessors until no further ones appear.
        boolean reiterate = true;
        while (reiterate) {
            reiterate = false;
            postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
            for (String ppName : postProcessorNames) {
                if (!processedBeans.contains(ppName)) {
                    currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
                    processedBeans.add(ppName);
                    reiterate = true;
                }
            }
            sortPostProcessors(currentRegistryProcessors, beanFactory);
            registryProcessors.addAll(currentRegistryProcessors);
            invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry, beanFactory.getApplicationStartup());
            currentRegistryProcessors.clear();
        }

        // Now, invoke the postProcessBeanFactory callback of all processors handled so far.
        invokeBeanFactoryPostProcessors(registryProcessors, beanFactory);
        invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);
    }

    else {
        // Invoke factory processors registered with the context instance.
        invokeBeanFactoryPostProcessors(beanFactoryPostProcessors, beanFactory);
    }

    // Do not initialize FactoryBeans here: We need to leave all regular beans
    // uninitialized to let the bean factory post-processors apply to them!
    String[] postProcessorNames =
            beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);

    // Separate between BeanFactoryPostProcessors that implement PriorityOrdered,
    // Ordered, and the rest.
    List<BeanFactoryPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
    List<String> orderedPostProcessorNames = new ArrayList<>();
    List<String> nonOrderedPostProcessorNames = new ArrayList<>();
    for (String ppName : postProcessorNames) {
        if (processedBeans.contains(ppName)) {
            // skip - already processed in first phase above
        }
        else if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
            priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class));
        }
        else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
            orderedPostProcessorNames.add(ppName);
        }
        else {
            nonOrderedPostProcessorNames.add(ppName);
        }
    }

    // First, invoke the BeanFactoryPostProcessors that implement PriorityOrdered.
    sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
    invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);

    // Next, invoke the BeanFactoryPostProcessors that implement Ordered.
    List<BeanFactoryPostProcessor> orderedPostProcessors = new ArrayList<>(orderedPostProcessorNames.size());
    for (String postProcessorName : orderedPostProcessorNames) {
        orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
    }
    sortPostProcessors(orderedPostProcessors, beanFactory);
    invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);

    // Finally, invoke all other BeanFactoryPostProcessors.
    List<BeanFactoryPostProcessor> nonOrderedPostProcessors = new ArrayList<>(nonOrderedPostProcessorNames.size());
    for (String postProcessorName : nonOrderedPostProcessorNames) {
        nonOrderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
    }
    invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory);

    // Clear cached merged bean definitions since the post-processors might have
    // modified the original metadata, e.g. replacing placeholders in values...
    beanFactory.clearMetadataCache();
}

这一大段代码，实话说看得我云里雾里的，但站在巨人的肩膀上之后，我大概明白 invokeBeanDefinitionRegistryPostProcessors 这个方法调用是里面的重点，那接下来就随着巨人的指引看看这个方法。

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private static void invokeBeanDefinitionRegistryPostProcessors(
        Collection<? extends BeanDefinitionRegistryPostProcessor> postProcessors, BeanDefinitionRegistry registry, ApplicationStartup applicationStartup) {

    for (BeanDefinitionRegistryPostProcessor postProcessor : postProcessors) {
        StartupStep postProcessBeanDefRegistry = applicationStartup.start("spring.context.beandef-registry.post-process")
                .tag("postProcessor", postProcessor::toString);
        postProcessor.postProcessBeanDefinitionRegistry(registry);
        postProcessBeanDefRegistry.end();
    }
}

debug 到 postProcessBeanDefinitionRegistry 这一行，发现 postProcessors 里面只有 ConfigurationClassPostProcessor，那么就进去看看这个类的 postProcessBeanDefinitionRegistry 在干什么。

下面代码会大量提到 configuration class 这个概念，而所谓 configuration class，就是带有 @Configuration 或者 @Component 注解的类。

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@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);
}

