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！

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    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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