①. getBean方法的入口-DefaultListableBeanFactory

• ①. getBean方法的入口:在Spring中,getBean方法的入口通常位于BeanFactory或ApplicationContext接口的实现类中。最常见的是DefaultListableBeanFactory类,它是Spring容器的核心实现,如下是三个API

// 根据name获取bean	
@Override
public Object getBean(String name) throws BeansException {return doGetBean(name, null, null, false);
}// 根据name获取bean,如果获取到的bean和指定类型不匹配,则抛出异常
@Override
public <T> T getBean(String name, Class<T> requiredType) throws BeansException {return doGetBean(name, requiredType, null, false);
}// 根据name获取bean,如果是一个有参构造器生成的bean,则将参数对象注入到构造器中
@Override
public Object getBean(String name, Object... args) throws BeansException {return doGetBean(name, null, args, false);
}

②. DefaultListableBeanFactory调用getBean

• ②. DefaultListableBeanFactory中先获取到所有的bean的名称,for循环调用getBean方法

DefaultListableBeanFactory类
public void preInstantiateSingletons() throws BeansException {if (logger.isTraceEnabled()) {logger.trace("Pre-instantiating singletons in " + this);}// 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<String> beanNames = new ArrayList<>(this.beanDefinitionNames);// Trigger initialization of all non-lazy singleton beans...for (String beanName : beanNames) {RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);// 是非抽象的并且是单例的并且不是懒加载的if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {// 如果是FactoryBean执行下面逻辑(如果是实现了FactoryBean,isFactoryBean的标识为true)if (isFactoryBean(beanName)) {Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);// beanName = & + beanNameif (bean instanceof FactoryBean) {FactoryBean<?> factory = (FactoryBean<?>) bean;boolean isEagerInit;if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>) ((SmartFactoryBean<?>) factory)::isEagerInit,getAccessControlContext());}else {isEagerInit = (factory instanceof SmartFactoryBean &&((SmartFactoryBean<?>) factory).isEagerInit());}if (isEagerInit) {getBean(beanName);}}}else {// 不是FactoryBean执行下面逻辑,普通的单实例非懒加载beangetBean(beanName);}}}
}

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

③. 进入到doGetBean方法

• ③. 进入到doGetBean方法

1. transformedBeanName,处理别名BeanName、处理带&符的工厂BeanName。
2. getSingleton,先尝试从缓存中获取Bean实例,这个位置就是三级缓存解决循环依赖的方法(后面重点说)</font
3. getObjectForBeanInstance,如果sharedInstance是普通的Bean实例,则下面的方法会直接返回。另外sharedInstance是工厂Bean类型,则需要获取getObject方法,可以参考关于FactoryBean的实现类。
4. isPrototypeCurrentlyInCreation,循环依赖有三种,setter注入、多实例和构造函数,Spring只能解决setter注入,所以这里是Prototype则会抛出异常。
5. getParentBeanFactory,父bean工厂存在,当前bean不存在于当前bean工厂,则到父工厂查找bean实例。
6. mbd.getDependsOn,处理使用了depends-on注解的依赖创建bean实例。getBean(dep),加载depends-on依赖dep是depends-on缩写
7. mbd.isSingleton(),创建单例bean实例,这里的sharedInstance = getSingleton(beanName, () -> {})很重要</font
8. mbd.isPrototype(),创建其他类型的bean实例

protected <T> T doGetBean(String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)throws BeansException {//处理别名BeanName、处理带&符的工厂BeanName,工厂beanName带了&xxx 最后会变成xxxString beanName = transformedBeanName(name);Object beanInstance;// Eagerly check singleton cache for manually registered singletons.// 先检查单实例bean缓存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 {//默认第一次获取组件都会进入到else环节,如果第一次获取肯定是没有的// Fail if we're already creating this bean instance:// We're assumably within a circular reference.// 如果是一个多实例bean,抛出异常if (isPrototypeCurrentlyInCreation(beanName)) {throw new BeanCurrentlyInCreationException(beanName);}// 拿到整个bean工厂,拿到父工厂;看父工厂有没有,如果有从父工厂去获取 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) {// 标记当前bean已经被创建,用来阻止多线程引用的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) {// 看有没有依赖其他Bean,如果有先获取其他的那个Beanfor (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. 创建Bean试连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;}});// 看当前bean是否是使用了beanFactorybeanInstance = 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);
}

