SpringBoot自动装配源码解析

之前看过很多关于写SpringBoot自动装配源码解析的帖子，但是都觉得在耍流氓，他们直接就从启动类的@SpringbootApplication这个注解开始，然后找到@Import，然后巴拉巴拉一大段，其实这是直接逆推过来的，这只是说明了他自动装配的功能，但是没有涉及到更深入一点，就比如这个注解是什么时候被解析生效的，根本就没有说到点，所以就写了这一篇的文章（能力有限，如若有问题，请能及时指出）

下图是从springboot程序启动如何找到启动类的注解，然后怎么去引入Spring.factory配置文件里的时序图

 

首先我们从源码入手的,关于springboot的启动流程我们在其他文章在做详细描述。

第一步：我们先断点从主程序入口出发

    public static void main(String[] args) {
        //主程序的启动入口
        SpringApplication.run(Course02Application.class, args);
    }

 第二步：我们执行到run()方法，后面我们着重关注下

refreshContext(context)这个方法，其实调用到这个方法的时候，其实里面的流程就是spring的refresh()方法，可能有很多同学没有看过spring的源码，后续会出关于spring的源码的文章。

        //停表，用于监控启动时长
		StopWatch stopWatch = new StopWatch();
		//启动停表
		stopWatch.start();
		//可配置的应用上下文
		ConfigurableApplicationContext context = null;
		//启动失败的分析报告器
		Collection exceptionReporters = new ArrayList<>();
		//配置是否是缺少显示屏，键盘或者鼠标环境属性
		configureHeadlessProperty();
		//SpringApplicationRunListener的容器，用于监听SpringApplication的各个启动事件
		SpringApplicationRunListeners listeners = getRunListeners(args);
		//触发启动中事件
		listeners.starting();
		try {
			//通过启动参数生成应用参数
			ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
			//1. 准备可配置的环境，在ApplicationContext中使用此环境
			ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
			//从环境中获取spring.beaninfo.ignore并配置
			configureIgnoreBeanInfo(environment);
			//打印启动横幅
			Banner printedBanner = printBanner(environment);
			//2. 创建ApplicationContext
			context = createApplicationContext();
			//从spring.factorites中获取失败分析报告器，用于失败时详细分析失败原因并报告
			exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
					new Class[] { ConfigurableApplicationContext.class }, context);
			//3. 准备ApplicationContext，包括设置环境。注册一些必要的组件等
			prepareContext(context, environment, listeners, applicationArguments, printedBanner);
			//4. 执行context的refresh方法，初始化启动ApplicationContext
			refreshContext(context);
			//刷新之后执行一些方法
			afterRefresh(context, applicationArguments);
			//停止停表
			stopWatch.stop();
			if (this.logStartupInfo) {
				//打印启动日志
				new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
			}
			//调用启动完成的时间回调
			listeners.started(context);
			//从应用上下文中获取ApplicationRunner和CommandLinerRunner类型的Bean并调用他们的run方法
			callRunners(context, applicationArguments);
		}

 第三步：我们执行到spring 里面AbstractApplicationContext类里面的refresh()方法，然后我们在重点关注invokeBeanFactoryPostProcessors（beanFactory）这个方法，这个beanfactory 默认是DefaultListableBeanFactory，大家可以关注下spring的类关系图

public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			// 准备此上下文以进行刷新
			prepareRefresh();

			// 告诉子类刷新内部 bean 工厂
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// 准备 bean 工厂以在此上下文中使用
			prepareBeanFactory(beanFactory);

			try {
				// 允许在上下文子类中对 bean 工厂进行后处理
				postProcessBeanFactory(beanFactory);

				// 调用在上下文中注册为 bean 的工厂处理器
				invokeBeanFactoryPostProcessors(beanFactory);

				// 注册拦截 bean 创建的 bean 处理器
				registerBeanPostProcessors(beanFactory);

				// 为此上下文初始化消息源
				initMessageSource();

				//为此上下文初始化事件多播器
				initApplicationEventMulticaster();

				// 初始化特定上下文子类中的其他特殊 bean
				onRefresh();

				// Check for listener beans and register them.
				registerListeners();

				// Instantiate all remaining (non-lazy-init) singletons.
				finishBeanFactoryInitialization(beanFactory);

				// Last step: publish corresponding event.
				finishRefresh();
			}

	protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
		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 (beanFactory.getTempClassLoader() == null && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
			beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
			beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
		}
	}

我们从上图可以看出，在invokeBeanFactoryPostProcessors（。。）方法里，会委托AbstractApplicationContext 的这个后置处理器去调用invokeBeanFactoryPostProcessors（。。），注意，前面虽然这个方法名相同，但是是不归属于同一个类上，是spring 容器委托给AbstractApplicationContext 去调用的，在调invokeBeanFactoryPostProcessors（..）这个方法的时候，会传入两个参数，第一个是beanfactory，第二个就是上图执行的，里面包含了三个对象，但是这3个对象是什么时候放进去的，我们可以看下springboot的启动原理

接着，我们进入这个方法，重点来了，这边才真正的开始

	invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);

 继续跟源码走

    /**
     *这边是使用了策略模式，根据传入的具体对象，调用相应的方法
        这次我们传过来的参数是ConfigurationClassPostProcessor这个对象
	 * 调用给定的 BeanDefinitionRegistryPostProcessor bean
	 */
	private static void invokeBeanDefinitionRegistryPostProcessors(
			Collection postProcessors, BeanDefinitionRegistry registry) {

		for (BeanDefinitionRegistryPostProcessor postProcessor : postProcessors) {
			postProcessor.postProcessBeanDefinitionRegistry(registry);
		}
	}

