【资料图】
本文通过阅读Spring源码，分析Bean实例化流程。
上一篇文章已经介绍，Bean实例化入口在AbstractApplicationContext类的finishBeanFactoryInitialization方法：
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 bean post-processor// (such as a PropertyPlaceholderConfigurer 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.// 实例化BeanbeanFactory.preInstantiateSingletons();}public void preInstantiateSingletons() throws BeansException {// Iterate over a copy to allow for init methods which in turn register new bean definitions.// While this may not be part of the regular factory bootstrap, it does otherwise work fine.List beanNames = new ArrayList<>(this.beanDefinitionNames);// Trigger initialization of all non-lazy singleton beans...for (String beanName : beanNames) {RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);// 判断非抽象，单例且非懒加载if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {// 判断FactoryBeanif (isFactoryBean(beanName)) {// 使用"&beanName"格式作为beanName去创建Bean实例Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);if (bean instanceof FactoryBean) {FactoryBean factory = (FactoryBean) bean;boolean isEagerInit;if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {isEagerInit = AccessController.doPrivileged((PrivilegedAction) ((SmartFactoryBean) factory)::isEagerInit,getAccessControlContext());} else {isEagerInit = (factory instanceof SmartFactoryBean &&((SmartFactoryBean) factory).isEagerInit());}if (isEagerInit) {getBean(beanName);}}} else {// 普通的BeangetBean(beanName);}}}// Trigger post-initialization callback for all applicable beans...// 此分支暂时不做分析for (String beanName : beanNames) {Object singletonInstance = getSingleton(beanName);if (singletonInstance instanceof SmartInitializingSingleton) {SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;if (System.getSecurityManager() != null) {AccessController.doPrivileged((PrivilegedAction public Object getBean(String name) throws BeansException {return doGetBean(name, null, null, false);}protected T doGetBean(String name, Class requiredType, Object[] args, boolean typeCheckOnly)throws BeansException {// 去除name的&前缀String beanName = transformedBeanName(name);Object bean;// 依次从singletonObjects, earlySingletonObjects, singletonFactories获取单例Bean// 如果找到了就不需要创建了Object sharedInstance = getSingleton(beanName);if (sharedInstance != null && args == null) {// Get the object for the given bean instance,// either the bean instance itself or its created object in case of a FactoryBean.// name参数是调用getBean方法时传递的原始BeanName，可能前缀&符用于获取FactoryBean实例// beanName参数是去除&符前缀后的BeanNamebean = 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);}// 如果当前BeanFactory没有指定Bean则从父级BeanFactory获取Bean实例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);}}// whether the instance is obtained for a type check, not for actual useif (!typeCheckOnly) {markBeanAsCreated(beanName);}try {RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);checkMergedBeanDefinition(mbd, beanName, args);// 创建DependsOn的BeanString[] dependsOn = mbd.getDependsOn();if (dependsOn != null) {for (String dep : dependsOn) {// 判断循环依赖if (isDependent(beanName, dep)) {throw new BeanCreationException( mbd.getResourceDescription(), beanName, "循环依赖");}registerDependentBean(dep, beanName);try {getBean(dep);} catch (NoSuchBeanDefinitionException ex) {throw new BeanCreationException( mbd.getResourceDescription(), beanName, "依赖的Bean不存在", ex);}}}// 此分支获取单例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;}});bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);} else if (mbd.isPrototype()) {// 此分支获取Prototype类型的BeanObject prototypeInstance = null;try {beforePrototypeCreation(beanName);prototypeInstance = createBean(beanName, mbd, args);} finally {afterPrototypeCreation(beanName);}bean = 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");}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 is not active for the current thread", ex);}}} catch (BeansException ex) {cleanupAfterBeanCreationFailure(beanName);throw ex;}}// 类型转换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) {throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());}}return (T) bean;} 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;}});bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);返回指定beanName的(原始)单例对象，如果没有则创建一个新对象：
public Object getSingleton(String beanName, ObjectFactory singletonFactory) {synchronized (this.singletonObjects) {Object singletonObject = this.singletonObjects.get(beanName);if (singletonObject == null) {// 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 (this.singletonsCurrentlyInDestruction) {throw new BeanCreationNotAllowedException(beanName, "");}beforeSingletonCreation(beanName);boolean newSingleton = false;boolean recordSuppressedExceptions = (this.suppressedExceptions == null);if (recordSuppressedExceptions) {this.suppressedExceptions = new LinkedHashSet<>();}try {// 此处需要返回去看createBean(beanName, mbd, args)的代码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) {// 将创建的单例放入单例池addSingleton(beanName, singletonObject);}}return 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);}}创建Bean实例、填充属性、调用后置处理器等：
protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args)throws BeanCreationException {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 {// 调用BeanPostProcessor处理器// 调用postProcessBeforeInstantiation方法Object bean = resolveBeforeInstantiation(beanName, mbdToUse);// 如果后置处理器返回了Bean实例则直接返回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);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);}}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) {// 创建实例，使用BeanWrapper包装// 构造方法的@Autowired也在这里面实现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 {// 调用MergedBeanDefinitionPostProcessor处理器applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);} catch (Throwable ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"Post-processing of merged bean definition failed", ex);}mbd.postProcessed = true;}}// 这里使用三级缓存封装了一段代码，解决循环依赖问题// 这段代码会执行SmartInstantiationAwareBeanPostProcessor的getEarlyBeanReference方法// 依赖这个Bean的其他Bean在填充属性时，调用getSingleton时会执行getEarlyBeanReference方法// 此时可以对这个Bean实例做一些事情，比如创建AOP代理// 之后会将修改之后的对象放入到二级缓存boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&isSingletonCurrentlyInCreation(beanName));if (earlySingletonExposure) {addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));}// Initialize the bean instance.Object exposedObject = bean;try {// Populate the bean instance in the given BeanWrapper // with the property values from the bean definition.// 1. 调用InstantiationAwareBeanPostProcessor的postProcessAfterInstantiation方法// 2. 依赖注入populateBean(beanName, mbd, instanceWrapper);// Initialize the given bean instance, applying factory // callbacks as well as init methods and bean post processors.// 1. invokeAwareMethods// 2. 调用BeanPostProcessor的postProcessBeforeInitialization// 3. InitializingBean的afterPropertiesSet和initMethod// 4. 调用BeanPostProcessor的postProcessAfterInitializationexposedObject = 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) {// 获取提前暴露的Bean实例Object earlySingletonReference = getSingleton(beanName, false);if (earlySingletonReference != null) {if (exposedObject == bean) {// 使用提前暴露的Bean替换当前BeanexposedObject = earlySingletonReference;} else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {String[] dependentBeans = getDependentBeans(beanName);Set actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);for (String dependentBean : dependentBeans) {if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {actualDependentBeans.add(dependentBean);}}if (!actualDependentBeans.isEmpty()) {throw new BeanCurrentlyInCreationException(beanName, "");}}}}// Register bean as disposable.try {registerDisposableBeanIfNecessary(beanName, bean, mbd);} catch (BeanDefinitionValidationException ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);}return exposedObject;} // It"s a prototype -> create a new instance.Object prototypeInstance = null;try {// 把beanName注册到ThreadLocal prototypesCurrentlyInCreation中beforePrototypeCreation(beanName);// 这里的逻辑与singleton一样了prototypeInstance = createBean(beanName, mbd, args);} finally {// 把beanName从ThreadLocal prototypesCurrentlyInCreation清除afterPrototypeCreation(beanName);}bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);标签：
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