使用@import導入實現了ImportBeanDefinitionRegistrar接口的類，不能被注冊為bean

今天在調試公司spring項目的時候發現了這樣一個問題，由於我們的項目使用的是springboot就以springboot為例，代碼如下：

 

@Import({DataSourceRegister.class,A.class})
@SpringBootApplication
@ComponentScan("com.viewhigh.bi")
//@EnableCaching
public class BiApplication {
    public static void main(String[] args) {
        LogUtil.setEnabled(true);//開啟日志輸出

        SpringApplication sa = new SpringApplication(BiApplication.class);
        sa.setBannerMode(Banner.Mode.LOG);
        sa.run(args);
    }
}

 

在springboot啟動的時候，loder模塊會根據“清單文件”加載該BIApplication類，並反射調用psvm入口函數main，但是一個很有意思的問題出現了，項目正常運行之后，在springcontext中可以找到Bean類A，但是無法找到DataSourceRegister這個類；

我們知道在spring4.2以后@Import注解也可以導入一個常規類，並將其注冊為bean，那么為什么DataSourceRegister沒有被注冊為Bean呢？

 

DataSourceRegister類是用來進行初始化數據源和並提供了執行動態切換數據源的工具類

public class DataSourceRegister<T> implements EnvironmentAware, ImportBeanDefinitionRegistrar {
    private javax.sql.DataSource defaultTargetDataSource;
    static final String MAINDATASOURCE = "mainDataSource";

    public final void setEnvironment(Environment environment) {
        DruidEntity druidEntity = FileUtil.readYmlByClassPath("db_info", DruidEntity.class);

        defaultTargetDataSource = DataSourceUtil.createMainDataSource(druidEntity);
    }

    public final void registerBeanDefinitions(AnnotationMetadata annotationMetadata, BeanDefinitionRegistry beanDefinitionRegistry) {
        // 0.將主數據源添加到數據源集合中
        DataSourceSet.putTargetDataSourcesMap(MAINDATASOURCE, defaultTargetDataSource);
        //1.創建DataSourceBean
        GenericBeanDefinition beanDefinition = new GenericBeanDefinition();
        beanDefinition.setBeanClass(DataSource.class);
        beanDefinition.setSynthetic(true);
        MutablePropertyValues mpv = beanDefinition.getPropertyValues();
        //spring名稱約定為defaultTargetDataSource和targetDataSources
        mpv.addPropertyValue("defaultTargetDataSource", defaultTargetDataSource);
        mpv.addPropertyValue("targetDataSources", DataSourceSet.getTargetDataSourcesMap());
        beanDefinitionRegistry.registerBeanDefinition("dataSource", beanDefinition);
    }
}

 看到代碼后相信大家已經明白了，這個動態數據注冊類實現了ImportBeanDefinitionRegistrar 接口，沒錯就是這個原因，由於實現了該接口讓該類成為了擁有注冊bean的能力。從原理上也能說得通作為一個Bean的注冊類是沒有必要和A類一樣都被注冊為Bean的！

 雖然這樣解釋也不為過但我仍然想一探究竟，本來想大概找找spring涉及關鍵類如：ConfigurationClass，ConfigurationClassParser等，可能由於不熟悉看到類中的代碼就呵呵了，似乎無從下手！

 所以准備調試下spring啟動部分的代碼 ，這樣會更清晰些！（以spring boot V1.5.6為例）

 ======

 spring boot啟動時使用了SpringApplication類的run方法來牽引整個spring的初始化過程！！！

 沒錯了就是從run開始吧！

 代碼如下：

 

public ConfigurableApplicationContext run(String... args) {
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        ConfigurableApplicationContext context = null;
        FailureAnalyzers analyzers = null;
        this.configureHeadlessProperty();
        SpringApplicationRunListeners listeners = this.getRunListeners(args);
        listeners.starting();

        try {
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
            ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments);
            Banner printedBanner = this.printBanner(environment);
            context = this.createApplicationContext();
            new FailureAnalyzers(context);
            this.prepareContext(context, environment, listeners, applicationArguments, printedBanner);
            this.refreshContext(context);
            this.afterRefresh(context, applicationArguments);
            listeners.finished(context, (Throwable)null);
            stopWatch.stop();
            if(this.logStartupInfo) {
                (new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);
            }

            return context;
        } catch (Throwable var9) {
            this.handleRunFailure(context, listeners, (FailureAnalyzers)analyzers, var9);
            throw new IllegalStateException(var9);
        }
    }

