深入解析spring的@Component注解扫描

在这之前我们先要了解一下ConfigurationClassPostProcessor这个类，它实现了PriorityOrdered，BeanDefinitionRegistryPostProcessor等接口

public class ConfigurationClassPostProcessor implements BeanDefinitionRegistryPostProcessor,PriorityOrdered, ResourceLoaderAware, ApplicationStartupAware, BeanClassLoaderAware, EnvironmentAware {

ConfigurationClassPostProcessor的类图如下：

在AbstractApplicationContext中的refresh方法中，执行invokeBeanFactoryPostProcessors方法，开篇提到的ConfigurationClassPostProcessor 是 BeanFactoryPostProcessor的子类，所以

postProcessBeanFactory(beanFactory);
StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);

// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);

让我们结合下面的类图来走这个invokeBeanFactoryPostProcessors方法，先判断beanFactory 是否是BeanDefinitionRegistry的实例，结合下面的类图，成立

public static void invokeBeanFactoryPostProcessors(
			ConfigurableListableBeanFactory beanFactory, List beanFactoryPostProcessors) {

		// Invoke BeanDefinitionRegistryPostProcessors first, if any.
		Set processedBeans = new HashSet<>();

		//判断是否是beanFactory 是否是BeanDefinitionRegistry的实例
		if (beanFactory instanceof BeanDefinitionRegistry) {
			BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;

进入 if 语句，先取出所有的实现了BeanDefinitionRegistryPostProcessor接口的bean，在过滤出实现了PriorityOrdered 接口的bean，由上面的ConfigurationClassPostProcessor的类图，我们可以知道ConfigurationClassPostProcessor是满足这个条件的。

List currentRegistryProcessors = new ArrayList<>();

// First, invoke the BeanDefinitionRegistryPostProcessors that implement PriorityOrdered.
String[] postProcessorNames =
		beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
for (String ppName : postProcessorNames) {
	if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
		currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
		processedBeans.add(ppName);
	}
}
sortPostProcessors(currentRegistryProcessors, beanFactory);
registryProcessors.addAll(currentRegistryProcessors);
//执行方法
invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry, beanFactory.getApplicationStartup());
currentRegistryProcessors.clear();

private static void invokeBeanDefinitionRegistryPostProcessors(
			Collection postProcessors, BeanDefinitionRegistry registry, ApplicationStartup applicationStartup) {

		for (BeanDefinitionRegistryPostProcessor postProcessor : postProcessors) {
			StartupStep postProcessBeanDefRegistry = applicationStartup.start("spring.context.beandef-registry.post-process")
					.tag("postProcessor", postProcessor::toString);
			postProcessor.postProcessBeanDefinitionRegistry(registry);
			postProcessBeanDefRegistry.end();
		}
	}

当postProcessor = ConfigurationClassPostProcessor时，进入相应的实现类

@Override
	public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
		int registryId = System.identityHashCode(registry);
	
		this.registriesPostProcessed.add(registryId);
		//处理配置类的定义信息
		processConfigBeanDefinitions(registry);
	}

进入方法，默认获取到的是6个spring内部类和一个启动类

String[] candidateNames = registry.getBeanDefinitionNames();
for (String beanName : candidateNames) {
	BeanDefinition beanDef = registry.getBeanDefinition(beanName);
	//判断BeanDefinition的中attrabutes属性里是否有configurationClass属性值是否为full或者lite或者null，
	//如果是full则代表是@Configuration修饰的配置类，lite代表是Component，ComponentScan，import，importResource修饰的类或者有Bean修饰的方法
	if (beanDef.getAttribute(ConfigurationClassUtils.CONFIGURATION_CLASS_ATTRIBUTE) != null) {

	}
	//检测是否添加了@Configuration注解，只有启动类才有这个注解
	else if (ConfigurationClassUtils.checkConfigurationClassCandidate(beanDef, this.metadataReaderFactory)) {
		configCandidates.add(new BeanDefinitionHolder(beanDef, beanName));
	}
}
//对启动类进行解析
Set candidates = new linkedHashSet<>(configCandidates);
parser.parse(candidates);

递归的查询出启动类包含的所有注解

// Recursively process the configuration class and its superclass hierarchy.
SourceClass sourceClass = asSourceClass(configClass, filter);
do {
	sourceClass = doProcessConfigurationClass(configClass, sourceClass, filter);
}
while (sourceClass != null);

this.configurationClasses.put(configClass, configClass);

进入 ConfigurationClassParser的doProcessConfigurationClass方法 ,发现对@PropertySource ，@ComponentScan，@Bean ， @import， @importResource注解进行了处理

@Nullable
	protected final SourceClass doProcessConfigurationClass(
			ConfigurationClass configClass, SourceClass sourceClass, Predicate filter)
			throws IOException {
		//处理加了@Component注解所在类的成员变量
		if (configClass.getmetadata().isAnnotated(Component.class.getName())) {
			// Recursively process any member (nested) classes first
			processMemberClasses(configClass, sourceClass, filter);
		}

