Spring源码-ImportSelector实现分析

public interface ImportSelector {

    /**
     * Select and return the names of which class(es) should be imported based on
     * the {@link AnnotationMetadata} of the importing @{@link Configuration} class.
     */
    String[] selectImports(AnnotationMetadata importingClassMetadata);

}

看一下该接口，注释说明，实现该接口的selectImport则返回的String[]的 class字符串 类将被注册为@Configuration，即只要实现该接口返回的类将被注入Spring容器。@see ImportBeanDefinitionRegistrar则说明是使用 ImportBeanDefinitionRegistrar的registerBeanDefinitions方法进行注入。看一下其直接继承的子类关系图：

 

大致分为五类（后续分析其他源码需要）：
Spring boot的自动装配、测试、缓存自动装配、web相关。
Spring的aop相关的：@EnableTransactionManagement、@Async、@EnableCaching
下面以@EnableTransactionManagement主线进行梳理
只是从注释看是通过ImportBeanDefinitionRegistrar进行注入的，但是今天梳理了一下，发现源头还是从ApplicationContext的refresh模板模式方法（可以参见模板方法模式-简单实现和Spring中的使用分析）开始。

@Override
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);
 
            // Invoke factory processors registered as beans in the context.
            invokeBeanFactoryPostProcessors(beanFactory);
 
            // Register bean processors that intercept bean creation.
            registerBeanPostProcessors(beanFactory);
 
            // Initialize message source for this context.
            initMessageSource();
 
            // Initialize event multicaster for this context.
            initApplicationEventMulticaster();
 
            // Initialize other special beans in specific context subclasses.
            onRefresh();
 
            // Check for listener beans and register them.
            registerListeners();
 
            // Instantiate all remaining (non-lazy-init) singletons.
            finishBeanFactoryInitialization(beanFactory);
 
            // Last step: publish corresponding event.
            finishRefresh();
        }
 
        catch (BeansException ex) {
            if (logger.isWarnEnabled()) {
                logger.warn("Exception encountered during context initialization - " +
                        "cancelling refresh attempt: " + ex);
            }
 
            // Destroy already created singletons to avoid dangling resources.
            destroyBeans();
 
            // Reset 'active' flag.
            cancelRefresh(ex);
 
            // Propagate exception to caller.
            throw ex;
        }
 
        finally {
            // Reset common introspection caches in Spring's core, since we
            // might not ever need metadata for singleton beans anymore...
            resetCommonCaches();
        }
    }
}

模板方法第五步就是：

// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);

委派给PostProcessorRegistrationDelegate进行处理，方法很长都在从beanFactory中获取

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<>();
 
        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);
        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);
 
        // 执行到该处，处理 BeanDefinitionRegistryPostProcessor 类型的注册时，当前
        // currentRegistryProcessors中包含了 ConfigurationClassPostParser 类型
        invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
        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);
        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);
            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();

}

重要的点在：

// 执行到该处，处理 BeanDefinitionRegistryPostProcessor 类型的注册时，当前
// currentRegistryProcessors中包含了 ConfigurationClassPostParser 类型
invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);

执行到该处是，先从beanFactory中获取 BeanDefinitionRegistryPostProcessor类型或者子类的bean名称，再判断是否为PriorityOrdered类型，最好获取到了 ConfigurationClassPostProcessor先看看结构，符合上面的判断：

private static void invokeBeanDefinitionRegistryPostProcessors(
        Collection<? extends BeanDefinitionRegistryPostProcessor> postProcessors, 
        BeanDefinitionRegistry registry) {
 
    for (BeanDefinitionRegistryPostProcessor postProcessor : postProcessors) {
        postProcessor.postProcessBeanDefinitionRegistry(registry);
    }
}

调用ConfigurationClassPostProcessor的postProcessBeanDefinitionRegistry方法：

@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);
}

执行： processConfigBeanDefinitions方法

public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
    List<BeanDefinitionHolder> configCandidates = new ArrayList();
    String[] candidateNames = registry.getBeanDefinitionNames();
    String[] var4 = candidateNames;
    int var5 = candidateNames.length;
 
    for(int var6 = 0; var6 < var5; ++var6) {
        String beanName = var4[var6];
        BeanDefinition beanDef = registry.getBeanDefinition(beanName);
        if (beanDef.getAttribute(ConfigurationClassUtils.CONFIGURATION_CLASS_ATTRIBUTE) != null) {
            if (this.logger.isDebugEnabled()) {
                this.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));
        }
    }
 
