[Java知识库]spring-boot源码解析之DefaultListableBeanFactory

功能

DefaultListableBeanFactory是几乎所有ApplicationContext默认的底层容器的实现。通常一个一个ApplicationContext至少包含一个BeanDefinitionReader和DefaultListableBeanFactory，BeanDefinitionReader负责读取并解析出BeanDefinition，然后注册到DefaultListableBeanFactory里面去。

• ConfigurableListableBeanFactory and BeanDefinitionRegistry
• 通过BeanPostProcessor扩展bean的实现

registerBeanDefinition

	@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);
			}
		}

		BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
		if (existingDefinition != null) {
			if (!isAllowBeanDefinitionOverriding()) {
				throw new BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);
			}
			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 + "]");
				}
			}
			else if (!beanDefinition.equals(existingDefinition)) {
				if (logger.isDebugEnabled()) {
					logger.debug("Overriding bean definition for bean '" + beanName +
							"' with a different definition: replacing [" + existingDefinition +
							"] with [" + beanDefinition + "]");
				}
			}
			else {
				if (logger.isTraceEnabled()) {
					logger.trace("Overriding bean definition for bean '" + beanName +
							"' with an equivalent definition: replacing [" + existingDefinition +
							"] with [" + beanDefinition + "]");
				}
			}
			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 {
				// Still in startup registration phase
				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();
		}
	}

其实就是更新beanDefinitionMap和beanDefinitionNames，如果又手动注册的singleton，会被清除。所以springFrameworkregisterSingleton的名字都很长，以至于不会被轻易覆盖。

registerSingleton

	@Override
	public void registerSingleton(String beanName, Object singletonObject) throws IllegalStateException {
		super.registerSingleton(beanName, singletonObject);
		updateManualSingletonNames(set -> set.add(beanName), set -> !this.beanDefinitionMap.containsKey(beanName));
		clearByTypeCache();
	}

手动注册的singleton只会做一个标记。真正singleton bean的处理是在ancestor class
DefaultSingletonBeanRegistry

	@Override
	public void registerSingleton(String beanName, Object singletonObject) throws IllegalStateException {
		Assert.notNull(beanName, "Bean name must not be null");
		Assert.notNull(singletonObject, "Singleton object must not be null");
		synchronized (this.singletonObjects) {
			Object oldObject = this.singletonObjects.get(beanName);
			if (oldObject != null) {
				throw new IllegalStateException("Could not register object [" + singletonObject +
						"] under bean name '" + beanName + "': there is already object [" + oldObject + "] bound");
			}
			addSingleton(beanName, singletonObject);
		}
	}
	protected void addSingleton(String beanName, Object singletonObject) {
		synchronized (this.singletonObjects) {
			this.singletonObjects.put(beanName, singletonObject);
			this.singletonFactories.remove(beanName);
			this.earlySingletonObjects.remove(beanName);
			this.registeredSingletons.add(beanName);
		}
	}

我们在分析Bean的初始化以及循环依赖的时候还会回来

preInstantiateSingletons

org.springframework.context.support.AbstractApplicationContext#refresh在最后会调用org.springframework.context.support.AbstractApplicationContext#finishBeanFactoryInitialization

	protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
		// Initialize conversion service for this context.
		if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
				beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
			beanFactory.setConversionService(
					beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
		}

		// Register a default embedded value resolver if no BeanFactoryPostProcessor
		// (such as a PropertySourcesPlaceholderConfigurer bean) registered any before:
		// at this point, primarily for resolution in annotation attribute values.
		if (!beanFactory.hasEmbeddedValueResolver()) {
			beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
		}

		// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
		String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
		for (String weaverAwareName : weaverAwareNames) {
			getBean(weaverAwareName);
		}

		// Stop using the temporary ClassLoader for type matching.
		beanFactory.setTempClassLoader(null);

		// Allow for caching all bean definition metadata, not expecting further changes.
		beanFactory.freezeConfiguration();

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

可以看到在最后preInstantiateSingletons会被调用，beanFactory的所有singleton bean会被初始化。

1， 对所有的singleton beanDefinition调用getBean(name)
2，调用SmartInitializingSingleton回调（没用到）

所以主要的任务逻辑就在于getBean

getBean(beanName)

在org.springframework.beans.factory.support.AbstractBeanFactory

	@Override
	public Object getBean(String name) throws BeansException {
		return doGetBean(name, null, null, false);
	}