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

    // 遍历所有的bean definition的名字
    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);
            }
        }
        // 检查这个bean definition是不是一个配置类
        // 1. 取得bean definition的metadata
        // 2. 设定configurationClass这个bean definition attribute的值，full或lite（暂不清楚是干什么用的）
        // 3. 如果指定了这个配置类的生效顺序，那么将其记录到order这个attribute中
        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) {
        sbr = _sbr;
        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();
    }

    // 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 {
        StartupStep processConfig = this.applicationStartup.start("spring.context.config-classes.parse");
        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());
        }
        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 = Set.of(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());
    }

    // Store the PropertySourceDescriptors to contribute them Ahead-of-time if necessary
    this.propertySourceDescriptors = parser.getPropertySourceDescriptors();

    if (this.metadataReaderFactory instanceof CachingMetadataReaderFactory 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.clearCache();
    }
}

首先这里会遍历所有的 bean definition，找到其中的配置类，然后调用 parse 方法解析它们。

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public void parse(Set<BeanDefinitionHolder> configCandidates) {
    for (BeanDefinitionHolder holder : configCandidates) {
        BeanDefinition bd = holder.getBeanDefinition();
        try {
            // 如果是从Spring Boot项目进来的，那么这里的bd就是主类封装成的AnnotatedGenericBeanDefinition
            if (bd instanceof AnnotatedBeanDefinition annotatedBeanDef) {
                parse(annotatedBeanDef.getMetadata(), holder.getBeanName());
            }
            else if (bd instanceof AbstractBeanDefinition abstractBeanDef && abstractBeanDef.hasBeanClass()) {
                parse(abstractBeanDef.getBeanClass(), holder.getBeanName());
            }
            else {
                parse(bd.getBeanClassName(), holder.getBeanName());
            }
        }
        catch (BeanDefinitionStoreException ex) {
            throw ex;
        }
        catch (Throwable ex) {
            throw new BeanDefinitionStoreException(
                    "Failed to parse configuration class [" + bd.getBeanClassName() + "]", ex);
        }
    }

    // 加载默认的配置，即自动装配的入口
    this.deferredImportSelectorHandler.process();
}

protected final void parse(AnnotationMetadata metadata, String beanName) throws IOException {
    processConfigurationClass(new ConfigurationClass(metadata, beanName), DEFAULT_EXCLUSION_FILTER);
}

protected void processConfigurationClass(ConfigurationClass configClass, Predicate<String> filter) throws IOException {
    if (this.conditionEvaluator.shouldSkip(configClass.getMetadata(), ConfigurationPhase.PARSE_CONFIGURATION)) {
        return;
    }

    ConfigurationClass existingClass = this.configurationClasses.get(configClass);
    if (existingClass != null) {
        if (configClass.isImported()) {
            if (existingClass.isImported()) {
                existingClass.mergeImportedBy(configClass);
            }
            // Otherwise ignore new imported config class; existing non-imported class overrides it.
            return;
        }
        else {
            // Explicit bean definition found, probably replacing an import.
            // Let's remove the old one and go with the new one.
            this.configurationClasses.remove(configClass);
            this.knownSuperclasses.values().removeIf(configClass::equals);
        }
    }

    // Recursively process the configuration class and its superclass hierarchy.
    // 递归处理当前的configuration class及其父类
    SourceClass sourceClass = null;
    try {

        sourceClass = asSourceClass(configClass, filter);
        do {
            sourceClass = doProcessConfigurationClass(configClass, sourceClass, filter);
        }
        while (sourceClass != null);
    }
    catch (IOException ex) {
        throw new BeanDefinitionStoreException(
                "I/O failure while processing configuration class [" + sourceClass + "]", ex);
    }

    this.configurationClasses.put(configClass, configClass);
}

// ConfigurationClassParser
@Nullable
protected final SourceClass doProcessConfigurationClass(
        ConfigurationClass configClass, SourceClass sourceClass, Predicate<String> filter)
        throws IOException {

