// DefaultListableBeanFactory
@Override
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
		throws BeanDefinitionStoreException {

	Assert.hasText(beanName, "Bean name must not be empty");
	Assert.notNull(beanDefinition, "BeanDefinition must not be null");

	if (beanDefinition instanceof AbstractBeanDefinition) {
		try {
			((AbstractBeanDefinition) beanDefinition).validate();
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
					"Validation of bean definition failed", ex);
		}
	}

	// 注冊(cè)時(shí)發(fā)現(xiàn)已經(jīng)存在
	// 除了 allowBeanDefinitionOverriding 為 false 時(shí)會(huì)拋出異常外，其余都會(huì)覆蓋
	BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
	if (existingDefinition != null) {
		// allowBeanDefinitionOverriding 默認(rèn) true，為 false 執(zhí)行到此直接拋出異常
		if (!isAllowBeanDefinitionOverriding()) {
			throw new BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);
		}
		// 已存在的角色等級(jí)低，會(huì)覆蓋
		else if (existingDefinition.getRole() < beanDefinition.getRole()) {
			// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
			if (logger.isInfoEnabled()) {
				logger.info("Overriding user-defined bean definition for bean '" + beanName +
						"' with a framework-generated bean definition: replacing [" +
						existingDefinition + "] with [" + beanDefinition + "]");
			}
		}
		// equals 方法判定不相等，會(huì)覆蓋
		else if (!beanDefinition.equals(existingDefinition)) {
			if (logger.isDebugEnabled()) {
				logger.debug("Overriding bean definition for bean '" + beanName +
						"' with a different definition: replacing [" + existingDefinition +
						"] with [" + beanDefinition + "]");
			}
		}
		// 其它情況打印覆蓋信息，之后覆蓋
		else {
			if (logger.isTraceEnabled()) {
				logger.trace("Overriding bean definition for bean '" + beanName +
						"' with an equivalent definition: replacing [" + existingDefinition +
						"] with [" + beanDefinition + "]");
			}
		}
		// 覆蓋
		this.beanDefinitionMap.put(beanName, beanDefinition);
	}
	else {
		if (hasBeanCreationStarted()) {
			// Cannot modify startup-time collection elements anymore (for stable iteration)
			synchronized (this.beanDefinitionMap) {
				this.beanDefinitionMap.put(beanName, beanDefinition);
				List<String> updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
				updatedDefinitions.addAll(this.beanDefinitionNames);
				updatedDefinitions.add(beanName);
				this.beanDefinitionNames = updatedDefinitions;
				removeManualSingletonName(beanName);
			}
		}
		else {
			// 仍在注冊(cè)階段
			this.beanDefinitionMap.put(beanName, beanDefinition);
			this.beanDefinitionNames.add(beanName);
			removeManualSingletonName(beanName);
		}
		this.frozenBeanDefinitionNames = null;
	}

	if (existingDefinition != null || containsSingleton(beanName)) {
		resetBeanDefinition(beanName);
	}
	else if (isConfigurationFrozen()) {
		clearByTypeCache();
	}
}

// ClassPathBeanDefinitionScanner
protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
	Assert.notEmpty(basePackages, "At least one base package must be specified");
	Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<>();
	for (String basePackage : basePackages) {
		Set<BeanDefinition> 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);
			}
			// 返回 false，并不會(huì)進(jìn)行 candidate 的注冊(cè)
			if (checkCandidate(beanName, candidate)) {
				BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
				definitionHolder =
						AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
				beanDefinitions.add(definitionHolder);
				registerBeanDefinition(definitionHolder, this.registry);
			}
		}
	}
	return beanDefinitions;
}
// ClassPathBeanDefinitionScanner
protected boolean checkCandidate(String beanName, BeanDefinition beanDefinition) throws IllegalStateException {
	if (!this.registry.containsBeanDefinition(beanName)) {
		return true;
	}
	BeanDefinition existingDef = this.registry.getBeanDefinition(beanName);
	BeanDefinition originatingDef = existingDef.getOriginatingBeanDefinition();
	if (originatingDef != null) {
		existingDef = originatingDef;
	}
	if (isCompatible(beanDefinition, existingDef)) {
		return false;
	}
	throw new ConflictingBeanDefinitionException("Annotation-specified bean name '" + beanName +
			"' for bean class [" + beanDefinition.getBeanClassName() + "] conflicts with existing, " +
			"non-compatible bean definition of same name and class [" + existingDef.getBeanClassName() + "]");
}

// ClassPathBeanDefinitionScanner
protected boolean isCompatible(BeanDefinition newDefinition, BeanDefinition existingDefinition) {
	return (!(existingDefinition instanceof ScannedGenericBeanDefinition) ||  // explicitly registered overriding bean
			(newDefinition.getSource() != null && newDefinition.getSource().equals(existingDefinition.getSource())) ||  // scanned same file twice
			newDefinition.equals(existingDefinition));  // scanned equivalent class twice
}

// ConfigurationClassBeanDefinitionReader
// 返回 true，表明被已經(jīng)存的 BeanDefinition 覆蓋了，便不在執(zhí)行配置類中 Bean 的定義和注冊(cè)
protected boolean isOverriddenByExistingDefinition(BeanMethod beanMethod, String beanName) {
	if (!this.registry.containsBeanDefinition(beanName)) {
		return false;
	}
	BeanDefinition existingBeanDef = this.registry.getBeanDefinition(beanName);

	// 已經(jīng)存在 ConfigurationClassBeanDefinition，比較工廠名和工廠方法名，只要工廠名相同就會(huì)返回 true
	if (existingBeanDef instanceof ConfigurationClassBeanDefinition) {
		ConfigurationClassBeanDefinition ccbd = (ConfigurationClassBeanDefinition) existingBeanDef;
		if (ccbd.getMetadata().getClassName().equals(
				beanMethod.getConfigurationClass().getMetadata().getClassName())) {
			if (ccbd.getFactoryMethodMetadata().getMethodName().equals(ccbd.getFactoryMethodName())) {
				ccbd.setNonUniqueFactoryMethodName(ccbd.getFactoryMethodMetadata().getMethodName());
			}
			return true;
		}
		else {
			return false;
		}
	}

	// 通過 component-scan 掃描到的 BeanDefinition 會(huì)被 @Bean 定義的 BeanDefinition 覆蓋
	if (existingBeanDef instanceof ScannedGenericBeanDefinition) {
		return false;
	}

	// 比較角色，默認(rèn) 0，即 ROLE_APPLICATION
	if (existingBeanDef.getRole() > BeanDefinition.ROLE_APPLICATION) {
		return false;
	}

	// 執(zhí)行到此，證明已經(jīng)存在的 BeanDefinition 是在配置類之前由 xml 定義的
	// DefaultListableBeanFactory 中 allowBeanDefinitionOverriding 默認(rèn) true，如果 allowBeanDefinitionOverriding 為 false，不允許覆蓋，出現(xiàn)了兩個(gè) BeanDefinition 直接拋出異常
	if (this.registry instanceof DefaultListableBeanFactory &&
			!((DefaultListableBeanFactory) this.registry).isAllowBeanDefinitionOverriding()) {
		throw new BeanDefinitionStoreException(beanMethod.getConfigurationClass().getResource().getDescription(),
				beanName, "@Bean definition illegally overridden by existing bean definition: " + existingBeanDef);
	}
	// 輸出被覆蓋的日志
	if (logger.isDebugEnabled()) {
		logger.debug(String.format("Skipping bean definition for %s: a definition for bean '%s' " +
				"already exists. This top-level bean definition is considered as an override.",
				beanMethod, beanName));
	}
	return true;
}