@Component@Import@Bean加载顺序解析

【前言】

我们在使用Spring注入Bean对象时，会使用不同注解，比如@Component @Service @Controller @Import @Bean等。由于@Service @Controller 等都可以归为@Component，那么@Component 和@Import 、@Bean是何时被加载的，以及他们之间的顺序呢，下面就来分析一下。

【源码解析】

首先Spring的启动肯定是由AbstractApplicationContext.refresh()方法开始的。之后直接进入invokeBeanFactoryPostProcessors(beanFactory)方法，该方法用来处理要载入的Bean对象。会进入PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors())方法。

之后会进入ConfigurationClassPostProcessor类的postProcessBeanDefinitionRegistry方法。

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);
	}

进入processConfigBeanDefinitions(registry);该方法主要加载就是如下这段

........................
ConfigurationClassParser parser = new ConfigurationClassParser(
				this.metadataReaderFactory, this.problemReporter, this.environment,
				this.resourceLoader, this.componentScanBeanNameGenerator, registry);
       
		Set<BeanDefinitionHolder> candidates = new LinkedHashSet<>(configCandidates);
		Set<ConfigurationClass> alreadyParsed = new HashSet<>(configCandidates.size());
		do {
			parser.parse(candidates);
			parser.validate();

			Set<ConfigurationClass> 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);
    }
........................

 这段parser.parse(candidates);中candidates就是最初启动的bean对象名字，比如springboot应用中的主类带@Springboot标识的，如下

@SpringBootApplication
public class DemoApplication {
    public static void main(String[] args) {
        new SpringApplication(DemoApplication.class).run(args);
    }
}

那么candidates 就会包含这个DemoApplication 。进入parser.parse(candidates)方法

public void parse(Set<BeanDefinitionHolder> configCandidates) {
		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);
			}
		}

		this.deferredImportSelectorHandler.process();
	}

会继续进行解析，根据不同类型，但一般都是AnnotatedBeanDefinition。之后继续进入

	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);
			}
		}

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

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

主要就是将 DemoApplication 封装成ConfigurationClass类。然后通过asSourceClass(configClass)解析出原始类DemoApplication.class。之后调用doProcessConfigurationClass方法

protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)
			throws IOException {
        //  **1**
		if (configClass.getMetadata().isAnnotated(Component.class.getName())) {
			// Recursively process any member (nested) classes first
			processMemberClasses(configClass, sourceClass);
		}

		// Process any @PropertySource annotations
		for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(
				sourceClass.getMetadata(), PropertySources.class,
				org.springframework.context.annotation.PropertySource.class)) {
			if (this.environment instanceof ConfigurableEnvironment) {
				processPropertySource(propertySource);
			}
			else {
				logger.info("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +
						"]. Reason: Environment must implement ConfigurableEnvironment");
			}
		}

		//   **2**
		Set<AnnotationAttributes> 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
                // ** 2.1 **
				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 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());
					}
				}
			}
		}

		// Process any @Import annotations
        //  **3**
		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<? extends BeanDefinitionReader> readerClass = importResource.getClass("reader");
			for (String resource : resources) {
				String resolvedResource = this.environment.resolveRequiredPlaceholders(resource);
				configClass.addImportedResource(resolvedResource, readerClass);
			}
		}

		// Process individual @Bean methods
        //  **4**
		Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);
		for (MethodMetadata methodMetadata : beanMethods) {
			configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
		}

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

		// Process superclass, if any
		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;
	}

  //  **1** ：处理内部类。比如我有一个@Component的类，里面有内部类A，且内部类中也有包含@Bean、@Component、@Import的注解，那么会继续处理内部类A。

private void processMemberClasses(ConfigurationClass configClass, SourceClass sourceClass) throws IOException {
		Collection<SourceClass> memberClasses = sourceClass.getMemberClasses();
		if (!memberClasses.isEmpty()) {
			List<SourceClass> candidates = new ArrayList<>(memberClasses.size());
			for (SourceClass memberClass : memberClasses) {
				if (ConfigurationClassUtils.isConfigurationCandidate(memberClass.getMetadata()) &&
						!memberClass.getMetadata().getClassName().equals(configClass.getMetadata().getClassName())) {
					candidates.add(memberClass);
				}
			}
			OrderComparator.sort(candidates);
			for (SourceClass candidate : candidates) {
				if (this.importStack.contains(configClass)) {
					this.problemReporter.error(new CircularImportProblem(configClass, this.importStack));
				}
				else {
					this.importStack.push(configClass);
					try {
						processConfigurationClass(candidate.asConfigClass(configClass));
					}
					finally {
						this.importStack.pop();
					}
				}
			}
		}
	}