    // 如果这个configuration class带有@Component注解
    // 那么就递归处理其内部类
    if (configClass.getMetadata().isAnnotated(Component.class.getName())) {
        // Recursively process any member (nested) classes first
        processMemberClasses(configClass, sourceClass, filter);
    }

    // Process any @PropertySource annotations
    // 针对@PropertySource注解的处理
    for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(
            sourceClass.getMetadata(), org.springframework.context.annotation.PropertySource.class,
            PropertySources.class, true)) {
        if (this.propertySourceRegistry != null) {
            this.propertySourceRegistry.processPropertySource(propertySource);
        }
        else {
            logger.info("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +
                    "]. Reason: Environment must implement ConfigurableEnvironment");
        }
    }

    // Search for locally declared @ComponentScan annotations first.
    // 根据@ComponentScan注解扫描项目中的bean
    Set<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable(
            sourceClass.getMetadata(), ComponentScan.class, ComponentScans.class,
            MergedAnnotation::isDirectlyPresent);

    // Fall back to searching for @ComponentScan meta-annotations (which indirectly
    // includes locally declared composed annotations).
    // 如果没找到直接使用的@ComponentScan注解，那么就尝试找间接的引用
    // 如 @SpringBootApplication -> @ComponentScan
    if (componentScans.isEmpty()) {
        componentScans = AnnotationConfigUtils.attributesForRepeatable(sourceClass.getMetadata(),
                ComponentScan.class, ComponentScans.class, MergedAnnotation::isMetaPresent);
    }

    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
            // 在这里立即开始扫描
            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();
                }
                // 检查扫描到的这个bean是不是一个configuration class
                // 如果是，那么递归查找与它关脸的configuration class，如@Bean定义的bean，或带有@Import注解的@Component类
                if (ConfigurationClassUtils.checkConfigurationClassCandidate(bdCand, this.metadataReaderFactory)) {
                    parse(bdCand.getBeanClassName(), holder.getBeanName());
                }
            }
        }
    }

    // Process any @Import annotations
    // 递归处理@Import注解
    processImports(configClass, sourceClass, getImports(sourceClass), filter, 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
    // 处理带有@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;
}

Resource 定位

这里的重点是 Set<BeanDefinitionHolder> scannedBeanDefinitions = this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName()); 这个方法调用，它负责处理扫描的具体过程，所以点进 parse 方法看它干了什么。

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public Set<BeanDefinitionHolder> parse(AnnotationAttributes componentScan, String declaringClass) {
    // 取得扫描器，并设置它的各个属性
    ClassPathBeanDefinitionScanner scanner = new ClassPathBeanDefinitionScanner(this.registry,
            componentScan.getBoolean("useDefaultFilters"), this.environment, this.resourceLoader);

    Class<? extends BeanNameGenerator> generatorClass = componentScan.getClass("nameGenerator");
    boolean useInheritedGenerator = (BeanNameGenerator.class == generatorClass);
    scanner.setBeanNameGenerator(useInheritedGenerator ? this.beanNameGenerator :
            BeanUtils.instantiateClass(generatorClass));

    ScopedProxyMode scopedProxyMode = componentScan.getEnum("scopedProxy");
    if (scopedProxyMode != ScopedProxyMode.DEFAULT) {
        scanner.setScopedProxyMode(scopedProxyMode);
    }
    else {
        Class<? extends ScopeMetadataResolver> resolverClass = componentScan.getClass("scopeResolver");
        scanner.setScopeMetadataResolver(BeanUtils.instantiateClass(resolverClass));
    }

    scanner.setResourcePattern(componentScan.getString("resourcePattern"));

