Spring Boot自动配置原理

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 1、在Spring Boot的启动过程中，要运行Spring应用程序，创建并刷新一个新的ApplicationContext上下文。org.springframework.boot.SpringApplication#run(java.lang.String…)参数是应用程序参数，通常从Java主方法传递过来。

public ConfigurableApplicationContext run(String... args) {
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    DefaultBootstrapContext bootstrapContext = createBootstrapContext();
    ConfigurableApplicationContext context = null;
    configureHeadlessProperty();
    SpringApplicationRunListeners listeners = getRunListeners(args);
    listeners.starting(bootstrapContext, this.mainApplicationClass);
    try {
        ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
        ConfigurableEnvironment environment = prepareEnvironment(listeners, bootstrapContext, applicationArguments);
        configureIgnoreBeanInfo(environment);
        Banner printedBanner = printBanner(environment);
        context = createApplicationContext();
        context.setApplicationStartup(this.applicationStartup);
        // 此处完成自动装配的过程
        prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);
        refreshContext(context);
        afterRefresh(context, applicationArguments);
        stopWatch.stop();
        if (this.logStartupInfo) {
            new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
        }
        listeners.started(context);
        callRunners(context, applicationArguments);
    }
    catch (Throwable ex) {
        handleRunFailure(context, ex, listeners);
        throw new IllegalStateException(ex);
    }

    try {
        listeners.running(context);
    }
    catch (Throwable ex) {
        handleRunFailure(context, ex, null);
        throw new IllegalStateException(ex);
    }
    return context;
}

2、在org.springframework.boot.SpringApplication#prepareContext方法中查找load方法，一层一层向内点击，找到最终的load方法

private void prepareContext(DefaultBootstrapContext bootstrapContext, ConfigurableApplicationContext context,
                            ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
                            ApplicationArguments applicationArguments, Banner printedBanner) {
    context.setEnvironment(environment);
    postProcessApplicationContext(context);
    applyInitializers(context);
    listeners.contextPrepared(context);
    bootstrapContext.close(context);
    if (this.logStartupInfo) {
        logStartupInfo(context.getParent() == null);
        logStartupProfileInfo(context);
    }
    // Add boot specific singleton beans
    ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
    beanFactory.registerSingleton("springApplicationArguments", applicationArguments);
    if (printedBanner != null) {
        beanFactory.registerSingleton("springBootBanner", printedBanner);
    }
    if (beanFactory instanceof DefaultListableBeanFactory) {
        ((DefaultListableBeanFactory) beanFactory)
                .setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
    }
    if (this.lazyInitialization) {
        context.addBeanFactoryPostProcessor(new LazyInitializationBeanFactoryPostProcessor());
    }
    // Load the sources
    Set<Object> sources = getAllSources();
    Assert.notEmpty(sources, "Sources must not be empty");
    // load方法完成该功能
    load(context, sources.toArray(new Object[0]));
    listeners.contextLoaded(context);
}

/**
 * 将bean加载到应用程序上下文中。
 * @param context 加载bean的上下文
 * @param sources 要加载的资源
 */
protected void load(ApplicationContext context, Object[] sources) {
    if (logger.isDebugEnabled()) {
        logger.debug("Loading source " + StringUtils.arrayToCommaDelimitedString(sources));
    }
    // 获取bean对象定义的加载器
    BeanDefinitionLoader loader = createBeanDefinitionLoader(getBeanDefinitionRegistry(context), sources);
    if (this.beanNameGenerator != null) {
        loader.setBeanNameGenerator(this.beanNameGenerator);
    }
    if (this.resourceLoader != null) {
        loader.setResourceLoader(this.resourceLoader);
    }
    if (this.environment != null) {
        loader.setEnvironment(this.environment);
    }
    loader.load();
}