手一懶，就把整個方法直接copy了。這就是springboot在啟動時的完整足跡。。。閑話少說我們直擊關鍵點；

context = this.createApplicationContext();
this.refreshContext(context);

這兩段是最關鍵的地方，閱讀過一些spring書籍的兄弟都知道refreshContext就是在做spring運行后的初始化工作。那么在createApplicationContext當中，由於我們是web項目，則spring默認給我們創建了一個AnnotationConfigEmbeddedWebApplicationContext

當然它也是繼承GenericWebApplicationContext類和GenericApplicationContext類的，那么他默認會持有一個DefaultListableBeanFactory對象，這個對象可以用來創建Bean，吼吼，這個塊說的似乎沒有意義哈！！！

接着往下走，進入refreshContext中會調用一系列的refresh方法，最終進入AbstractApplicationContext中：

@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();
            }
        }
    }

 invokeBeanFactoryPostProcessors()方法就是Bean在注冊前期做的一系列數據收集工作！

 跟着堆棧繼續深入，會進入到這個方法中，這個方法就是初始化bean前的所有軌跡：

在invokeBeanFactoryPostProcessors方法中繼續跟進一系列方法就會看到在一開始的時候spring會初始化幾個系統固有的Bean：

繼續調試后的關鍵點出現在這個方法中：

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

        for (String beanName : candidateNames) {
            BeanDefinition beanDef = registry.getBeanDefinition(beanName);
            if (ConfigurationClassUtils.isFullConfigurationClass(beanDef) ||
                    ConfigurationClassUtils.isLiteConfigurationClass(beanDef)) {
                if (logger.isDebugEnabled()) {
                    logger.debug("Bean definition has already been processed as a configuration class: " + beanDef);
                }
            }
            else if (ConfigurationClassUtils.checkConfigurationClassCandidate(beanDef, this.metadataReaderFactory)) {
                configCandidates.add(new BeanDefinitionHolder(beanDef, beanName));
            }
        }

        // Return immediately if no @Configuration classes were found
        if (configCandidates.isEmpty()) {
            return;
        }

        // Sort by previously determined @Order value, if applicable
        Collections.sort(configCandidates, new Comparator<BeanDefinitionHolder>() {
            @Override
            public int compare(BeanDefinitionHolder bd1, BeanDefinitionHolder bd2) {
                int i1 = ConfigurationClassUtils.getOrder(bd1.getBeanDefinition());
                int i2 = ConfigurationClassUtils.getOrder(bd2.getBeanDefinition());
                return (i1 < i2) ? -1 : (i1 > i2) ? 1 : 0;
            }
        });

        // Detect any custom bean name generation strategy supplied through the enclosing application context
        SingletonBeanRegistry sbr = null;
        if (registry instanceof SingletonBeanRegistry) {
            sbr = (SingletonBeanRegistry) registry;
            if (!this.localBeanNameGeneratorSet && sbr.containsSingleton(CONFIGURATION_BEAN_NAME_GENERATOR)) {
                BeanNameGenerator generator = (BeanNameGenerator) sbr.getSingleton(CONFIGURATION_BEAN_NAME_GENERATOR);
                this.componentScanBeanNameGenerator = generator;
                this.importBeanNameGenerator = generator;
            }
        }

        // Parse each @Configuration class
        ConfigurationClassParser parser = new ConfigurationClassParser(
                this.metadataReaderFactory, this.problemReporter, this.environment,
                this.resourceLoader, this.componentScanBeanNameGenerator, registry);

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

            Set<ConfigurationClass> configClasses = new LinkedHashSet<ConfigurationClass>(parser.getConfigurationClasses());
            configClasses.removeAll(alreadyParsed);