    for (AnnotationAttributes includeFilterAttributes : componentScan.getAnnotationArray("includeFilters")) {
        List<TypeFilter> typeFilters = TypeFilterUtils.createTypeFiltersFor(includeFilterAttributes, this.environment,
                this.resourceLoader, this.registry);
        for (TypeFilter typeFilter : typeFilters) {
            scanner.addIncludeFilter(typeFilter);
        }
    }
    for (AnnotationAttributes excludeFilterAttributes : componentScan.getAnnotationArray("excludeFilters")) {
        List<TypeFilter> typeFilters = TypeFilterUtils.createTypeFiltersFor(excludeFilterAttributes, this.environment,
            this.resourceLoader, this.registry);
        for (TypeFilter typeFilter : typeFilters) {
            scanner.addExcludeFilter(typeFilter);
        }
    }

    boolean lazyInit = componentScan.getBoolean("lazyInit");
    if (lazyInit) {
        scanner.getBeanDefinitionDefaults().setLazyInit(true);
    }

    Set<String> basePackages = new LinkedHashSet<>();
    // 取得basePackages的值，默认情况下为空，因为@SpringBootApplication里面就没给@ComponentScan配置这个值
    String[] basePackagesArray = componentScan.getStringArray("basePackages");
    for (String pkg : basePackagesArray) {
        String[] tokenized = StringUtils.tokenizeToStringArray(this.environment.resolvePlaceholders(pkg),
                ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
        Collections.addAll(basePackages, tokenized);
    }
    // 通过basePackageClasses里面指定的类找到它们所在的包，
    // 并将其加入basePackages
    // 默认情况下这个也是空的
    for (Class<?> clazz : componentScan.getClassArray("basePackageClasses")) {
        basePackages.add(ClassUtils.getPackageName(clazz));
    }
    // 查到最后basePackages还是空的话
    // 那就把带着@ComponentScan这个注解的类所在的包加到basePackages里面
    // 因为启动类带着这个注解，所以启动类所在的包就会被加进去
    if (basePackages.isEmpty()) {
        basePackages.add(ClassUtils.getPackageName(declaringClass));
    }

    scanner.addExcludeFilter(new AbstractTypeHierarchyTraversingFilter(false, false) {
        @Override
        protected boolean matchClassName(String className) {
            return declaringClass.equals(className);
        }
    });

    // 根据basePackages扫描
    return scanner.doScan(StringUtils.toStringArray(basePackages));
}

走到 scanner.doScan 的时候，就完成了定位这一步。接下来继续走进 ClassPathBeanDefinitionScanner#doScan 看它是怎么载入和注册各个 BeanDefinition 的。

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protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
    Assert.notEmpty(basePackages, "At least one base package must be specified");
    Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<>();
    for (String basePackage : basePackages) {
        // 从指定的包中扫描需要装载的Bean
        Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
        for (BeanDefinition candidate : candidates) {
            // 得到这个bean的scope，如singleton、prototype等
            ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
            // 并将其设置到BeanDefinition中
            candidate.setScope(scopeMetadata.getScopeName());
            String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
            if (candidate instanceof AbstractBeanDefinition abstractBeanDefinition) {
                postProcessBeanDefinition(abstractBeanDefinition, beanName);
            }
            if (candidate instanceof AnnotatedBeanDefinition annotatedBeanDefinition) {
                AnnotationConfigUtils.processCommonDefinitionAnnotations(annotatedBeanDefinition);
            }
            // 检查与这个bean名字对应的BeanDefinition是否已经被注册过
            if (checkCandidate(beanName, candidate)) {
                BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
                definitionHolder =
                        AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
                beanDefinitions.add(definitionHolder);
                // 将这个bean注册到IoC容器
                registerBeanDefinition(definitionHolder, this.registry);
            }
        }
    }
    return beanDefinitions;
}

BeanDefinition 载入

上面代码的 for 循环里首先会执行 findCandidateComponents，这里就是负责载入这个 basePackage 下的所有 BeanDefinition 的。

