spring獲取bean的源碼解析

介紹

前面一章說(shuō)了AbstractApplicationContext中的refresh方法中的invokeBeanFactoryPostProcessors。主要是調用BeanFactoryPostProcessor。其中也有獲取bean的過(guò)程，就是beanFactory.getBean的方法。這一章就說(shuō)下getBean這個(gè)方法。由于spring中獲取bean的方法比較復雜，涉及到的流程也非常多，這一章就先說(shuō)下整個(gè)大體的流程。其中的細節會(huì )在后面也會(huì )慢慢說(shuō)。

源碼

直接看源碼吧

@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轉化成beanName，也就是把FactoryBean的名稱(chēng)轉化成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(lèi)型的bean是否存在循環(huán)引用
			if (isPrototypeCurrentlyInCreation(beanName)) {
				throw new BeanCurrentlyInCreationException(beanName);
			}
 
			// 校驗父類(lèi)BeanFactory
			BeanFactory parentBeanFactory = getParentBeanFactory();
			if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
				// 父類(lèi)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);
				}
			}
            
            // 標記成已創(chuàng  )建
			if (!typeCheckOnly) {
				markBeanAsCreated(beanName);
			}
 
			try {
                // 把原來(lái)BeanDefinition轉換成RootBeanDefinition
				final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
				checkMergedBeanDefinition(mbd, beanName, args);
 
				// 獲取依賴(lài)的bean，也就是通過(guò)@DependsOn注入進(jìn)來(lái)的bean
				String[] dependsOn = mbd.getDependsOn();
				if (dependsOn != null) {
					for (String dep : dependsOn) {
                        // 校驗dependsOn的bean是否存在循環(huán)應用
						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(lèi)型不是這個(gè)bean的實(shí)例類(lèi)型，則進(jìn)行轉換
		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;
	}

獲取bean的整體流程就像上面源碼所示，這里再梳理下spring獲取bean的整個(gè)流程

1.先轉換bean的名稱(chēng)，轉換成beanName。這里意思就是，我們在獲取bean的時(shí)候，可能是FactoryBean的名稱(chēng)（&開(kāi)頭），這里轉成不帶&開(kāi)頭的名稱(chēng)，如果有別名，再獲取別名。

2.從緩存中獲取bean，這里的緩存分為一二三級緩存，也就是我們常常被問(wèn)到了spring三級緩存，具體邏輯下面再說(shuō)。

3.根據獲取的到對象再去獲取我們想要的bean，因為這里獲取到的對象可能是我們需要的bean，也可能是FactoryBean。

4.如果緩存中沒(méi)有，那么我們就要自己去創(chuàng )建bean了。

5.查看有沒(méi)有父類(lèi)的BeanFactory，如果有，那么就父類(lèi)去創(chuàng )建bean。

6.獲取要創(chuàng )建的bean對象的@DependsOn注解上的名稱(chēng)，先去創(chuàng )建DependsOn的bean，并且校驗是否存在循環(huán)引用

7.創(chuàng )建bean，根據類(lèi)型創(chuàng )建不同的bean，比如singleton，prototype，request，session等。

8.如果需要轉換類(lèi)型，則進(jìn)行類(lèi)型轉換。

整體的獲取bean的流程就是這樣了，其中有些具體流程接著(zhù)分析。

從緩存中獲取bean對象

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

spring通過(guò)三級緩存來(lái)解決循環(huán)依賴(lài)的問(wèn)題。簡(jiǎn)單來(lái)介紹下三級緩存。

1. singletonObjects為一級緩存，我們實(shí)例化的bean都在這個(gè)map里，俠義的說(shuō)singletonObjects才是我們真正的spring容器，存放bean的地方。

2. earlySingletonObjects為二級緩存，是存放未完成的bean的緩存，如果有代理的話(huà)，存放的是代理對象。

3. singletonFactories為三級緩存，存放的是一個(gè)ObjectFactory，數據通過(guò)getObject方法獲得。

從BeanInstance中獲取對象

接下來(lái)看getObjectForBeanInstance方法。

	protected Object getObjectForBeanInstance(
			Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
 