	@Override
	public <T> T getBean(String name, Class<T> requiredType) throws BeansException {
		return doGetBean(name, requiredType, null, false);
	}

	@Override
	public Object getBean(String name, Object... args) throws BeansException {
		return doGetBean(name, null, args, false);
	}

	/**
	 * Return an instance, which may be shared or independent, of the specified bean.
	 * @param name the name of the bean to retrieve
	 * @param requiredType the required type of the bean to retrieve
	 * @param args arguments to use when creating a bean instance using explicit arguments
	 * (only applied when creating a new instance as opposed to retrieving an existing one)
	 * @return an instance of the bean
	 * @throws BeansException if the bean could not be created
	 */
	public <T> T getBean(String name, @Nullable Class<T> requiredType, @Nullable Object... args)
			throws BeansException {

		return doGetBean(name, requiredType, args, false);
	}

	/**
	 * Return an instance, which may be shared or independent, of the specified bean.
	 * @param name the name of the bean to retrieve
	 * @param requiredType the required type of the bean to retrieve
	 * @param args arguments to use when creating a bean instance using explicit arguments
	 * (only applied when creating a new instance as opposed to retrieving an existing one)
	 * @param typeCheckOnly whether the instance is obtained for a type check,
	 * not for actual use
	 * @return an instance of the bean
	 * @throws BeansException if the bean could not be created
	 */
	@SuppressWarnings("unchecked")
	protected <T> T doGetBean(
			String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
			throws BeansException {

		String beanName = transformedBeanName(name);
		Object beanInstance;

		// Eagerly check singleton cache for manually registered singletons.
		Object sharedInstance = getSingleton(beanName);
		if (sharedInstance != null && args == null) {
			if (logger.isTraceEnabled()) {
				if (isSingletonCurrentlyInCreation(beanName)) {
					logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
							"' that is not fully initialized yet - a consequence of a circular reference");
				}
				else {
					logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
				}
			}
			beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);
		}

		else {
			// Fail if we're already creating this bean instance:
			// We're assumably within a circular reference.
			if (isPrototypeCurrentlyInCreation(beanName)) {
				throw new BeanCurrentlyInCreationException(beanName);
			}

			// Check if bean definition exists in this factory.
			BeanFactory parentBeanFactory = getParentBeanFactory();
			if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
				// Not found -> check parent.
				String nameToLookup = originalBeanName(name);
				if (parentBeanFactory instanceof AbstractBeanFactory) {
					return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
							nameToLookup, requiredType, args, typeCheckOnly);
				}
				else if (args != null) {
					// Delegation to parent with explicit args.
					return (T) parentBeanFactory.getBean(nameToLookup, args);
				}
				else if (requiredType != null) {
					// No args -> delegate to standard getBean method.
					return parentBeanFactory.getBean(nameToLookup, requiredType);
				}
				else {
					return (T) parentBeanFactory.getBean(nameToLookup);
				}
			}

			if (!typeCheckOnly) {
				markBeanAsCreated(beanName);
			}

			StartupStep beanCreation = this.applicationStartup.start("spring.beans.instantiate")
					.tag("beanName", name);
			try {
				if (requiredType != null) {
					beanCreation.tag("beanType", requiredType::toString);
				}
				RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
				checkMergedBeanDefinition(mbd, beanName, args);

				// Guarantee initialization of beans that the current bean depends on.
				String[] dependsOn = mbd.getDependsOn();
				if (dependsOn != null) {
					for (String dep : dependsOn) {
						if (isDependent(beanName, dep)) {
							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
						}
						registerDependentBean(dep, beanName);
						try {
							getBean(dep);
						}
						catch (NoSuchBeanDefinitionException ex) {
							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
						}
					}
				}

				// Create bean instance.
				if (mbd.isSingleton()) {
					sharedInstance = getSingleton(beanName, () -> {
						try {
							return createBean(beanName, mbd, args);
						}
						catch (BeansException ex) {
							// Explicitly remove instance from singleton cache: It might have been put there
							// eagerly by the creation process, to allow for circular reference resolution.
							// Also remove any beans that received a temporary reference to the bean.
							destroySingleton(beanName);
							throw ex;
						}
					});
					beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
				}

				else if (mbd.isPrototype()) {
					// It's a prototype -> create a new instance.
					Object prototypeInstance = null;
					try {
						beforePrototypeCreation(beanName);
						prototypeInstance = createBean(beanName, mbd, args);
					}
					finally {
						afterPrototypeCreation(beanName);
					}
					beanInstance = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
				}