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public Set<BeanDefinition> findCandidateComponents(String basePackage) {
    if (this.componentsIndex != null && indexSupportsIncludeFilters()) {
        return addCandidateComponentsFromIndex(this.componentsIndex, basePackage);
    }
    else {
        return scanCandidateComponents(basePackage);
    }
}

private Set<BeanDefinition> scanCandidateComponents(String basePackage) {
    Set<BeanDefinition> candidates = new LinkedHashSet<>();
    try {
        // 根据basePackages生成classpath，
        // 如classpath*:com/example/demo/**/*.class
        String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +
                resolveBasePackage(basePackage) + '/' + this.resourcePattern;
        // 从这个classpath下面扫描所有匹配的资源，也就是类
        Resource[] resources = getResourcePatternResolver().getResources(packageSearchPath);
        boolean traceEnabled = logger.isTraceEnabled();
        boolean debugEnabled = logger.isDebugEnabled();
        // 循环处理上面找到的类，将符合条件的类包装成ScannedGenericBeanDefinition并加到candidates中
        for (Resource resource : resources) {
            String filename = resource.getFilename();
            if (filename != null && filename.contains(ClassUtils.CGLIB_CLASS_SEPARATOR)) {
                // Ignore CGLIB-generated classes in the classpath
                continue;
            }
            if (traceEnabled) {
                logger.trace("Scanning " + resource);
            }
            try {
                MetadataReader metadataReader = getMetadataReaderFactory().getMetadataReader(resource);
                // 检查这个类是不是既没有命中任何一个exclude filter，并至少命中了一个include filter
                // 其中一个include filter就是根据@Component注解来过滤的
                if (isCandidateComponent(metadataReader)) {
                    ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader);
                    sbd.setSource(resource);
                    if (isCandidateComponent(sbd)) {
                        // 打日志，略
                        candidates.add(sbd);
                    }
                    else {
                        // 打日志，略
                    }
                }
                else {
                    // 打日志，略
                }
            }
            catch (FileNotFoundException ex) {
                // 打日志，略
            }
            catch (ClassFormatException ex) {
                if (shouldIgnoreClassFormatException) {
                    // 打日志，略
                }
                else {
                    throw new BeanDefinitionStoreException("Incompatible class format in " + resource +
                            ": set system property 'spring.classformat.ignore' to 'true' " +
                            "if you mean to ignore such files during classpath scanning", ex);
                }
            }
            catch (Throwable ex) {
                throw new BeanDefinitionStoreException("Failed to read candidate component class: " + resource, ex);
            }
        }
    }
    catch (IOException ex) {
        throw new BeanDefinitionStoreException("I/O failure during classpath scanning", ex);
    }
    return candidates;
}

注册 BeanDefinition

回到 doScan 方法，在得到所有的 BeanDefinition 之后，就会调用 registerBeanDefinition 方法来完成 BeanDefinition 的注册。

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protected void registerBeanDefinition(BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry) {
    BeanDefinitionReaderUtils.registerBeanDefinition(definitionHolder, registry);
}

// BeanDefinitionReaderUtils
public static void registerBeanDefinition(
        BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
        throws BeanDefinitionStoreException {

    // Register bean definition under primary name.
    String beanName = definitionHolder.getBeanName();
    // 将这个bean注册到DefaultListableBeanFactory的BeanDefinitionMap
    registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());

    // Register aliases for bean name, if any.
    String[] aliases = definitionHolder.getAliases();
    if (aliases != null) {
        for (String alias : aliases) {
            registry.registerAlias(beanName, alias);
        }
    }
}

这里就是把传进来的 BeanDefinition 注册到 DefaultListableBeanFactory 的 BeanDefinitionMap 中。这部分在上一篇博文中已经提过，就不重复了。

在 doScan 方法的 for 循环遍历完所有的 BeanDefinition 之后，所有带有 @Component 注解的 bean 就全部注册好了。接下来会回到 doProcessConfigurationClass 方法，继续处理带有 @Import 注解的类和带有 @Bean 注解的方法。

参考文章

  • SpringBoot 启动流程分析（四）：IoC 容器的初始化过程