SpringBoot2 | @SpringBootApplication注解 自动化配置流程源码分析

先从@SpringBootApplication开始。

在启动流程章节中，我们讲述了SpringBoot2大致的启动步骤，并进行了源码详解。但是在刷新容器这块并未展开，refreshContext(context);简单的一行代码，背后却做了太多事情。所以为了不喧宾夺主，本篇也尽量选取和注解@SpringBootApplication有关的方法讲解。

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1）springBoot启动类加载

首先加载springBoot启动类注入到spring容器中bean map中，看下prepareContext方法中的load方法：
load(context, sources.toArray(new Object[0]));
跟进该方法最终会执行BeanDefinitionLoader的load方法：

	private int load(Object source) {
		Assert.notNull(source, "Source must not be null");
		//如果是class类型，启用注解类型
		if (source instanceof Class) {
			return load((Class) source);
		}
		//如果是resource类型，启用xml解析
		if (source instanceof Resource) {
			return load((Resource) source);
		}
		//如果是package类型，启用扫描包，例如：@ComponentScan
		if (source instanceof Package) {
			return load((Package) source);
		}
		//如果是字符串类型，直接加载
		if (source instanceof CharSequence) {
			return load((CharSequence) source);
		}
		throw new IllegalArgumentException("Invalid source type " + source.getClass());
	}
继续跟进load(Class source)方法：

上述方法判断启动类中是否包含@component注解，可我们的启动类并没有该注解。继续跟进会发现，AnnotationUtils判断是否包含该注解是通过递归实现，注解上的注解若包含指定类型也是可以的。

启动类中包含@SpringBootApplication注解，进一步查找到@SpringBootConfiguration注解，然后查找到@Component注解，最后会查找到@Component注解：
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Configuration {
}
在查找到@Component注解后，表面该对象为spring bean，然后会将其信息包装成 beanDefinitaion ，添加到容器的 beanDefinitionMap中。如下：



如此一来，我们的启动类就被包装成AnnotatedGenericBeanDefinition了，后续启动类的处理都基于该对象了。

  
  
    
    
    
    

2）自动装配的入口：
从刷新容器开始：
public void refresh() throws BeansException, IllegalStateException {
		//...
		invokeBeanFactoryPostProcessors(beanFactory);
		//...
	}

上述省去了不相关的代码，继续跟进invokeBeanFactoryPostProcessors方法：

	protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
		//开始执行beanFactoryPostProcessor对应实现类
		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()));
	}

首先我们要知道beanFactoryPostProcessor接口是spring的扩展接口，从名字也可以看出，是 beanFactory的扩展接口。在刷新容器之前，该接口可用来修改bean元数据信息。具体实现方式，我们继续跟着上述执行逻辑便知。
继续跟进上面invokeBeanFactoryPostProcessors方法，第一行很关键：

PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());

一个比较核心的代理类出现了，AbstractApplicationContext委托执行post processors任务的工具类。
而在项目启动时会委托什么任务呢？

或许你还记得第一篇博客中介绍的SpringApplication类中applyInitializers(context);方法吧，它会将三个默认的内部类加入到 spring 容器DefaultListableBeanFactory中，如下：

//设置配置警告
ConfigurationWarningsApplicationContextInitializer$ConfigurationWarningsPostProcessor
SharedMetadataReaderFactoryContextInitializer$CachingMetadataReaderFactoryPostProcessor
ConfigFileApplicationListener$PropertySourceOrderingPostProcessor

来看一下具体任务执行细节，跟进invokeBeanFactoryPostProcessors方法：

if (beanFactory instanceof BeanDefinitionRegistry) {
			BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
			List regularPostProcessors = new LinkedList<>();
			List registryProcessors = new LinkedList<>();
		//这里开始遍历上面三个内部类，如果属于BeanDefinitionRegistryPostProcessor 子类，
		//加入到bean注册的集合，否则加入到 regularPostProcessors中，从名字可以看出是有规律集合。
		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);
				
		//首先执行类型为PriorityOrdered的BeanDefinitionRegistryPostProcessor
		//PriorityOrdered类型表明为优先执行
		for (String ppName : postProcessorNames) {
			if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
				//获取对应的bean
				currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
				//用来存储已经执行过的`BeanDefinitionRegistryPostProcessor`				
				processedBeans.add(ppName);
			}
		}
		sortPostProcessors(currentRegistryProcessors, beanFactory);
		registryProcessors.addAll(currentRegistryProcessors);
		invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
		currentRegistryProcessors.clear();