				else {
					String scopeName = mbd.getScope();
					if (!StringUtils.hasLength(scopeName)) {
						throw new IllegalStateException("No scope name defined for bean '" + beanName + "'");
					}
					Scope scope = this.scopes.get(scopeName);
					if (scope == null) {
						throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
					}
					try {
						Object scopedInstance = scope.get(beanName, () -> {
							beforePrototypeCreation(beanName);
							try {
								return createBean(beanName, mbd, args);
							}
							finally {
								afterPrototypeCreation(beanName);
							}
						});
						beanInstance = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
					}
					catch (IllegalStateException ex) {
						throw new ScopeNotActiveException(beanName, scopeName, ex);
					}
				}
			}
			catch (BeansException ex) {
				beanCreation.tag("exception", ex.getClass().toString());
				beanCreation.tag("message", String.valueOf(ex.getMessage()));
				cleanupAfterBeanCreationFailure(beanName);
				throw ex;
			}
			finally {
				beanCreation.end();
			}
		}

		return adaptBeanInstance(name, beanInstance, requiredType);
	}

	@SuppressWarnings("unchecked")
	<T> T adaptBeanInstance(String name, Object bean, @Nullable Class<?> requiredType) {
		// Check if required type matches the type of the actual bean instance.
		if (requiredType != null && !requiredType.isInstance(bean)) {
			try {
				Object convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
				if (convertedBean == null) {
					throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
				}
				return (T) convertedBean;
			}
			catch (TypeMismatchException ex) {
				if (logger.isTraceEnabled()) {
					logger.trace("Failed to convert bean '" + name + "' to required type '" +
							ClassUtils.getQualifiedName(requiredType) + "'", ex);
				}
				throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
			}
		}
		return (T) bean;
	}

transformedBeanName:

	public static String transformedBeanName(String name) {
		Assert.notNull(name, "'name' must not be null");
		if (!name.startsWith(BeanFactory.FACTORY_BEAN_PREFIX)) {
			return name;
		}
		return transformedBeanNameCache.computeIfAbsent(name, beanName -> {
			do {
				beanName = beanName.substring(BeanFactory.FACTORY_BEAN_PREFIX.length());
			}
			while (beanName.startsWith(BeanFactory.FACTORY_BEAN_PREFIX));
			return beanName;
		});
	}

大多数直接返回，只有factoryBean会以&开头，其实也就是把&去掉而已

getSingleton：返回的对象有可能是已经初始化好的，也有可能是还未初始化好的（循环以来的场景，总有一个会在为完全初始化的时候注入到另外一个）。假设我们这是刚开始初始化遇到的bean，那么getSingleton会返回null。

假设没有parentFactory，只在并且beanFactory有这个beanName

getMergedLocalBeanDefinition： 因为beanDefinition也会有parent（只在xml的情况下），所以需要merge， spring boot大多数场景可以忽略

getDependsOn：指定的强制的先后顺序，并不是Autowire确定的 隐含的依赖关系。大多是是null
org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#getSingleton

	public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
		Assert.notNull(beanName, "Bean name must not be null");
		synchronized (this.singletonObjects) {
			Object singletonObject = this.singletonObjects.get(beanName);
			if (singletonObject == null) {
				if (this.singletonsCurrentlyInDestruction) {
					throw new BeanCreationNotAllowedException(beanName,
							"Singleton bean creation not allowed while singletons of this factory are in destruction " +
							"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
				}
				if (logger.isDebugEnabled()) {
					logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
				}
				beforeSingletonCreation(beanName);
				boolean newSingleton = false;
				boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
				if (recordSuppressedExceptions) {
					this.suppressedExceptions = new LinkedHashSet<>();
				}
				try {
					singletonObject = singletonFactory.getObject();
					newSingleton = true;
				}
				catch (IllegalStateException ex) {
					// Has the singleton object implicitly appeared in the meantime ->
					// if yes, proceed with it since the exception indicates that state.
					singletonObject = this.singletonObjects.get(beanName);
					if (singletonObject == null) {
						throw ex;
					}
				}
				catch (BeanCreationException ex) {
					if (recordSuppressedExceptions) {
						for (Exception suppressedException : this.suppressedExceptions) {
							ex.addRelatedCause(suppressedException);
						}
					}
					throw ex;
				}
				finally {
					if (recordSuppressedExceptions) {
						this.suppressedExceptions = null;
					}
					afterSingletonCreation(beanName);
				}
				if (newSingleton) {
					addSingleton(beanName, singletonObject);
				}
			}
			return singletonObject;
		}
	}

beforeSingletonCreation: singletonsCurrentlyInCreation.add(beanName)
afterSingletonCreation: singletonsCurrentlyInCreation.remove(beanName)
addSingleton