		// 处理添加了注解@PropertySource 
		for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(
				sourceClass.getmetadata(), PropertySources.class,
				org.springframework.context.annotation.PropertySource.class)) {
			if (this.environment instanceof ConfigurableEnvironment) {
				processPropertySource(propertySource);
			}
			
		}

		// 处理 @ComponentScan ，ComponentScans 注解
		Set componentScans = AnnotationConfigUtils.attributesForRepeatable(
				sourceClass.getmetadata(), ComponentScans.class, ComponentScan.class);
		
		// 处理@import注解
		processimports(configClass, sourceClass, getimports(sourceClass), filter, true);

		//处理 @importResource 注解
		AnnotationAttributes importResource =
				AnnotationConfigUtils.attributesFor(sourceClass.getmetadata(), importResource.class);
		
		//处理单独的 @Bean 方法上的注解
		Set beanMethods = retrieveBeanMethodmetadata(sourceClass);
		for (Methodmetadata methodmetadata : beanMethods) {
			configClass.addBeanMethod(new BeanMethod(methodmetadata, configClass));
		}

		// 处理接口中默认的方法
		processInterfaces(configClass, sourceClass);

		// 处理父类，在递归处理
		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();
			}
		}

		// 没有父类，处理结束，结束递归
		return null;
	}

扫描@Component注解

启动类继承了来自@SpringBootApplication注解的所有注解，其中有@ComponentScan，也就是对@Component进行扫描

@SpringBootApplication
public class SpringSourceApplication {

    public static void main(String[] args) {
        SpringApplication.run(SpringSourceApplication.class, args);
    }
}

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
		@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {

进入到扫描@Componen的代码区域，扫描也是要有一个范围的，比如扫描一个包，或者多个包，现在还不知道，看spring是则么处理的

// Process any @ComponentScan annotations
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) {
		// 执行扫描 
		Set scannedBeanDefinitions =
			this.componentScanParser.parse(componentScan, sourceClass.getmetadata().getClassName());

先获取@ComponentScan注解的属性

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@documented
@Repeatable(ComponentScans.class)
public @interface ComponentScan {

	@AliasFor("basePackages")
	String[] value() default {};

	@AliasFor("value")
	String[] basePackages() default {};

	Class[] basePackageClasses() default {};
	
	Filter[] includeFilters() default {};
	
	Filter[] excludeFilters() default {};
	........

我们看到了basePackages属性，在启动类的@SpringBootApplication注解中并没有对basePackages进行指定，默认为空

@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),

进入this.componentScanParser.parse(componentScan, sourceClass.getmetadata().getClassName()) 方法，里面是对注解属性进行处理，其中就有我们关注的basePackages

Set basePackages = new linkedHashSet<>();
String[] basePackagesArray = componentScan.getStringArray("basePackages");
for (String pkg : basePackagesArray) {
	String[] tokenized = StringUtils.tokenizeToStringArray(this.environment.resolvePlaceholders(pkg),
			ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
	Collections.addAll(basePackages, tokenized);
}
for (Class clazz : componentScan.getClassArray("basePackageClasses")) {
	basePackages.add(ClassUtils.getPackageName(clazz));
}
//满足这个条件，
if (basePackages.isEmpty()) {
	//ClassUtils.getPackageName(declaringClass)是获取当前类所在的包路径，这里就是为什么把类放在启动类包外，无法扫描到的原因
	basePackages.add(ClassUtils.getPackageName(declaringClass));
}
//真正开始对包进行扫描
return scanner.doScan(StringUtils.toStringArray(basePackages));

进入ClassPathBeanDefinitionScanner 的 doScan(String... basePackages) 方法

protected Set doScan(String... basePackages) {
	Set beanDefinitions = new linkedHashSet<>();
	for (String basePackage : basePackages) {
		//获取所有加了@Component的 BeanDefinition
		Set candidates = findCandidateComponents(basePackage);
		for (BeanDefinition candidate : candidates) {
			Scopemetadata scopemetadata = this.scopemetadataResolver.resolveScopemetadata(candidate);
			candidate.setScope(scopemetadata.getScopeName());
			String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
			if (candidate instanceof AbstractBeanDefinition) {
				postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
			}
			if (candidate instanceof AnnotatedBeanDefinition) {
				AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
			}
			if (checkCandidate(beanName, candidate)) {
				BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
				definitionHolder =
						AnnotationConfigUtils.applyScopedProxyMode(scopemetadata, definitionHolder, this.registry);
				beanDefinitions.add(definitionHolder);
				//把所有的BeanDefinition注册到BeanDefinitionRegistry中
				registerBeanDefinition(definitionHolder, this.registry);
			}
		}
	}
	return beanDefinitions;
}

1、先扫描启动类所在包下的所有class文件
2、获取直接或者间接加了@Component注解的class文件，并包装成BeanDefinition，在包装成BeanDefinitionHolder
3、通过BeanDefinitionRegistry进行注册

深入解析spring的@Component注解扫描

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