    if (!configCandidates.isEmpty()) {
        configCandidates.sort((bd1, bd2) -> {
            int i1 = ConfigurationClassUtils.getOrder(bd1.getBeanDefinition());
            int i2 = ConfigurationClassUtils.getOrder(bd2.getBeanDefinition());
            return Integer.compare(i1, i2);
        });
        SingletonBeanRegistry sbr = null;
        if (registry instanceof SingletonBeanRegistry) {
            sbr = (SingletonBeanRegistry)registry;
            if (!this.localBeanNameGeneratorSet) {
                BeanNameGenerator generator = (BeanNameGenerator)sbr.getSingleton("org.springframework.context.annotation.internalConfigurationBeanNameGenerator");
                if (generator != null) {
                    this.componentScanBeanNameGenerator = generator;
                    this.importBeanNameGenerator = generator;
                }
            }
        }
 
        if (this.environment == null) {
            this.environment = new StandardEnvironment();
        }
 
        ConfigurationClassParser parser = new ConfigurationClassParser(this.metadataReaderFactory, this.problemReporter, this.environment, this.resourceLoader, this.componentScanBeanNameGenerator, registry);
        Set<BeanDefinitionHolder> candidates = new LinkedHashSet(configCandidates);
        HashSet alreadyParsed = new HashSet(configCandidates.size());
 
        do {
            parser.parse(candidates);
            parser.validate();
            Set<ConfigurationClass> configClasses = new LinkedHashSet(parser.getConfigurationClasses());
            configClasses.removeAll(alreadyParsed);
            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<String> oldCandidateNames = new HashSet(Arrays.asList(candidateNames));
                Set<String> alreadyParsedClasses = new HashSet();
                Iterator var12 = alreadyParsed.iterator();
 
                while(var12.hasNext()) {
                    ConfigurationClass configurationClass = (ConfigurationClass)var12.next();
                    alreadyParsedClasses.add(configurationClass.getMetadata().getClassName());
                }
 
                String[] var23 = newCandidateNames;
                int var24 = newCandidateNames.length;
 
                for(int var14 = 0; var14 < var24; ++var14) {
                    String candidateName = var23[var14];
                    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());
 
        if (sbr != null && !sbr.containsSingleton(IMPORT_REGISTRY_BEAN_NAME)) {
            sbr.registerSingleton(IMPORT_REGISTRY_BEAN_NAME, parser.getImportRegistry());
        }
 
        if (this.metadataReaderFactory instanceof CachingMetadataReaderFactory) {
            ((CachingMetadataReaderFactory)this.metadataReaderFactory).clearCache();
        }
 
    }
}

则真正执行的是  ConfigurationClassParser的parse方法

public void parse(Set<BeanDefinitionHolder> configCandidates) {
    for (BeanDefinitionHolder holder : configCandidates) {
        BeanDefinition bd = holder.getBeanDefinition();
        try {
            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);
        }
    }
 
    this.deferredImportSelectorHandler.process();
}

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<>();
    }
}

handler.processGroupImports();

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

processImports(configurationClass, asSourceClass(configurationClass),asSourceClasses(entry.getImportClassName()), false);

private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
                            Collection<SourceClass> importCandidates, boolean checkForCircularImports) {
 
    if (importCandidates.isEmpty()) {
        return;
    }
 
    if (checkForCircularImports && isChainedImportOnStack(configClass)) {
        this.problemReporter.error(new CircularImportProblem(configClass, this.importStack));
    }
    else {
        this.importStack.push(configClass);
        try {
            for (SourceClass candidate : importCandidates) {
                if (candidate.isAssignable(ImportSelector.class)) {
                    // Candidate class is an ImportSelector -> delegate to it to determine imports
                    Class<?> candidateClass = candidate.loadClass();
                    ImportSelector selector = ParserStrategyUtils.instantiateClass(candidateClass, ImportSelector.class,
                            this.environment, this.resourceLoader, this.registry);
                    if (selector instanceof DeferredImportSelector) {
                        this.deferredImportSelectorHandler.handle(configClass, (DeferredImportSelector) selector);
                    }
                    else {
                        String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
                        Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames);
                        processImports(configClass, currentSourceClass, importSourceClasses, false);
                    }
                }
                else if (candidate.isAssignable(ImportBeanDefinitionRegistrar.class)) {
                    // Candidate class is an ImportBeanDefinitionRegistrar ->
                    // delegate to it to register additional bean definitions
                    Class<?> candidateClass = candidate.loadClass();
                    ImportBeanDefinitionRegistrar registrar =
                            ParserStrategyUtils.instantiateClass(candidateClass, ImportBeanDefinitionRegistrar.class,
                                    this.environment, this.resourceLoader, this.registry);
                    configClass.addImportBeanDefinitionRegistrar(registrar, currentSourceClass.getMetadata());
                }
                else {
                    // Candidate class not an ImportSelector or ImportBeanDefinitionRegistrar ->
                    // process it as an @Configuration class
                    this.importStack.registerImport(
                            currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());
                    processConfigurationClass(candidate.asConfigClass(configClass));
                }
            }
        }
        catch (BeanDefinitionStoreException ex) {
            throw ex;
        }
        catch (Throwable ex) {
            throw new BeanDefinitionStoreException(
                    "Failed to process import candidates for configuration class [" +
                            configClass.getMetadata().getClassName() + "]", ex);
        }
        finally {
            this.importStack.pop();
        }
    }
}