		// Next, invoke the BeanDefinitionRegistryPostProcessors that implement Ordered.
		postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
		//其次执行类型为Ordered的BeanDefinitionRegistryPostProcessor
		//Ordered表明按顺序执行
		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;
		//循环中执行类型不为PriorityOrdered,Ordered类型的BeanDefinitionRegistryPostProcessor
		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.
		//执行父类方法，优先执行注册处理类
		invokeBeanFactoryPostProcessors(registryProcessors, beanFactory);
		//执行有规则处理类
		invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);
	}

来分析一下核心代码：

String[] postProcessorNames =beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);

这行代码通过类型BeanDefinitionRegistryPostProcessor获取的处理类名称为：
"org.springframework.context.annotation.internalConfigurationAnnotationProcessor"
而在源码中却搜不到internalConfigurationAnnotationProcessor类，为什么呢？最初看这块代码确实迷惑了半天。
在第一篇博客中，当启动springBoot，创建springBoot容器上下文AnnotationConfigEmbeddedWebApplicationContext时，会装配几个默认bean：

	public AnnotationConfigEmbeddedWebApplicationContext() {
		//在这里装配
		this.reader = new AnnotatedBeanDefinitionReader(this);
		this.scanner = new ClassPathBeanDefinitionScanner(this);
	}

继续跟进会执行registerAnnotationConfigProcessors方法：

public static final String CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME =
			"org.springframework.context.annotation.internalConfigurationAnnotationProcessor";
//将 internalConfigurationAnnotationProcessor 对应的类包装成 RootBeanDefinition 加载到容器
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));
	}

到这里，答案清晰浮现。internalConfigurationAnnotationProcessor为bean名称，容器中真正的类则是ConfigurationClassPostProcessor。

继续后面流程，获取ConfigurationClassPostProcessor后，开始执行BeanDefinitionRegistryPostProcessor:

//开始执行装配逻辑
invokeBeanDefinitionRegistryPostProcessors(priorityOrderedPostProcessors, registry);

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3）开始执行自动配置逻辑（启动类指定的配置，非默认配置）：

如上如：首先获得ConfigurationClassParser，这个是所有配置类的解析类，比较核心。所有的解析逻辑在parser.parse(candidates);中，我们详细的来看一下：

	public void parse(Set configCandidates) {
		this.deferredImportSelectors = new LinkedList();
	for (BeanDefinitionHolder holder : configCandidates) {
		BeanDefinition bd = holder.getBeanDefinition();
		try {
			//是否是注解类
			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 (Exception ex) {
			throw new BeanDefinitionStoreException(
					"Failed to parse configuration class [" + bd.getBeanClassName() + "]", ex);
		}
	}

	//执行配置类
	processDeferredImportSelectors();
}

继续跟进parse方法：

protected void processConfigurationClass(ConfigurationClass configClass) throws IOException {
		//...省略不核心代码
		// Recursively process the configuration class and its superclass hierarchy.
		SourceClass sourceClass = asSourceClass(configClass);
		do {
			//循环处理bean，如果有父类，则处理父类。直至结束。
			sourceClass = doProcessConfigurationClass(configClass, sourceClass);
		}
		while (sourceClass != null);
	this.configurationClasses.put(configClass, configClass);
}

继续跟进doProcessConfigurationClass方法，该方法可以说是 spring 框架支持注解配置的核心逻辑了，来看看：

 protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass) throws IOException {
        //处理内部类逻辑，由于传来的参数是我们的启动类，不含内部类，所以跳过。
        processMemberClasses(configClass, sourceClass);
    // Process any @PropertySource annotations
    //针对属性配置的解析
    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");
        }
    }

    //这里是根据启动类 @ComponentScan 注解来扫描项目中的bean
    AnnotationAttributes componentScan = AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ComponentScan.class);
    if (componentScan != null && !this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) {
        // 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 necessary

        //遍历我们项目中的bean，如果是注解定义的bean，则进一步解析
        for (BeanDefinitionHolder holder : scannedBeanDefinitions) {
            //判断是否是注解bean
            if (ConfigurationClassUtils.checkConfigurationClassCandidate(holder.getBeanDefinition(), this.metadataReaderFactory)) {
                //这里是关键，递归解析。所有的bean，如果有注解，会进一步解析注解中包含的bean
                parse(holder.getBeanDefinition().getBeanClassName(), holder.getBeanName());
            }
        }
    }