其实就是createBean的一个wrapper
1， 更新singletonsCurrentlyInCreation，用于循环依赖检测
2，把新生成的bean addSingleton

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#createBean

@Override
	protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
			throws BeanCreationException {

		if (logger.isTraceEnabled()) {
			logger.trace("Creating instance of bean '" + beanName + "'");
		}
		RootBeanDefinition mbdToUse = mbd;

		// Make sure bean class is actually resolved at this point, and
		// clone the bean definition in case of a dynamically resolved Class
		// which cannot be stored in the shared merged bean definition.
		Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
		if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
			mbdToUse = new RootBeanDefinition(mbd);
			mbdToUse.setBeanClass(resolvedClass);
		}

		// Prepare method overrides.
		try {
			mbdToUse.prepareMethodOverrides();
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
					beanName, "Validation of method overrides failed", ex);
		}

		try {
			// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
			Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
			if (bean != null) {
				return bean;
			}
		}
		catch (Throwable ex) {
			throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
					"BeanPostProcessor before instantiation of bean failed", ex);
		}

		try {
			Object beanInstance = doCreateBean(beanName, mbdToUse, args);
			if (logger.isTraceEnabled()) {
				logger.trace("Finished creating instance of bean '" + beanName + "'");
			}
			return beanInstance;
		}
		catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
			// A previously detected exception with proper bean creation context already,
			// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
			throw ex;
		}
		catch (Throwable ex) {
			throw new BeanCreationException(
					mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
		}
	}

	/**
	 * Actually create the specified bean. Pre-creation processing has already happened
	 * at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.
	 * <p>Differentiates between default bean instantiation, use of a
	 * factory method, and autowiring a constructor.
	 * @param beanName the name of the bean
	 * @param mbd the merged bean definition for the bean
	 * @param args explicit arguments to use for constructor or factory method invocation
	 * @return a new instance of the bean
	 * @throws BeanCreationException if the bean could not be created
	 * @see #instantiateBean
	 * @see #instantiateUsingFactoryMethod
	 * @see #autowireConstructor
	 */
	protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
			throws BeanCreationException {

		// Instantiate the bean.
		BeanWrapper instanceWrapper = null;
		if (mbd.isSingleton()) {
			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
		}
		if (instanceWrapper == null) {
			instanceWrapper = createBeanInstance(beanName, mbd, args);
		}
		Object bean = instanceWrapper.getWrappedInstance();
		Class<?> beanType = instanceWrapper.getWrappedClass();
		if (beanType != NullBean.class) {
			mbd.resolvedTargetType = beanType;
		}

		// Allow post-processors to modify the merged bean definition.
		synchronized (mbd.postProcessingLock) {
			if (!mbd.postProcessed) {
				try {
					applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
				}
				catch (Throwable ex) {
					throw new BeanCreationException(mbd.getResourceDescription(), beanName,
							"Post-processing of merged bean definition failed", ex);
				}
				mbd.postProcessed = true;
			}
		}

		// Eagerly cache singletons to be able to resolve circular references
		// even when triggered by lifecycle interfaces like BeanFactoryAware.
		boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
				isSingletonCurrentlyInCreation(beanName));
		if (earlySingletonExposure) {
			if (logger.isTraceEnabled()) {
				logger.trace("Eagerly caching bean '" + beanName +
						"' to allow for resolving potential circular references");
			}
			addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
		}

		// Initialize the bean instance.
		Object exposedObject = bean;
		try {
			populateBean(beanName, mbd, instanceWrapper);
			exposedObject = initializeBean(beanName, exposedObject, mbd);
		}
		catch (Throwable ex) {
			if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
				throw (BeanCreationException) ex;
			}
			else {
				throw new BeanCreationException(
						mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
			}
		}

		if (earlySingletonExposure) {
			Object earlySingletonReference = getSingleton(beanName, false);
			if (earlySingletonReference != null) {
				if (exposedObject == bean) {
					exposedObject = earlySingletonReference;
				}
				else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
					String[] dependentBeans = getDependentBeans(beanName);
					Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
					for (String dependentBean : dependentBeans) {
						if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
							actualDependentBeans.add(dependentBean);
						}
					}
					if (!actualDependentBeans.isEmpty()) {
						throw new BeanCurrentlyInCreationException(beanName,
								"Bean with name '" + beanName + "' has been injected into other beans [" +
								StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
								"] in its raw version as part of a circular reference, but has eventually been " +
								"wrapped. This means that said other beans do not use the final version of the " +
								"bean. This is often the result of over-eager type matching - consider using " +
								"'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");
					}
				}
			}
		}

