如何解析SpringFramework的@Configuration/@Import注解

这篇文章主要介绍"如何解析SpringFramework的@Configuration/@Import注解"，在日常操作中，相信很多人在如何解析SpringFramework的@Configuration/@Import注解问题上存在疑惑，小编查阅了各式资料，整理出简单好用的操作方法，希望对大家解答"如何解析SpringFramework的@Configuration/@Import注解"的疑惑有所帮助！接下来，请跟着小编一起来学习吧！

Spring版本是5.0.9.release，Springboot版本是2.0.3.release

Springboot中，Servlet web应用，使用的是AnnotationConfigServletWebServerApplicationContext，它的构造方法中实例化了AnnotatedBeanDefinitionReader，如下List-1，调用了AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry)，如List-2所示:

List-1

public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry, Environment environment) {    Assert.notNull(registry, "BeanDefinitionRegistry must not be null");    Assert.notNull(environment, "Environment must not be null");    this.registry = registry;    this.conditionEvaluator = new ConditionEvaluator(registry, environment, null);    AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);}

List-2

public static Set registerAnnotationConfigProcessors(        BeanDefinitionRegistry registry, @Nullable Object source) {    ...    if (!registry.containsBeanDefinition(CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME)) {        RootBeanDefinition def = new RootBeanDefinition(ConfigurationClassPostProcessor.class);        def.setSource(source);        beanDefs.add(registerPostProcessor(registry, def, CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME));    }    ...

如List-2所示，ConfigurationClassPostProcessor转换为BeanDefinition，注册到registry中。ConfigurationClassPostProcessor实现了BeanFacotryPostProcessor接口，如果熟悉SpringIOC，应该知道这个的作用。

图1

AnnotationConfigServletWebServerApplicationContext继承了AbstractApplicationContext，AbstractApplicationContext的refresh()如下List-3所示，refresh()中调用了this.invokeBeanFactoryPostProcessors(beanFactory)，接着调用了PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, this.getBeanFactoryPostProcessors())，从BeanFactory方法中获取所有BeanFactoryPostProcessor，之后调用BeanFactoryPostProcessor的postProcessBeanFactory()。

List-3

public void refresh() throws BeansException, IllegalStateException {    synchronized(this.startupShutdownMonitor) {        this.prepareRefresh();        ConfigurableListableBeanFactory beanFactory = this.obtainFreshBeanFactory();        this.prepareBeanFactory(beanFactory);        try {            this.postProcessBeanFactory(beanFactory);            this.invokeBeanFactoryPostProcessors(beanFactory);            this.registerBeanPostProcessors(beanFactory);            this.initMessageSource();            this.initApplicationEventMulticaster();            this.onRefresh();            this.registerListeners();            this.finishBeanFactoryInitialization(beanFactory);            this.finishRefresh();        } catch (BeansException var9) {            if (this.logger.isWarnEnabled()) {                this.logger.warn("Exception encountered during context initialization - cancelling refresh attempt: " + var9);            }            this.destroyBeans();            this.cancelRefresh(var9);            throw var9;        } finally {            this.resetCommonCaches();        }    }}

回到ConfigurationClassPostProcessor，看它的postProcessBeanFactory()，调用了processConfigBeanDefinitions()，如下List-4，代码很多，不过看关键部分就好了，1处的ConfigurationClassUtils.checkConfigurationClassCandidate要重点看，这个方法中判断类上是否有Configuration/ComponentScan/Import/ImportResource注解，如果有这些注解，那么会加入到configCandidates中，循环处理configCandidates，交给ConfigurationClassParser.pase()进行解析。