3、实际执行load的是org.springframework.boot.BeanDefinitionLoader#load(java.lang.Object)方法，如下：

private void load(Object source) {
    Assert.notNull(source, "Source must not be null");
    // 如果是 Class 类型，启用注解类型
    if (source instanceof Class<?>) {
        load((Class<?>) source);
        return;
    }
    // 如果是 Resource 类型，启动 XML 解析
    if (source instanceof Resource) {
        load((Resource) source);
        return;
    }
    // 如果是 Package 类型，启用扫描包，例如 @ComponentScan
    if (source instanceof Package) {
        load((Package) source);
        return;
    }
    // 如果是字符串类型，直接加载
    if (source instanceof CharSequence) {
        load((CharSequence) source);
        return;
    }
    throw new IllegalArgumentException("Invalid source type " + source.getClass());
}

4、下面方法将用来判断是否资源的类型，是使用groovy加载还是使用注解的方式

private void load(Class<?> source) {
    // 判断使用 groovy 脚本
    if (isGroovyPresent() && BeanDefinitionLoader.GroovyBeanDefinitionSource.class.isAssignableFrom(source)) {
        // Any GroovyLoaders added in beans{} DSL can contribute beans here
        BeanDefinitionLoader.GroovyBeanDefinitionSource loader = BeanUtils.instantiateClass(source, BeanDefinitionLoader.GroovyBeanDefinitionSource.class);
        ((GroovyBeanDefinitionReader) this.groovyReader).beans(loader.getBeans());
    }
    // 使用注解加载
    if (isEligible(source)) {
        this.annotatedReader.register(source);
    }
}

5、下面方法判断启动类中是否包含@Component注解，但是会神奇地发现我们的启动类中并没有该注解，继续跟进发现MergedAnnotations类传入了一个参数SearchStrategy.TYPE_HIERARCHY，会查找继承关系中是否包含这个注解，@SpringBootApplication –> @SpringBootConfiguration –> @Configuration –> @Component，当找到@Component注解之后，会把该对象注册到AnnotatedBeanDefinitionReader对象中

// org.springframework.boot.context.properties.ConfigurationPropertiesScanRegistrar#isComponent
private boolean isComponent(Class<?> type) {
    return MergedAnnotations.from(type, SearchStrategy.TYPE_HIERARCHY).isPresent(Component.class);
}

/**
 * 从给定的bean类注册一个bean，从类声明的注释派生它的元数据。
 * org.springframework.context.annotation.AnnotatedBeanDefinitionReader#doRegisterBean(java.lang.Class, java.lang.String, java.lang.Class[], java.util.function.Supplier, org.springframework.beans.factory.config.BeanDefinitionCustomizer[])
 * @param beanClass bean的类
 * @param name bean的显式名称
 * @param qualifiers 除了bean类级别的限定符之外，还要考虑特定的限定符注释(如果有的话)
 * @param supplier 用于创建bean实例的回调(可能为空)
 * @param customizers 一个或多个回调函数用于自定义工厂的bean定义，例如设置lazy-init或primary标志
 */
private <T> void doRegisterBean(Class<T> beanClass, @Nullable String name,
                                @Nullable Class<? extends Annotation>[] qualifiers, @Nullable Supplier<T> supplier,
                                @Nullable BeanDefinitionCustomizer[] customizers) {

    AnnotatedGenericBeanDefinition abd = new AnnotatedGenericBeanDefinition(beanClass);
    if (this.conditionEvaluator.shouldSkip(abd.getMetadata())) {
        return;
    }

    abd.setInstanceSupplier(supplier);
    ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(abd);
    abd.setScope(scopeMetadata.getScopeName());
    String beanName = (name != null ? name : this.beanNameGenerator.generateBeanName(abd, this.registry));

    AnnotationConfigUtils.processCommonDefinitionAnnotations(abd);
    if (qualifiers != null) {
        for (Class<? extends Annotation> qualifier : qualifiers) {
            if (Primary.class == qualifier) {
                abd.setPrimary(true);
            }
            else if (Lazy.class == qualifier) {
                abd.setLazyInit(true);
            }
            else {
                abd.addQualifier(new AutowireCandidateQualifier(qualifier));
            }
        }
    }
    if (customizers != null) {
        for (BeanDefinitionCustomizer customizer : customizers) {
            customizer.customize(abd);
        }
    }

    BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(abd, beanName);
    definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
    BeanDefinitionReaderUtils.registerBeanDefinition(definitionHolder, this.registry);
}

/**
 * 将给定的bean定义注册到给定的bean工厂。
 * org.springframework.beans.factory.support.BeanDefinitionReaderUtils#registerBeanDefinition(org.springframework.beans.factory.config.BeanDefinitionHolder, org.springframework.beans.factory.support.BeanDefinitionRegistry)
 * @param definitionHolder bean定义包括名称和别名
 * @param registry 要注册的bean工厂
 * @throws BeanDefinitionStoreException 如果注册失败
 */
public static void registerBeanDefinition(
        BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
        throws BeanDefinitionStoreException {

    // Register bean definition under primary name.
    String beanName = definitionHolder.getBeanName();
    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);
        }
    }
}

// org.springframework.boot.autoconfigure.SpringBootApplication
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
		@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {...}

// org.springframework.boot.SpringBootConfiguration
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
@Indexed
public @interface SpringBootConfiguration {...}

// org.springframework.context.annotation.Configuration
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Configuration {...}

当看完上述代码之后，只是完成了启动对象的注入，自动装配还没有开始，下面开始进入到自动装配。

6、自动装配入口，从刷新容器开始

// org.springframework.context.support.AbstractApplicationContext#refresh
@Override
public void refresh() throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
        StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");

        // 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);

            StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
            // Invoke factory processors registered as beans in the context.
            // 此处是自动装配的入口
            invokeBeanFactoryPostProcessors(beanFactory);
            ...
        }

        catch (BeansException ex) {
            ...
        }

        finally {
            ...
        }
    }
}

7、在invokeBeanFactoryPostProcessors方法中完成bean的实例化和执行

/**
 * 实例化和调用所有已注册的BeanFactoryPostProcessor bean，如果给定了明确的顺序。
 * 必须在单例实例化之前调用。
 */
protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
    // 开始执行 beanFactoryPostProcessors 对应实现类，需要知道的是 beanFactoryPostProcessors 是 Spring 的扩展接口，在刷新容器之前，该接口可以用来修改 bean 元数据信息
    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()));
    }
}

8、查看invokeBeanFactoryPostProcessors的具体执行方法

// org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(org.springframework.beans.factory.config.ConfigurableListableBeanFactory, java.util.List<org.springframework.beans.factory.config.BeanFactoryPostProcessor>)
public static void invokeBeanFactoryPostProcessors(
        ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {

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

    if (beanFactory instanceof BeanDefinitionRegistry) {
        BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
        List<BeanFactoryPostProcessor> regularPostProcessors = new ArrayList<>();
        List<BeanDefinitionRegistryPostProcessor> registryProcessors = new ArrayList<>();
        // 开始遍历三个内部类，如果属于 BeanDefinitionRegistryPostProcessor 子类，加入到 bean 注册的集合，否则加入到 regularPostProcessors
        for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
            if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
                BeanDefinitionRegistryPostProcessor registryProcessor =
                        (BeanDefinitionRegistryPostProcessor) postProcessor;
                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);
        // 首先执行类型为 PriorityOrdered 的 BeanDefinitionRegistryPostProcessor
        // PriorityOrdered 类型表明为优先执行
        for (String ppName : postProcessorNames) {
            if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
                // 获取对应的 bean
                currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
                // 用来存储已经执行过的 BeanDefinitionRegistryPostProcessor
                processedBeans.add(ppName);
            }
        }
        sortPostProcessors(currentRegistryProcessors, beanFactory);
        registryProcessors.addAll(currentRegistryProcessors);
        // 开始执行装配逻辑
        invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry, beanFactory.getApplicationStartup());
        currentRegistryProcessors.clear();