        // name是不是factoryBean的name（&開(kāi)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(chēng)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(guò)FactoryFBean中獲取需要的beanInstance
			object = getObjectFromFactoryBean(factory, beanName, !synthetic);
		}
		return object;
	}

這里是通過(guò)BeanInstance獲取我們想要的bean，這里也簡(jiǎn)單說(shuō)下流程

1. 首先判斷name是不是FactoryBean的name，也就是&開(kāi)頭的name，如果是去判斷beanInstance是不是FactoryBean，如果beanInstance不是FactoryBean則拋異常。

2. 由于上面已經(jīng)判斷過(guò)，如果name是FactoryBeanName，但是BeanInstance不是FactoryBean的話(huà)，就會(huì )拋出異常。所以如果BeanInstance如果不是FactoryBean的話(huà)，那么name一定不是FactoryBeanName。那么就直接返回BeanInstance就是我們需要的了。

如果name是FactoryBeanName，那么我們需要獲取的就是FactoryBean，也直接返回就可以了。

3. 如果都沒(méi)有返回，那么已經(jīng)可以確定我們此時(shí)的已經(jīng)可以確定BeanInstance是FactoryBean了，因為如果不是FactoryBean的話(huà)，在!(beanInstance instanceof FactoryBean)就已經(jīng)返回了。

4. 通過(guò)FactoryBean的getObject方法獲取我們需要的bean實(shí)例。

創(chuàng )建bean

根據@dependsOn查找依賴(lài)的bean并且加到依賴(lài)里面去沒(méi)有什么好說(shuō)的，代碼邏輯也很簡(jiǎn)單，接下來(lái)看創(chuàng )建單例bean。其他類(lèi)型的bean的創(chuàng )建也都差別不大?？丛创a

	public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
		Assert.notNull(beanName, "Bean name must not be null");
		synchronized (this.singletonObjects) {
            // 直接從一級緩存中取
			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(méi)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  )建結束之后的工作
					afterSingletonCreation(beanName);
				}
				if (newSingleton) {
                    // 加到一級緩存中，其實(shí)也就是真正的容器中了
					addSingleton(beanName, singletonObject);
				}
			}
			return singletonObject;
		}
	}

對于創(chuàng )建單例bean的主要流程就是如此，傳入一個(gè)beanName，和一個(gè)ObjectFactory。ObjectFactory中具體實(shí)現了創(chuàng )建bean的邏輯。在看具體創(chuàng )建bean的邏輯之前，我們還需要去看下getSingleton中的創(chuàng )建bean之前的工作和創(chuàng )建bean之后的工作。這里面就是查找bean的循環(huán)依賴(lài)的方法（和dependsOn不同）。主要是查找根據filed，set，構造器方法的循環(huán)依賴(lài)。

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");
		}
	}

可以看到代碼非常的簡(jiǎn)單，就是創(chuàng )建bean之前，如果沒(méi)有排除依賴(lài)檢查，那么就加入到正在創(chuàng )建的Set中，如果加入不進(jìn)去，說(shuō)明之前已經(jīng)加過(guò)，這就產(chǎn)生了循環(huán)依賴(lài)，從而拋出異常。

如果在創(chuàng )建bean之后，沒(méi)有排除檢查依賴(lài)，并且移除失敗，說(shuō)明已經(jīng)不在Set中，也會(huì )拋出異常。

好了，既然明白了spring是如何校驗循環(huán)依賴(lài)的，也看到了三級緩存，后面再說(shuō)為什么不能解決構造器依賴(lài)就很好說(shuō)了。接著(zhù)看創(chuàng )建bean的方法。

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;
        
        // 獲取要創(chuàng  )建bean的class
		Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
		if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
            // 如果沒(méi)有beanclass，設置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);
		}
	}

這里其實(shí)還是沒(méi)有到創(chuàng )建bean的過(guò)程，還是在創(chuàng )建bean的一些準備工作。其實(shí)我們可以發(fā)現，spring中，真正做事的都是do開(kāi)頭的方法。

這邊的流程就是設置beanClass，后面需要根據反射來(lái)創(chuàng )建bean。然后會(huì )根據spring里面的beanPostProcessor，看看有沒(méi)有能產(chǎn)生代理bean的，如果有就返回，沒(méi)有就去創(chuàng )建bean。