            // Read the model and create bean definitions based on its content
            if (this.reader == null) {
                this.reader = new ConfigurationClassBeanDefinitionReader(
                        registry, this.sourceExtractor, this.resourceLoader, this.environment,
                        this.importBeanNameGenerator, parser.getImportRegistry());
            }
            this.reader.loadBeanDefinitions(configClasses);
            alreadyParsed.addAll(configClasses);

            candidates.clear();
            if (registry.getBeanDefinitionCount() > candidateNames.length) {
                String[] newCandidateNames = registry.getBeanDefinitionNames();
                Set<String> oldCandidateNames = new HashSet<String>(Arrays.asList(candidateNames));
                Set<String> alreadyParsedClasses = new HashSet<String>();
                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) {
            if (!sbr.containsSingleton(IMPORT_REGISTRY_BEAN_NAME)) {
                sbr.registerSingleton(IMPORT_REGISTRY_BEAN_NAME, parser.getImportRegistry());
            }
        }

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

而通過不斷重復調試確定獲得注冊Bean的列表應該發生在配置的“剖析階段”，也就是parser.parse(candidates);這個方法的內部，到了這里基本問題的答案已經要浮出水面了，我也不再粘貼無用的代碼，如果你真的對這個問題比驕傲好奇可以自己跟蹤並練習調試的源碼技巧！

當然在ConfigurationClassParser這個類中parse方法也是不少，只要靜下心來逐漸分析，馬上就能准確的找到Override的parse方法。。。

現在直接貼出最最關鍵的代碼：

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);
                for (Iterator<ConfigurationClass> it = this.knownSuperclasses.values().iterator(); it.hasNext();) {
                    if (configClass.equals(it.next())) {
                        it.remove();
                    }
                }
            }
        }

        // Recursively process the configuration class and its superclass hierarchy.
        SourceClass sourceClass = asSourceClass(configClass);
        do {
            sourceClass = doProcessConfigurationClass(configClass, sourceClass);//處理定義的配置類
        }
        while (sourceClass != null);

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

protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)
            throws IOException {

        // 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.warn("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) {
                    if (ConfigurationClassUtils.checkConfigurationClassCandidate(
                            holder.getBeanDefinition(), this.metadataReaderFactory)) {
                        parse(holder.getBeanDefinition().getBeanClassName(), holder.getBeanName());
                    }
                }
            }
        }

        // Process any @Import annotations
        processImports(configClass, sourceClass, getImports(sourceClass), true);//處理注解導入的類型

        // Process any @ImportResource annotations
        if (sourceClass.getMetadata().isAnnotated(ImportResource.class.getName())) {
            AnnotationAttributes importResource =
                    AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);
            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
        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.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;
    }

 以上兩個方法中標紅的就是關鍵點。而且spring的大師們也把注釋寫的十分明顯：”//Process any @Import annotations“，到這里已經徹底豁然開朗！

spring會先去處理scan，將你程序內部的所有要注冊的Bean全部獲得（自然包括那些configuration），這里統稱為ConfigurationClass，scan全部整理完畢后才會去處理@Import注解時導入的類！

我們回到最初的問題 DataSourceRegister和A兩個類為什么A成為了Bean但DataSourceRegister卻未成為Bean呢？

答案就在processImports方法中，很明顯candidate.isAssignable(ImportBeanDefinitionRegistrar.class)時操作為：configClass.addImportBeanDefinitionRegistrar(registrar, currentSourceClass.getMetadata());而普通的類通過processConfigurationClass(candidate.asConfigClass(configClass));方法，最終會被放在ConfigurationClassParser類的成員變量configurationClasses中，最終被初始化為Bean。。。

private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
            Collection<SourceClass> importCandidates, boolean checkForCircularImports) throws IOException {

        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 = BeanUtils.instantiateClass(candidateClass, ImportSelector.class);
                        ParserStrategyUtils.invokeAwareMethods(
                                selector, this.environment, this.resourceLoader, this.registry);
                        if (this.deferredImportSelectors != null && selector instanceof DeferredImportSelector) {
                            this.deferredImportSelectors.add(
                                    new DeferredImportSelectorHolder(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 =
                                BeanUtils.instantiateClass(candidateClass, ImportBeanDefinitionRegistrar.class);
                        ParserStrategyUtils.invokeAwareMethods(
                                registrar, 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();
            }
        }
    }

簡單了剖析了下spring-context代碼，心情還是很不錯滴！你明白這個問題了嗎？