前面一章說了AbstractApplicationContext中的refresh方法中的invokeBeanFactoryPostProcessors。主要是調(diào)用BeanFactoryPostProcessor。其中也有獲取bean的過程，就是beanFactory.getBean的方法。這一章就說下getBean這個(gè)方法。由于spring中獲取bean的方法比較復(fù)雜，涉及到的流程也非常多，這一章就先說下整個(gè)大體的流程。其中的細(xì)節(jié)會(huì)在后面也會(huì)慢慢說。

直接看源碼吧

@Overridepublic Object getBean(String name) throws BeansException {return doGetBean(name, null, null, false);} @Overridepublic <T> T getBean(String name, @Nullable Class<T> requiredType) throws BeansException {return doGetBean(name, requiredType, null, false);} @Overridepublic 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FactoryBeanObject 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FactoryBeanbean = 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BeanFactoryBeanFactory parentBeanFactory = getParentBeanFactory();if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {// 父類去獲取beanString 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RootBeanDefinitionfinal 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beanString[] 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beangetBean(dep);}} // 創(chuàng)建單例的beanif (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FactoryBeanbean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);}// 創(chuàng)建prototype的beanelse 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;}

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(zhuǎn)換bean的名稱，轉(zhuǎn)換成beanName。這里意思就是，我們?cè)讷@取bean的時(shí)候，可能是FactoryBean的名稱（&開頭），這里轉(zhuǎn)成不帶&開頭的名稱，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(jí)緩存，也就是我們常常被問到了spring三級(jí)緩存，具體邏輯下面再說。

3.根據(jù)獲取的到對(duì)象再去獲取我們想要的bean，因?yàn)檫@里獲取到的對(duì)象可能是我們需要的bean，也可能是FactoryBean。

4.如果緩存中沒有，那么我們就要自己去創(chuàng)建bean了。

5.查看有沒有父類的BeanFactory，如果有，那么就父類去創(chuàng)建bean。

6.獲取要?jiǎng)?chuàng)建的bean對(duì)象的@DependsOn注解上的名稱，先去創(chuàng)建DependsOn的bean，并且校驗(yàn)是否存在循環(huán)引用

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

8.如果需要轉(zhuǎn)換類型，則進(jìn)行類型轉(zhuǎn)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分析。

從緩存中獲取bean對(duì)象

public Object getSingleton(String beanName) {return getSingleton(beanName, true);} @Nullableprotected 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;}

spring通過三級(jí)緩存來解決循環(huán)依賴的問題。簡(jiǎn)單來介紹下三級(jí)緩存。

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

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

3. singletonFactories為三級(jí)緩存，存放的是一個(gè)ObjectFactory，數(shù)據(jù)通過getObject方法獲得。

接下來看getObjectForBeanInstance方法。

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BeanInstaceif (!(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beanInstanceobject = getObjectFromFactoryBean(factory, beanName, !synthetic);}return object;}

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

1. 首先判斷name是不是FactoryBean的name，也就是&開頭的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，如果name是FactoryBeanName，但是BeanInstance不是FactoryBean的話，就會(huì)拋出異常。所以如果BeanInstance如果不是FactoryBean的話，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?yàn)槿绻皇荈actoryBean的話，在!(beanInstance instanceof FactoryBean)就已經(jīng)返回了。

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

根據(jù)@dependsOn查找依賴的bean并且加到依賴?yán)锩嫒]有什么好說的，代碼邏輯也很簡(jiǎn)單，接下來看創(chuàng)建單例bean。其他類型的bean的創(chuàng)建也都差別不大。看源碼

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beansingletonObject = 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;}}

對(duì)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(xiàn)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)依賴的方法（和dependsOn不同）。主要是查找根據(jù)filed，set，構(gòu)造器方法的循環(huán)依賴。

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，如果沒有排除依賴檢查，那么就加入到正在創(chuàng)建的Set中，如果加入不進(jìn)去，說明之前已經(jīng)加過，這就產(chǎn)生了循環(huán)依賴，從而拋出異常。

如果在創(chuàng)建bean之后，沒有排除檢查依賴，并且移除失敗，說明已經(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(yàn)循環(huán)依賴的，也看到了三級(jí)緩存，后面再說為什么不能解決構(gòu)造器依賴就很好說了。接著看創(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?jiǎng)?chuàng)建bean的classClass<?> resolvedClass = resolveBeanClass(mbd, beanName);if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) { // 如果沒有beanclass，設(shè)置beanclassmbdToUse = 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beanObject 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beanObject 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)還是沒有到創(chuàng)建bean的過程，還是在創(chuàng)建bean的一些準(zhǔn)備工作。其實(shí)我們可以發(fā)現(xiàn)，spring中，真正做事的都是do開頭的方法。

這邊的流程就是設(shè)置beanClass，后面需要根據(jù)反射來創(chuàng)建bean。然后會(huì)根據(jù)spring里面的beanPostProcessor，看看有沒有能產(chǎn)生代理bean的，如果有就返回，沒有就去創(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(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beaninstanceWrapper = 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beanpopulateBean(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)disposableBeantry {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(fù)雜的。這里再進(jìn)行一個(gè)簡(jiǎn)單梳理。

1. 根據(jù)class還有bean以及參數(shù)創(chuàng)建bean。

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

3. 將bean加入到三級(jí)緩存中去。

4. 填充bean需要注入的其他bean。

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

6. 接著進(jìn)行校驗(yàn)。這里稍微比較復(fù)雜一點(diǎn)。如果從二級(jí)緩存能取到，那就說明之前被別人從三級(jí)緩存拿出來過了。可能是因?yàn)檠h(huán)依賴，也可能是因?yàn)閯e的地方調(diào)用了getBean方法了。從三級(jí)緩存拿出來的時(shí)候有個(gè)getEarlyBeanReference的方法，就是查看是否要生成代理的bean。如果生成過了，那么在調(diào)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(jí)緩存生成代理之后，再去調(diào)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(xiàn)有循環(huán)調(diào)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。

到這里spring的創(chuàng)建bean就結(jié)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)行類型轉(zhuǎn)換了。

本篇大概說下spring獲取bean和加到容器里面的流程。其實(shí)廣義上來說Bean的容器是BeanFactory或者applicationContext。狹義上說就是一個(gè)map。也就是一級(jí)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)要的說了下bean獲取和加入容器的整個(gè)流程，具體的根據(jù)無參構(gòu)造器創(chuàng)建bean，有參數(shù)構(gòu)造器創(chuàng)建bean。還有對(duì)于創(chuàng)建bean中依賴的bean的查找還有創(chuàng)建，三級(jí)緩存如何解決循環(huán)依賴還有為何不能解決構(gòu)造器依賴，以及bean調(diào)用初始化的等等操作都沒有說。因?yàn)橐黄f起來確實(shí)太長(zhǎng)了。后面都會(huì)一一去分析。

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