@Override
	public Object getBean(String name) throws BeansException {
		return doGetBean(name, null, null, false);
	}
 
	@Override
	public <T> T getBean(String name, @Nullable 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);
	}
 
 
	public <T> T getBean(String name, @Nullable Class<T> requiredType, @Nullable Object... args)
			throws BeansException {
 
		return doGetBean(name, requiredType, args, false);
	}
 
 
	@SuppressWarnings("unchecked")
	protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
			@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
        
        // 把name轉(zhuǎn)化成beanName，也就是把FactoryBean的名稱轉(zhuǎn)化成beanName如果有別名則用別名
		final String beanName = transformedBeanName(name);
		Object bean;
 
		// 從緩存中獲取實(shí)例
        // 可能是需要的Bean實(shí)例，也可能是FactoryBean
		Object sharedInstance = getSingleton(beanName);
		if (sharedInstance != null && args == null) {
			if (logger.isDebugEnabled()) {
				if (isSingletonCurrentlyInCreation(beanName)) {
					logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
							"' that is not fully initialized yet - a consequence of a circular reference");
				}
				else {
					logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
				}
			}
            // 獲取需要的bean或者FactoryBean
			bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
		}
 
		else {
			// 判斷prototype類型的bean是否存在循環(huán)引用
			if (isPrototypeCurrentlyInCreation(beanName)) {
				throw new BeanCurrentlyInCreationException(beanName);
			}
 
			// 校驗(yàn)父類BeanFactory
			BeanFactory parentBeanFactory = getParentBeanFactory();
			if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
				// 父類去獲取bean
				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 {
					// No args -> delegate to standard getBean method.
					return parentBeanFactory.getBean(nameToLookup, requiredType);
				}
			}
            
            // 標(biāo)記成已創(chuàng)建
			if (!typeCheckOnly) {
				markBeanAsCreated(beanName);
			}
 
			try {
                // 把原來BeanDefinition轉(zhuǎn)換成RootBeanDefinition
				final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
				checkMergedBeanDefinition(mbd, beanName, args);
 
				// 獲取依賴的bean，也就是通過@DependsOn注入進(jìn)來的bean
				String[] dependsOn = mbd.getDependsOn();
				if (dependsOn != null) {
					for (String dep : dependsOn) {
                        // 校驗(yàn)dependsOn的bean是否存在循環(huán)應(yīng)用
						if (isDependent(beanName, dep)) {
							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
						}
                        // 加入到引用的緩存中，由于校驗(yàn)dependsOn循環(huán)引用
						registerDependentBean(dep, beanName);
                        // 獲取@dependsOn的bean
						getBean(dep);
					}
				}
 
				// 創(chuàng)建單例的bean
				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;
						}
					});
                    // 獲取需要的bean或者FactoryBean
					bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
				}
                // 創(chuàng)建prototype的bean
				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);
					}
					bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
				}
                // 創(chuàng)建其他的bean，比如session，request等
				else {
					String scopeName = mbd.getScope();
					final 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);
							}
						});
						bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
					}
					catch (IllegalStateException ex) {
						throw new BeanCreationException(beanName,
								"Scope '" + scopeName + "' is not active for the current thread; consider " +
								"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
								ex);
					}
				}
			}
			catch (BeansException ex) {
				cleanupAfterBeanCreationFailure(beanName);
				throw ex;
			}
		}
 
		// 如果要求的類型不是這個(gè)bean的實(shí)例類型，則進(jìn)行轉(zhuǎn)換
		if (requiredType != null && !requiredType.isInstance(bean)) {
			try {
				T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
				if (convertedBean == null) {
					throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
				}
				return convertedBean;
			}
			catch (TypeMismatchException ex) {
				if (logger.isDebugEnabled()) {
					logger.debug("Failed to convert bean '" + name + "' to required type '" +
							ClassUtils.getQualifiedName(requiredType) + "'", ex);
				}
				throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
			}
		}
		return (T) bean;
	}

public Object getSingleton(String beanName) {
		return getSingleton(beanName, true);
	}
 
 
	@Nullable
	protected Object getSingleton(String beanName, boolean allowEarlyReference) {
        // 從一級(jí)緩存中獲取
		Object singletonObject = this.singletonObjects.get(beanName);
		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
			synchronized (this.singletonObjects) {
                // 從二級(jí)緩存中獲取
				singletonObject = this.earlySingletonObjects.get(beanName);
				if (singletonObject == null && allowEarlyReference) {
                    // 從三級(jí)緩存中獲取
					ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
					if (singletonFactory != null) {
						singletonObject = singletonFactory.getObject();
						this.earlySingletonObjects.put(beanName, singletonObject);
						this.singletonFactories.remove(beanName);
					}
				}
			}
		}
		return singletonObject;
	}

	protected Object getObjectForBeanInstance(
			Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
 
        // name是不是factoryBean的name（&開頭的）
		if (BeanFactoryUtils.isFactoryDereference(name)) {
			if (beanInstance instanceof NullBean) {
				return beanInstance;
			}
            // 如果是FactoryBeanName,但是獲取到的bean不是FactoryBean，則拋異常
			if (!(beanInstance instanceof FactoryBean)) {
				throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
			}
		}
 
        // 如果bean不是FactoryBean，或者名稱是FactoryBeanName，直接返回BeanInstace
		if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
			return beanInstance;
		}
 