processConfigurationClass(candidate.asConfigClass(configClass));

protected void processConfigurationClass(ConfigurationClass configClass) 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.
    SourceClass sourceClass = asSourceClass(configClass);
    do {
        sourceClass = doProcessConfigurationClass(configClass, sourceClass);
    }
    while (sourceClass != null);
 
    this.configurationClasses.put(configClass, configClass);
}

do {
   sourceClass = doProcessConfigurationClass(configClass, sourceClass);
}

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
......

最后：

private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
                            Collection<SourceClass> importCandidates, boolean checkForCircularImports) {
 
    if (importCandidates.isEmpty()) {
        return;
    }
 
    if (checkForCircularImports && isChainedImportOnStack(configClass)) {
        this.problemReporter.error(new CircularImportProblem(configClass, this.importStack));
    }
    else {
        this.importStack.push(configClass);
        try {
            for (SourceClass candidate : importCandidates) {
                if (candidate.isAssignable(ImportSelector.class)) {
                    // Candidate class is an ImportSelector -> delegate to it to determine imports
                    Class<?> candidateClass = candidate.loadClass();
                    ImportSelector selector = ParserStrategyUtils.instantiateClass(candidateClass, ImportSelector.class,
                            this.environment, this.resourceLoader, this.registry);
                    if (selector instanceof DeferredImportSelector) {
                        this.deferredImportSelectorHandler.handle(configClass, (DeferredImportSelector) selector);
                    }
                    else {
                        String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
                        Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames);
                        processImports(configClass, currentSourceClass, importSourceClasses, false);
                    }
                }
                else if (candidate.isAssignable(ImportBeanDefinitionRegistrar.class)) {
                    // Candidate class is an ImportBeanDefinitionRegistrar ->
                    // delegate to it to register additional bean definitions
                    Class<?> candidateClass = candidate.loadClass();
                    ImportBeanDefinitionRegistrar registrar =
                            ParserStrategyUtils.instantiateClass(candidateClass, ImportBeanDefinitionRegistrar.class,
                                    this.environment, this.resourceLoader, this.registry);
                    configClass.addImportBeanDefinitionRegistrar(registrar, currentSourceClass.getMetadata());
                }
                else {
                    // Candidate class not an ImportSelector or ImportBeanDefinitionRegistrar ->
                    // process it as an @Configuration class
                    this.importStack.registerImport(
                            currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());
                    processConfigurationClass(candidate.asConfigClass(configClass));
                }
            }
        }
        catch (BeanDefinitionStoreException ex) {
            throw ex;
        }
        catch (Throwable ex) {
            throw new BeanDefinitionStoreException(
                    "Failed to process import candidates for configuration class [" +
                            configClass.getMetadata().getClassName() + "]", ex);
        }
        finally {
            this.importStack.pop();
        }
    }
}

重点是：
1、当前的importCandidates对象为：TransactionManagementConfigurationSelector
2、查找ImportSelect.class类型
3、使用了工具类型进行处理 ParserStrategyUtils.instantiateClass
4、注入容器中

流程总结：
invokeBeanFactoryPostProcessors
PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors
invokeBeanDefinitionRegistryPostProcessors
ConfigurationClassPostParser.postProcessBeanDefinitionRegistry
processConfigBeanDefinitions
ConfigurationClassParser.parse
parse(@Nullable String className, String beanName)
processConfigurationClass
doProcessConfigurationClass
processImports
ParserStrategyUtils.instantiateClass(candidateClass, ImportSelector.class,this.environment, this.resourceLoader, this.registry);

String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames);
processImports(configClass, currentSourceClass, importSourceClasses, false);