        // Next, invoke the BeanDefinitionRegistryPostProcessors that implement Ordered.
        // 其次执行类型为 Ordered 的 BeanDefinitionRegistryPostProcessor
        // Ordered 表明按顺序执行
        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.
        // 循环中执行类型不为 PriorityOrdered、Ordered 类型的 BeanDefinitionRegistryPostProcessor
        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.
        // 执行父类方法，优先执行注册处理类
        invokeBeanFactoryPostProcessors(registryProcessors, beanFactory);
        // 执行有规则处理类
        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();
}

9、开始执行自动配置逻辑（启动类指定的配置，非默认配置），可以通过debug的方式一层层向里进行查找，会发现最终会在ConfigurationClassParser类中，此类是所有配置类的解析类，所有的解析逻辑在org.springframework.context.annotation.ConfigurationClassParser#parse(java.util.Set<org.springframework.beans.factory.config.BeanDefinitionHolder>)中

public void parse(Set<BeanDefinitionHolder> configCandidates) {
    for (BeanDefinitionHolder holder : configCandidates) {
        BeanDefinition bd = holder.getBeanDefinition();
        try {
            // 是否是注解类
            if (bd instanceof AnnotatedBeanDefinition) {
                parse(((AnnotatedBeanDefinition) bd).getMetadata(), holder.getBeanName());
            }
            else if (bd instanceof AbstractBeanDefinition && ((AbstractBeanDefinition) bd).hasBeanClass()) {
                parse(((AbstractBeanDefinition) bd).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);
        }
    }
    // 执行配置类
    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.
    ConfigurationClassParser.SourceClass sourceClass = asSourceClass(configClass, filter);
    do {
        // 循环处理 bean，如果有父类，则处理父类，直至结束
        sourceClass = doProcessConfigurationClass(configClass, sourceClass, filter);
    }
    while (sourceClass != null);

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

10、继续跟进doProcessConfigurationClass方法，此方式是支持注解配置的核心逻辑

/**
 * 通过从源类读取注释、成员和方法，应用处理并构建一个完整的ConfigurationClass。当发现相关源时，可以多次调用该方法。
 * @param configClass 正在构建的配置类
 * @return 超类，如果没有发现或之前处理过，则为空
 */
@Nullable
protected final SourceClass doProcessConfigurationClass(
        ConfigurationClass configClass, SourceClass sourceClass, Predicate<String> filter)
        throws IOException {

    // 处理内部类逻辑，由于传来的参数是启动类，并不包含内部类，所以踢啊偶哦 
    if (configClass.getMetadata().isAnnotated(Component.class.getName())) {
        // Recursively process any member (nested) classes first
        processMemberClasses(configClass, sourceClass, filter);
    }

    // Process any @PropertySource annotations
    // 针对属性配置的解析
    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
    // 这里是根据启动类 @ComponentScan 注解来扫描项目中的 bean
    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
            // 遍历项目中的 bean，如果是注解定义的bean，则进一步解析
            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)) {
                    // 递归解析，所有的 bean，如果有注解，会进一步解析注解中包含的 bean
                    parse(bdCand.getBeanClassName(), holder.getBeanName());
                }
            }
        }
    }

    // Process any @Import annotations
    // 递归解析，获取导入的配置类，很多情况下，导入的配置类中会同样包含导入类注解
    processImports(configClass, sourceClass, getImports(sourceClass), filter, true);

    // Process any @ImportResource annotations
    // 解析 @ImportResource 配置类
    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 @Bean 注解修饰的类
    Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);
    for (MethodMetadata methodMetadata : beanMethods) {
        configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
    }

    // Process default methods on interfaces
    // 处理接口中的默认方法
    processInterfaces(configClass, sourceClass);

    // Process superclass, if any
    // 如果该类有父类，则继续返回，上层方法判断不为空，则继续递归执行
    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;
}

11、查看获取配置类的逻辑

processImports(configClass, sourceClass, getImports(sourceClass), filter, true);

/**
 * 返回@Import类，考虑所有的元注解。
 */
private Set<SourceClass> getImports(SourceClass sourceClass) throws IOException {
    Set<SourceClass> imports = new LinkedHashSet<>();
    Set<SourceClass> visited = new LinkedHashSet<>();
    collectImports(sourceClass, imports, visited);
    return imports;
}