		Object object = null;
		if (mbd == null) {
            // 從緩存中獲取
			object = getCachedObjectForFactoryBean(beanName);
		}
		if (object == null) {
			// 這里可以確定beanInstance是FactoryBean了
			FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
			// Caches object obtained from FactoryBean if it is a singleton.
			if (mbd == null && containsBeanDefinition(beanName)) {
				mbd = getMergedLocalBeanDefinition(beanName);
			}
			boolean synthetic = (mbd != null && mbd.isSynthetic());
            // 通過FactoryFBean中獲取需要的beanInstance
			object = getObjectFromFactoryBean(factory, beanName, !synthetic);
		}
		return object;
	}

	public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
		Assert.notNull(beanName, "Bean name must not be null");
		synchronized (this.singletonObjects) {
            // 直接從一級(jí)緩存中取
			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 + "'");
				}
                // 在沒創(chuàng)建bean之前的處理
				beforeSingletonCreation(beanName);
				boolean newSingleton = false;
				boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
				if (recordSuppressedExceptions) {
					this.suppressedExceptions = new LinkedHashSet<>();
				}
				try {
                    // 獲取創(chuàng)建的bean
					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;
					}
                    // 創(chuàng)建結(jié)束之后的工作
					afterSingletonCreation(beanName);
				}
				if (newSingleton) {
                    // 加到一級(jí)緩存中，其實(shí)也就是真正的容器中了
					addSingleton(beanName, singletonObject);
				}
			}
			return singletonObject;
		}
	}

protected void beforeSingletonCreation(String beanName) {
		if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
			throw new BeanCurrentlyInCreationException(beanName);
		}
	}
 
	protected void afterSingletonCreation(String beanName) {
		if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.remove(beanName)) {
			throw new IllegalStateException("Singleton '" + beanName + "' isn't currently in creation");
		}
	}

protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
			throws BeanCreationException {
 
		if (logger.isDebugEnabled()) {
			logger.debug("Creating instance of bean '" + beanName + "'");
		}
		RootBeanDefinition mbdToUse = mbd;
        
        // 獲取要?jiǎng)?chuàng)建bean的class
		Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
		if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
            // 如果沒有beanclass，設(shè)置beanclass
			mbdToUse = new RootBeanDefinition(mbd);
			mbdToUse.setBeanClass(resolvedClass);
		}
 
		// 配置方法重載
		try {
			mbdToUse.prepareMethodOverrides();
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
					beanName, "Validation of method overrides failed", ex);
		}
 
		try {
			// 那些beanPostProcessor如果能產(chǎn)生代理，則直接返回bean
			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 {
            // 創(chuàng)建bean
			Object beanInstance = doCreateBean(beanName, mbdToUse, args);
			if (logger.isDebugEnabled()) {
				logger.debug("Finished creating instance of bean '" + beanName + "'");
			}
			return beanInstance;
		}
		catch (BeanCreationException ex) {
			// A previously detected exception with proper bean creation context already...
			throw ex;
		}
		catch (ImplicitlyAppearedSingletonException ex) {
			// An IllegalStateException to be communicated up to DefaultSingletonBeanRegistry...
			throw ex;
		}
		catch (Throwable ex) {
			throw new BeanCreationException(
					mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
		}
	}

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
			throws BeanCreationException {
 
		// 裝飾Bean的對(duì)象
		BeanWrapper instanceWrapper = null;
		if (mbd.isSingleton()) {
            // 通過緩存獲取
			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
		}
		if (instanceWrapper == null) {
            // 創(chuàng)建bean
			instanceWrapper = createBeanInstance(beanName, mbd, args);
		}
		final 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 {
                    // 調(diào)用MergedBeanDefinitionPostProcessor的方法
					applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
				}
				catch (Throwable ex) {
					throw new BeanCreationException(mbd.getResourceDescription(), beanName,
							"Post-processing of merged bean definition failed", ex);
				}
				mbd.postProcessed = true;
			}
		}
 
		// 加入到三級(jí)緩存中去
		boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
				isSingletonCurrentlyInCreation(beanName));
		if (earlySingletonExposure) {
			if (logger.isDebugEnabled()) {
				logger.debug("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 {
            // 填充依賴的bean，field注入，和方法注入的bean
			populateBean(beanName, mbd, instanceWrapper);
            // 調(diào)用初始化的方法
			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) {
            // 獲取二級(jí)緩存的值
			Object earlySingletonReference = getSingleton(beanName, false);
			if (earlySingletonReference != null) {
                // 如果一致，則直接使用二級(jí)緩存的對(duì)象
				if (exposedObject == bean) {
					exposedObject = earlySingletonReference;
				}
                // 如果調(diào)用初始化后的bean和之前的bean不一致，并且有依賴
				else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                    // 查找循環(huán)依賴
					String[] dependentBeans = getDependentBeans(beanName);
					Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
					for (String dependentBean : dependentBeans) {
                        // 如果有循環(huán)依賴并且在創(chuàng)建中，則拋出異常
						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 " +
								"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
					}
				}
			}
		}
 
		// 注冊(cè)disposableBean
		try {
			registerDisposableBeanIfNecessary(beanName, bean, mbd);
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanCreationException(
					mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
		}
 
		return exposedObject;
	}