    // Process any @Import annotations
    //这里又是一个递归解析，获取导入的配置类。很多情况下，导入的配置类中会同样包含导入类注解。
    processImports(configClass, sourceClass, getImports(sourceClass), true);

    // Process any @ImportResource annotations
    //解析导入的 xml 配置类
    if (sourceClass.getMetadata().isAnnotated(ImportResource.class.getName())) {
        AnnotationAttributes importResource = AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);
        String[] resources = importResource.getAliasedStringArray("locations", ImportResource.class, sourceClass);
        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 = sourceClass.getMetadata().getAnnotatedMethods(Bean.class.getName());
    for (MethodMetadata methodMetadata : beanMethods) {
        configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
    }

    // 获取接口中的默认方法，1.8以上的处理逻辑
    for (SourceClass ifc : sourceClass.getInterfaces()) {
        beanMethods = ifc.getMetadata().getAnnotatedMethods(Bean.class.getName());
        for (MethodMetadata methodMetadata : beanMethods) {
            if (!methodMetadata.isAbstract()) {
                // A default method or other concrete method on a Java 8+ interface...
                configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
            }
        }
    }

    // Process superclass, if any
    //如果该类有父类，则继续返回。上层方法判断不为空，则继续递归执行。
    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
    //递归实现，superclass为空，则结束递归中的循环
    return null;
}

来看一下获取导入配置类的逻辑：

processImports(configClass, sourceClass, getImports(sourceClass), true);
跟进getImports方法：



可以看到我自定义的bean以导入的方式被加载进去了。另外processImports方法执行逻辑和上述parse方法类似，同样采用递归处理，这里就不做展开了。

 
 
   
   
   
   

4）开始执行 SpringBoot 默认配置逻辑
继续回到ConfigurationClassParser中的parse方法，回到该方法的最后一步：
public void parse(Set configCandidates) {
		//...
		//开始执行默认配置
		processDeferredImportSelectors();
	}
继续跟进该方法processDeferredImportSelectors：
private void processDeferredImportSelectors() {
		List deferredImports = this.deferredImportSelectors;
		this.deferredImportSelectors = null;
		Collections.sort(deferredImports, DEFERRED_IMPORT_COMPARATOR);
	for (DeferredImportSelectorHolder deferredImport : deferredImports) {
		ConfigurationClass configClass = deferredImport.getConfigurationClass();
		try {
			//获取配置类
			String[] imports = deferredImport.getImportSelector().selectImports(configClass.getMetadata());
			//再次递归解析配置类
			processImports(configClass, asSourceClass(configClass), asSourceClasses(imports), false);
		}
		catch (BeanDefinitionStoreException ex) {
			throw ex;
		}
		catch (Throwable ex) {
			throw new BeanDefinitionStoreException("Failed to process import candidates for configuration class [" +
					configClass.getMetadata().getClassName() + "]", ex);
		}
	}
}

getImportSelector()方法获取的 selector对象为EnableAutoConfigurationImportSelector，继续跟进该对象的selectImports方法：

	@Override
	public String[] selectImports(AnnotationMetadata metadata) {
		try {
			AnnotationAttributes attributes = getAttributes(metadata);
			//获取默认配置类
			List configurations = getCandidateConfigurations(metadata,
					attributes);
			configurations = removeDuplicates(configurations);
			Set exclusions = getExclusions(metadata, attributes);
			configurations.removeAll(exclusions);
			configurations = sort(configurations);
			recordWithConditionEvaluationReport(configurations, exclusions);
			return configurations.toArray(new String[configurations.size()]);
		}
		catch (IOException ex) {
			throw new IllegalStateException(ex);
		}
	}
这里的处理方式，前面的博客中已经详细介绍过了，通过class类型来获取spring.factories中的指定类，class类型为：EnableAutoConfiguration
	protected Class getSpringFactoriesLoaderFactoryClass() {
		return EnableAutoConfiguration.class;
	}
springBoot为我们提供的所有配置类如下,大概100多个：


在获取到springBoot提供的配置后，再次调用processImports方法进行递归解析，根据我们自定义的配置文件，进行选择性配置。
这么多的配置类，不可能全部进行加载，项目也用不了这么多。选择的规则是什么呢？
后续会有完整博文详细介绍。