然后我们就会进入到ConfigurationClassPostProcessor这个类

//从注册表中的配置类派生更多的 bean 定义	
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);
	}

继续跟着源码走，这个方法有点长，里面有相应的注释

/**
	 * 基于注册表构建和验证配置模型
	 * {@link Configuration} classes.
	 */
	public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
		List configCandidates = new ArrayList<>();
        //这边得到了bean的定义信息的名字，其中包含了启动类的，这个很重要，后面有图指示
		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)) {
                //这个集合里只添加了一个对象，就是刚刚那个启动类
				configCandidates.add(new BeanDefinitionHolder(beanDef, beanName));
			}
		}

		//这边判断启动类里有没有@Configuration，如果没有找到 @Configuration 类，则立即返回，            这边可以点进去看下，我这边就不进去看了
		if (configCandidates.isEmpty()) {
			return;
		}

		// 按先前确定的@Order 值排序（如果适用），只有一个，不重要
		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();
		}

		//  重点来了解析每个 @Configuration 类
		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();
		}
	}

 接着我们在上面方法里这一块的代码，我们跟进去看下，看看里面干了什么

	        parser.parse(candidates);
			parser.validate();

	public void parse(Set configCandidates) {
		this.deferredImportSelectors = new LinkedList<>();

		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);
			}
		}

		processDeferredImportSelectors();
	}

 接着上面方法里继续跟进

parse(((AnnotatedBeanDefinition) bd).getMetadata(), holder.getBeanName());

protected final void parse(AnnotationMetadata metadata, String beanName) throws IOException {
		processConfigurationClass(new ConfigurationClass(metadata, beanName));
	}

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);
			}
		}

		// 递归处理配置类及其超类层次结构
		SourceClass sourceClass = asSourceClass(configClass);
		do {
			sourceClass = doProcessConfigurationClass(configClass, sourceClass);
		}
		while (sourceClass != null);

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

	// 递归处理配置类及其超类层次结构
		SourceClass sourceClass = asSourceClass(configClass);
		do {
			sourceClass = doProcessConfigurationClass(configClass, sourceClass);
		}

重点来了：我们继续跟进

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

		// 首先递归处理任何成员（嵌套）类
		processMemberClasses(configClass, sourceClass);

		// 处理任何 @PropertySource 注释
		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");
			}
		}

		// 处理任何@ComponentScan 注释
		Set 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 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());
					}
				}
			}
		}

		// 处理任何 @Import 注释
		processImports(configClass, sourceClass, getImports(sourceClass), true);

		// Process any @ImportResource annotations
		AnnotationAttributes importResource =
				AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);
		if (importResource != null) {
			String[] resources = importResource.getStringArray("locations");
			Class readerClass = importResource.getClass("reader");
			for (String resource : resources) {
				String resolvedResource = this.environment.resolveRequiredPlaceholders(resource);
				configClass.addImportedResource(resolvedResource, readerClass);
			}
		}

		// 处理单个 @Bean 方法
		Set beanMethods = retrieveBeanMethodMetadata(sourceClass);
		for (MethodMetadata methodMetadata : beanMethods) {
			configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
		}

		// Process default methods on interfaces
		processInterfaces(configClass, sourceClass);

		// 处理接口上的默认方法
		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;
	}

从上图我们是不是发现了什么东西，是不是在对每一个注解在做解析了，然后我们单独把@import这个拎出来讲

private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
			Collection 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 = 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 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();
			}
		}
	}

从上图我们很清楚的可以看到我们在启动类上的@springBootApplication的父子注解上找到了@import  ,而且还找到了要引入的2个对象 ，然后分别对这两个对象进行处理工作

然后我们继续回到下面这个方法上，这个方法是在parse()上的

processDeferredImportSelectors();

我们跟进去看下

	private void processDeferredImportSelectors() {
		List deferredImports = this.deferredImportSelectors;
		this.deferredImportSelectors = null;
		if (deferredImports == null) {
			return;
		}

		deferredImports.sort(DEFERRED_IMPORT_COMPARATOR);
		Map groupings = new LinkedHashMap<>();
		Map configurationClasses = new HashMap<>();
		for (DeferredImportSelectorHolder deferredImport : deferredImports) {
			Class group = deferredImport.getImportSelector().getImportGroup();
			DeferredImportSelectorGrouping grouping = groupings.computeIfAbsent(
					(group == null ? deferredImport : group),
					(key) -> new DeferredImportSelectorGrouping(createGroup(group)));
			grouping.add(deferredImport);
			configurationClasses.put(deferredImport.getConfigurationClass().getMetadata(),
					deferredImport.getConfigurationClass());
		}
            //这边正式会执行AutoConfigurationImportSelector（）上
		for (DeferredImportSelectorGrouping grouping : groupings.values()) {
			grouping.getImports().forEach((entry) -> {
				ConfigurationClass configurationClass = 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);
				}
			});
		}
	}

根据上图的注释，我们在上面方法中，会进入到

AutoConfigurationImportSelector类的selectImports（）方法上

现在是不是清楚了我们的这个注解是什么时候生效的，然后是什么时候去执行这个类中的方法的