/**
 * 递归地收集所有声明的@Import值。与大多数元注释不同的是，使用不同的值声明几个@Imports是有效的;通常从类的第一个元注释返回值的过程是不够的。
 * 例如，除了源自@Enable注释的元导入之外，@Configuration类通常还声明直接的@Imports。
 * @param sourceClass 要搜索的类
 * @param imports 到目前为止的进口额
 * @param visited 用于跟踪访问的类，以防止无限递归
 * @throws IOException 如果从指定的类读取元数据有任何问题
 */
private void collectImports(SourceClass sourceClass, Set<SourceClass> imports, Set<SourceClass> visited)
        throws IOException {

    if (visited.add(sourceClass)) {
        for (SourceClass annotation : sourceClass.getAnnotations()) {
            String annName = annotation.getMetadata().getClassName();
            if (!annName.equals(Import.class.getName())) {
                collectImports(annotation, imports, visited);
            }
        }
        imports.addAll(sourceClass.getAnnotationAttributes(Import.class.getName(), "value"));
    }
}

12、继续返回到org.springframework.context.annotation.ConfigurationClassParser#parse(java.util.Set<org.springframework.beans.factory.config.BeanDefinitionHolder>)方法中的最后遗憾，继续跟进该方法：

this.deferredImportSelectorHandler.process();

public void process() {
    List<DeferredImportSelectorHolder> deferredImports = this.deferredImportSelectors;
    this.deferredImportSelectors = null;
    try {
        if (deferredImports != null) {
            DeferredImportSelectorGroupingHandler handler = new DeferredImportSelectorGroupingHandler();
            deferredImports.sort(DEFERRED_IMPORT_COMPARATOR);
            deferredImports.forEach(handler::register);
            handler.processGroupImports();
        }
    }
    finally {
        this.deferredImportSelectors = new ArrayList<>();
    }
}

public void processGroupImports() {
    for (DeferredImportSelectorGrouping grouping : this.groupings.values()) {
        Predicate<String> exclusionFilter = grouping.getCandidateFilter();
        grouping.getImports().forEach(entry -> {
            ConfigurationClass configurationClass = this.configurationClasses.get(entry.getMetadata());
            try {
                processImports(configurationClass, asSourceClass(configurationClass, exclusionFilter),
                        Collections.singleton(asSourceClass(entry.getImportClassName(), exclusionFilter)),
                        exclusionFilter, false);
            }
            catch (BeanDefinitionStoreException ex) {
                throw ex;
            }
            catch (Throwable ex) {
                throw new BeanDefinitionStoreException(
                        "Failed to process import candidates for configuration class [" +
                                configurationClass.getMetadata().getClassName() + "]", ex);
            }
        });
    }
}

/**
 * 返回组定义的导入。
 * @return 每个导入及其关联的配置类
 */
public Iterable<Group.Entry> getImports() {
    for (DeferredImportSelectorHolder deferredImport : this.deferredImports) {
        this.group.process(deferredImport.getConfigurationClass().getMetadata(),
                deferredImport.getImportSelector());
    }
    return this.group.selectImports();
}

public DeferredImportSelector getImportSelector() {
    return this.importSelector;
}

// org.springframework.boot.autoconfigure.AutoConfigurationImportSelector.AutoConfigurationGroup#process
@Override
public void process(AnnotationMetadata annotationMetadata, DeferredImportSelector deferredImportSelector) {
    Assert.state(deferredImportSelector instanceof AutoConfigurationImportSelector,
            () -> String.format("Only %s implementations are supported, got %s",
                    AutoConfigurationImportSelector.class.getSimpleName(),
                    deferredImportSelector.getClass().getName()));
    AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector)
            .getAutoConfigurationEntry(annotationMetadata);
    this.autoConfigurationEntries.add(autoConfigurationEntry);
    for (String importClassName : autoConfigurationEntry.getConfigurations()) {
        this.entries.putIfAbsent(importClassName, annotationMetadata);
    }
}

​