		// Register bean as disposable.
		try {
			registerDisposableBeanIfNecessary(beanName, bean, mbd);
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanCreationException(
					mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
		}

		return exposedObject;
	}

resolveBeanClass: 通常都是在beanDefinition注册的时候写入的，比如
org.springframework.context.annotation.AnnotatedBeanDefinitionReader#doRegisterBean
–》

	public AnnotatedGenericBeanDefinition(Class<?> beanClass) {
		setBeanClass(beanClass);
		this.metadata = AnnotationMetadata.introspect(beanClass);
	}

resolveBeforeInstantiation

/**
 * Subinterface of {@link BeanPostProcessor} that adds a before-instantiation callback,
 * and a callback after instantiation but before explicit properties are set or
 * autowiring occurs.
 *
 * <p>Typically used to suppress default instantiation for specific target beans,
 * for example to create proxies with special TargetSources (pooling targets,
 * lazily initializing targets, etc), or to implement additional injection strategies
 * such as field injection.
 *
 * <p><b>NOTE:</b> This interface is a special purpose interface, mainly for
 * internal use within the framework. It is recommended to implement the plain
 * {@link BeanPostProcessor} interface as far as possible.
 *
 * @author Juergen Hoeller
 * @author Rod Johnson
 * @since 1.2
 * @see org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator#setCustomTargetSourceCreators
 * @see org.springframework.aop.framework.autoproxy.target.LazyInitTargetSourceCreator
 */
public interface InstantiationAwareBeanPostProcessor extends BeanPostProcessor {}

			// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
			Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
			if (bean != null) {
				return bean;
			}
			
	@Nullable
	protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
		Object bean = null;
		if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
			// Make sure bean class is actually resolved at this point.
			if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
				Class<?> targetType = determineTargetType(beanName, mbd);
				if (targetType != null) {
					bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
					if (bean != null) {
						bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
					}
				}
			}
			mbd.beforeInstantiationResolved = (bean != null);
		}
		return bean;
	}

createBeanInstance: 这个非常复杂，全是反射获取constructor，如果constructor有参数，最终会调用到
org.springframework.beans.factory.support.DefaultListableBeanFactory#resolveDependency

	public Object resolveDependency(DependencyDescriptor descriptor, @Nullable String requestingBeanName,
			@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {

		descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());
		if (Optional.class == descriptor.getDependencyType()) {
			return createOptionalDependency(descriptor, requestingBeanName);
		}
		else if (ObjectFactory.class == descriptor.getDependencyType() ||
				ObjectProvider.class == descriptor.getDependencyType()) {
			return new DependencyObjectProvider(descriptor, requestingBeanName);
		}
		else if (javaxInjectProviderClass == descriptor.getDependencyType()) {
			return new Jsr330Factory().createDependencyProvider(descriptor, requestingBeanName);
		}
		else {
			Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(
					descriptor, requestingBeanName);
			if (result == null) {
				result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
			}
			return result;
		}
	}

在这里我们可以看到，我们的注入参数可以是Optional，ObjectFactory， ObjectProvider

	protected Map<String, Object> findAutowireCandidates(
			@Nullable String beanName, Class<?> requiredType, DependencyDescriptor descriptor) {