看真正的doCreateBean方法

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
			throws BeanCreationException {
 
		// 裝飾Bean的對象
		BeanWrapper instanceWrapper = null;
		if (mbd.isSingleton()) {
            // 通過(guò)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(lài)的bean，field注入，和方法注入的bean
			populateBean(beanName, mbd, instanceWrapper);
            // 調用初始化的方法
			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) {
            // 獲取二級緩存的值
			Object earlySingletonReference = getSingleton(beanName, false);
			if (earlySingletonReference != null) {
                // 如果一致，則直接使用二級緩存的對象
				if (exposedObject == bean) {
					exposedObject = earlySingletonReference;
				}
                // 如果調用初始化后的bean和之前的bean不一致，并且有依賴(lài)
				else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                    // 查找循環(huán)依賴(lài)
					String[] dependentBeans = getDependentBeans(beanName);
					Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
					for (String dependentBean : dependentBeans) {
                        // 如果有循環(huán)依賴(lài)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disposableBean
		try {
			registerDisposableBeanIfNecessary(beanName, bean, mbd);
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanCreationException(
					mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
		}
 
		return exposedObject;
	}

真正創(chuàng )建bean這里還是有點(diǎn)復雜的。這里再進(jìn)行一個(gè)簡(jiǎn)單梳理。

1. 根據class還有bean以及參數創(chuàng )建bean。

2. 調用beanPostprocessor的方法，調用屬于MergedBeanDefinitionPostProcessor的方法。對bean進(jìn)行一些處理，比如找到那些依賴(lài)的bean的field和method。

3. 將bean加入到三級緩存中去。

4. 填充bean需要注入的其他bean。

5. 調用初始化方法，先去調用@PostConstruct注解方法，然后調用InitializingBean的afterPropertiesSet，以及自定義的init-method方法。在bean調用初始化方法之后，再去調用后置接口看看是否需要生成Aop代理。

6. 接著(zhù)進(jìn)行校驗。這里稍微比較復雜一點(diǎn)。如果從二級緩存能取到，那就說(shuō)明之前被別人從三級緩存拿出來(lái)過(guò)了?？赡苁且驗檠h(huán)依賴(lài)，也可能是因為別的地方調用了getBean方法了。從三級緩存拿出來(lái)的時(shí)候有個(gè)getEarlyBeanReference的方法，就是查看是否要生成代理的bean。如果生成過(guò)了，那么在調用第五步的時(shí)候，就不會(huì )在生成代理了。這樣exposedObject ==bean，直接只用代理返回。

如果不相等：這里的情況就是如果是spring自己的@Async，在從二級緩存生成代理之后，再去調用第五步時(shí)候一樣會(huì )生成代理。所以exposedObject !=bean，所以在再往下發(fā)現有循環(huán)調用，并且bean還在創(chuàng )建時(shí)，就會(huì )拋出異常了。所以一般慎用spring的@Async。但是一般也可以使用@Lazy進(jìn)行處理。至于原理后面再說(shuō)。

到這里spring的創(chuàng )建bean就結束了。然后返回時(shí)候就到了入口方法getBean的getObjectForBeanInstance的方法，到底需要的bean還是FactoryBean。

最后就是如果requiredType和實(shí)例不一樣就要進(jìn)行類(lèi)型轉換了。

總結

本篇大概說(shuō)下spring獲取bean和加到容器里面的流程。其實(shí)廣義上來(lái)說(shuō)Bean的容器是BeanFactory或者applicationContext。狹義上說(shuō)就是一個(gè)map。也就是一級緩存SingletonObjects。我們獲取的真正需要的bean也就是從中獲取的。本篇只是簡(jiǎn)要的說(shuō)了下bean獲取和加入容器的整個(gè)流程，具體的根據無(wú)參構造器創(chuàng )建bean，有參數構造器創(chuàng )建bean。還有對于創(chuàng )建bean中依賴(lài)的bean的查找還有創(chuàng )建，三級緩存如何解決循環(huán)依賴(lài)還有為何不能解決構造器依賴(lài)，以及bean調用初始化的等等操作都沒(méi)有說(shuō)。因為一篇說(shuō)起來(lái)確實(shí)太長(cháng)了。后面都會(huì )一一去分析。

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