在处理内部类中，会继续调用processConfigurationClass方法。此时的入参会将内部类A封装成ConfigurationClass类。

  //  **2** ：处理@Component中包含@ComponentScans注解的，去扫描其他Component对象，并直接注册成Bean。在  //  ** 2.1 ** 方法中实现。

//  **3** ：处理@Import注解。该方法会将@Import分为三类。一类是ImportSelector，例如EnableAutoConfiguration中的AutoConfigurationImportSelector。第二类是ImportBeanDefinitionRegistrar。第三类就当作@Component继续处理。代码如下：

private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
			Collection<SourceClass> 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 (selector instanceof DeferredImportSelector) {
							this.deferredImportSelectorHandler.handle(configClass, (DeferredImportSelector) selector);
						}
						else {
							String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
							Collection<SourceClass> 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();
			}
		}
	}

//  **4** ：处理类中带@Bean的方法。Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);方法获取每个方法中带有@Bean注解的方法。

	private Set<MethodMetadata> retrieveBeanMethodMetadata(SourceClass sourceClass) {
		AnnotationMetadata original = sourceClass.getMetadata();
		Set<MethodMetadata> beanMethods = original.getAnnotatedMethods(Bean.class.getName());
        ..............
    }

并且加入到之前封装的configClass的beanMethod方法中

	Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);
		for (MethodMetadata methodMetadata : beanMethods) {
			configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
		}

至此，刚才的processConfigBeanDefinitions(registry);方法的parse方法就完事了。把上面的代码站下来。

	Set<BeanDefinitionHolder> candidates = new LinkedHashSet<>(configCandidates);
		Set<ConfigurationClass> alreadyParsed = new HashSet<>(configCandidates.size());
		do {
			parser.parse(candidates);
			parser.validate();

			Set<ConfigurationClass> 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);

执行完 parser.parse(candidates) ，此时Spring容器中装载的Bean有 DemoApplication 扫描下的所有@Component注解的类，以及该类中包含@ComponentScan路径下带有@Component注解的类。至于内部类和Import中的第二类ImportBeanDefinitionRegistrar以及BeanMethod还没有装载。

继续执行this.reader.loadBeanDefinitions(configClasses);

	public void loadBeanDefinitions(Set<ConfigurationClass> configurationModel) {
		TrackedConditionEvaluator trackedConditionEvaluator = new TrackedConditionEvaluator();
		for (ConfigurationClass configClass : configurationModel) {
			loadBeanDefinitionsForConfigurationClass(configClass, trackedConditionEvaluator);
		}
	}
	private void loadBeanDefinitionsForConfigurationClass(
			ConfigurationClass configClass, TrackedConditionEvaluator trackedConditionEvaluator) {
        // 1
		if (trackedConditionEvaluator.shouldSkip(configClass)) {
			String beanName = configClass.getBeanName();
			if (StringUtils.hasLength(beanName) && this.registry.containsBeanDefinition(beanName)) {
				this.registry.removeBeanDefinition(beanName);
			}
			this.importRegistry.removeImportingClass(configClass.getMetadata().getClassName());
			return;
		}
        // 2
		if (configClass.isImported()) {
			registerBeanDefinitionForImportedConfigurationClass(configClass);
		}
        // 3
		for (BeanMethod beanMethod : configClass.getBeanMethods()) {
			loadBeanDefinitionsForBeanMethod(beanMethod);
		}

		loadBeanDefinitionsFromImportedResources(configClass.getImportedResources());
		// 4
  loadBeanDefinitionsFromRegistrars(configClass.getImportBeanDefinitionRegistrars());
	}

//1：判断之前生成的configclass 是否符合注入条件，就是判断包含的@Conditional注解。如果不符合直接return

//2：判断是否为内部类，如果是且带@Component就注入。比如 A 类中有内部类B，且类B也有@Component注解，那么此时Spring容器才会将B注入。

//3：将之前类中包含@Bean的method方法，注入Spring容器中，当作Bean对象。

//4：注入实现ImportBeanDefinitionRegistrar接口的类，比如aop的AspectJAutoProxyRegistrar。

【总结】

1、执行完 parser.parse(candidates) ，此时Spring容器中装载的Bean有 DemoApplication 扫描下的所有@Component注解的类，以及该类中包含@ComponentScan路径下带有@Component注解的类。

2、注入内部类

3、注入Bean方法

4、注入实现ImportBeanDefinitionRegistrar接口的类