List-4

public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {    List configCandidates = new ArrayList<>();    String[] candidateNames = registry.getBeanDefinitionNames();    for (String beanName : candidateNames) {        BeanDefinition beanDef = registry.getBeanDefinition(beanName);        if (ConfigurationClassUtils.isFullConfigurationClass(beanDef) ||                ConfigurationClassUtils.isLiteConfigurationClass(beanDef)) {            if (logger.isDebugEnabled()) {                logger.debug("Bean definition has already been processed as a configuration class: " + beanDef);            }        }        //1        else if (ConfigurationClassUtils.checkConfigurationClassCandidate(beanDef, this.metadataReaderFactory)) {            configCandidates.add(new BeanDefinitionHolder(beanDef, beanName));        }    }    // Return immediately if no @Configuration classes were found    if (configCandidates.isEmpty()) {        return;    }    // Sort by previously determined @Order value, if applicable    configCandidates.sort((bd1, bd2) -> {        int i1 = ConfigurationClassUtils.getOrder(bd1.getBeanDefinition());        int i2 = ConfigurationClassUtils.getOrder(bd2.getBeanDefinition());        return Integer.compare(i1, i2);    });    // Detect any custom bean name generation strategy supplied through the enclosing application context    SingletonBeanRegistry sbr = null;    if (registry instanceof SingletonBeanRegistry) {        sbr = (SingletonBeanRegistry) registry;        if (!this.localBeanNameGeneratorSet) {            BeanNameGenerator generator = (BeanNameGenerator) sbr.getSingleton(CONFIGURATION_BEAN_NAME_GENERATOR);            if (generator != null) {                this.componentScanBeanNameGenerator = generator;                this.importBeanNameGenerator = generator;            }        }    }    if (this.environment == null) {        this.environment = new StandardEnvironment();    }    // Parse each @Configuration class    ConfigurationClassParser parser = new ConfigurationClassParser(            this.metadataReaderFactory, this.problemReporter, this.environment,            this.resourceLoader, this.componentScanBeanNameGenerator, registry);    Set candidates = new LinkedHashSet<>(configCandidates);    Set alreadyParsed = new HashSet<>(configCandidates.size());    do {        parser.parse(candidates);        parser.validate();        Set configClasses = new LinkedHashSet<>(parser.getConfigurationClasses());        configClasses.removeAll(alreadyParsed);        // Read the model and create bean definitions based on its content        if (this.reader == null) {            this.reader = new ConfigurationClassBeanDefinitionReader(                    registry, this.sourceExtractor, this.resourceLoader, this.environment,                    this.importBeanNameGenerator, parser.getImportRegistry());        }        this.reader.loadBeanDefinitions(configClasses);        alreadyParsed.addAll(configClasses);        candidates.clear();        if (registry.getBeanDefinitionCount() > candidateNames.length) {            String[] newCandidateNames = registry.getBeanDefinitionNames();            Set oldCandidateNames = new HashSet<>(Arrays.asList(candidateNames));            Set alreadyParsedClasses = new HashSet<>();            for (ConfigurationClass configurationClass : alreadyParsed) {                alreadyParsedClasses.add(configurationClass.getMetadata().getClassName());            }            for (String candidateName : newCandidateNames) {                if (!oldCandidateNames.contains(candidateName)) {                    BeanDefinition bd = registry.getBeanDefinition(candidateName);                    if (ConfigurationClassUtils.checkConfigurationClassCandidate(bd, this.metadataReaderFactory) &&                            !alreadyParsedClasses.contains(bd.getBeanClassName())) {                        candidates.add(new BeanDefinitionHolder(bd, candidateName));                    }                }            }            candidateNames = newCandidateNames;        }    }    while (!candidates.isEmpty());    // Register the ImportRegistry as a bean in order to support ImportAware @Configuration classes    if (sbr != null && !sbr.containsSingleton(IMPORT_REGISTRY_BEAN_NAME)) {        sbr.registerSingleton(IMPORT_REGISTRY_BEAN_NAME, parser.getImportRegistry());    }    if (this.metadataReaderFactory instanceof CachingMetadataReaderFactory) {        // Clear cache in externally provided MetadataReaderFactory; this is a no-op        // for a shared cache since it'll be cleared by the ApplicationContext.        ((CachingMetadataReaderFactory) this.metadataReaderFactory).clearCache();    }}

自动装配中EnableXX注解上的Import引入的Selector的处理就是在这里处理的，有趣的是Selector返回的String[]中的值需要是类的完全路径累名，这是因为使用Class.forName()将String类型的beanName转换为对应的Class类型。

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