		String[] candidateNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
				this, requiredType, true, descriptor.isEager());
		Map<String, Object> result = CollectionUtils.newLinkedHashMap(candidateNames.length);
		for (Map.Entry<Class<?>, Object> classObjectEntry : this.resolvableDependencies.entrySet()) {
			Class<?> autowiringType = classObjectEntry.getKey();
			if (autowiringType.isAssignableFrom(requiredType)) {
				Object autowiringValue = classObjectEntry.getValue();
				autowiringValue = AutowireUtils.resolveAutowiringValue(autowiringValue, requiredType);
				if (requiredType.isInstance(autowiringValue)) {
					result.put(ObjectUtils.identityToString(autowiringValue), autowiringValue);
					break;
				}
			}
		}
		for (String candidate : candidateNames) {
			if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate, descriptor)) {
				addCandidateEntry(result, candidate, descriptor, requiredType);
			}
		}
		if (result.isEmpty()) {
			boolean multiple = indicatesMultipleBeans(requiredType);
			// Consider fallback matches if the first pass failed to find anything...
			DependencyDescriptor fallbackDescriptor = descriptor.forFallbackMatch();
			for (String candidate : candidateNames) {
				if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate, fallbackDescriptor) &&
						(!multiple || getAutowireCandidateResolver().hasQualifier(descriptor))) {
					addCandidateEntry(result, candidate, descriptor, requiredType);
				}
			}
			if (result.isEmpty() && !multiple) {
				// Consider self references as a final pass...
				// but in the case of a dependency collection, not the very same bean itself.
				for (String candidate : candidateNames) {
					if (isSelfReference(beanName, candidate) &&
							(!(descriptor instanceof MultiElementDescriptor) || !beanName.equals(candidate)) &&
							isAutowireCandidate(candidate, fallbackDescriptor)) {
						addCandidateEntry(result, candidate, descriptor, requiredType);
					}
				}
			}
		}
		return result;
	}

我们可以看到在findAutowireCandidates中首先会考虑resolvableDependencies， 其中applicationContext，eventMulticaster就在里面

最终我们会看到，经过一些列的骚操作

	public Object resolveCandidate(String beanName, Class<?> requiredType, BeanFactory beanFactory)
   		throws BeansException {

   	return beanFactory.getBean(beanName);
   }

还是又回到beanFactory.getBean

earlySingletonExposure： 如果允许循环非constructor依赖，那么这个就是true，并且加入singletonFactories（第三级缓存）。isSingletonCurrentlyInCreation是在getBean的wrapper里面设置为true的
populateBean： 1， postProcessAfterInstantiation并调用， 2， 填充autowired field，这个过程也会调用到getBean， 3，postProcessPropertyValues 并调用。 @Autowired就是在这儿回调的

initializeBean：1， invokeAwareMethods： BeanNameAware， BeanClassLoaderAware， BeanFactoryAware。2，applyBeanPostProcessorsBeforeInitialization 3，调用afterPropertiesSet，invokeCustomInitMethod， 3， applyBeanPostProcessorsAfterInitialization

registerDisposableBeanIfNecessary：注册@PostConstruct回调

A<>B循环依赖典型场景分析

0->getBean(A)
1-> singletonsCurrentlyInCreation.add(A)
2-> singletonFactories.put(A) 三级缓存
3-> populateBean(A) 此时发现依赖B，出发B的初始化
-4> getBean(B)
5-> singletonsCurrentlyInCreation.add(B)
6-> singletonFactories.put(B) 三级缓存
7-> populateBean(B), initializeBean(B)
8-> getBean(A) 返回 A(early cached), 同时earlySingletonObjects.put(A), singletonFactories.remove(A)
9->singletonsCurrentlyInCreation.remove(B), singletonObjects.put(B), singletonFactories.remove(B),earlySingletonObjects.remove(B), registeredSingletons.add(B)
10 -> return from step 2, A = initializeBean(A)， 假设initializeBean新增加了一个proxy， A就变变为了A2
11 -> 因为A2 != A，所以已经注入到B的A是不正确的，正确的是A2

		if (earlySingletonExposure) {
			Object earlySingletonReference = getSingleton(beanName, false);
			if (earlySingletonReference != null) {
				if (exposedObject == bean) {
					exposedObject = earlySingletonReference;
				}
				else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
					String[] dependentBeans = getDependentBeans(beanName);
					Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
					for (String dependentBean : dependentBeans) {
						if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
							actualDependentBeans.add(dependentBean);
						}
					}
					if (!actualDependentBeans.isEmpty()) {
						throw new BeanCurrentlyInCreationException(beanName,
								"Bean with name '" + beanName + "' has been injected into other beans [" +
								StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
								"] in its raw version as part of a circular reference, but has eventually been " +
								"wrapped. This means that said other beans do not use the final version of the " +
								"bean. This is often the result of over-eager type matching - consider using " +
								"'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");
					}
				}
			}
		}

12 -> 如果11 没有发生 singletonsCurrentlyInCreation.remove(A), singletonObjects.put(A), singletonFactories.remove(A),earlySingletonObjects.remove(A), registeredSingletons.add(A)