④. getSingleton三级缓存方法

• ④. Object sharedInstance = getSingleton(beanName,第一次进来的时候,会直接返回null

1. 一级缓存 - singletonObjects
2. 二级缓存 - earlySingletonObjects,当前bean已经创建,未进行实例化
3. 三级缓存 - singletonFactories,用来保存BeanName和创建bean的工厂之间的关系

	/** * 一级缓存* Cache of singleton objects: bean name --> bean instance */private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);/** * 三级缓存* 用来保存BeanName和创建bean的工厂之间的关系* Cache of singleton factories: bean name --> ObjectFactory */private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);/** * 二级缓存* 保存BeanName和创建bean实例之间的关系,与singletonFactories的不同之处在于,当一个单例bean被放到这里之后,那么当bean还在创建过程中* 就可以通过getBean方法获取到,可以方便进行循环依赖的检测* Cache of early singleton objects: bean name --> bean instance */private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);public Object getSingleton(String beanName) { // allowEarlyReference 允许早期引用(循环依赖的核心)return getSingleton(beanName, true);}@Nullableprotected Object getSingleton(String beanName, boolean allowEarlyReference) {// 先检查单列缓存池,获取当前对象 Quick check for existing instance without full singleton lockObject singletonObject = this.singletonObjects.get(beanName);// 先看单例池有没有 并且 是否正在创建if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {// 如果当前bean正在创建过程中,而且缓存中没有继续往下//singletonObject = this.earlySingletonObjects.get(beanName);if (singletonObject == null && allowEarlyReference) {synchronized (this.singletonObjects) {// Consistent creation of early reference within full singleton locksingletonObject = this.singletonObjects.get(beanName);if (singletonObject == null) {singletonObject = this.earlySingletonObjects.get(beanName);if (singletonObject == null) {ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);if (singletonFactory != null) {singletonObject = singletonFactory.getObject();this.earlySingletonObjects.put(beanName, singletonObject);this.singletonFactories.remove(beanName);}}}}}}return singletonObject;}

⑤. getSingleton()方法分析

• ⑤. public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory)方法

1. beforeSingletonCreation(beanName)会将beanName加入到singletonsCurrentlyInCreation集合中去,表示正在创建的集合
2. singletonFactory.getObject()会调用lamda表达式的内容[这里之后要进入lamda方法流程]
3. afterSingletonCreation(beanName)会将当前beanName从singletonsCurrentlyInCreation中移除出去
4. addSingleton方法会将当前bean加入到singletonObjects池中去

	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 {// 会调用lamda表达式的内容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;}}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);}}

⑥. createBean创建对象方法

• ⑥. 进入到createBean(beanName, mbd, args);lamda流程,最重要的是doCreateBean方法,进入到创建对象流程中

	protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)throws BeanCreationException {// 提前给一个机会,去返回组件的代理对象(aop也没有在这里操作) Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.Object bean = resolveBeforeInstantiation(beanName, mbdToUse);if (bean != null) {return bean;}......Object beanInstance = doCreateBean(beanName, mbdToUse, args);if (logger.isTraceEnabled()) {logger.trace("Finished creating instance of bean '" + beanName + "'");}return beanInstance;}

⑦. 对象创建、属性赋值、初始化

• ⑦. doCreateBean方法详解

1. 利用无参构造方法创建实例
2. earlySingletonExposure表示:当前对象是单实列、并且singletonsCurrentlyInCreation中存在这个beanName,我们在上面的步骤中已经加入到了singletonsCurrentlyInCreation集合中了
3. addSingletonFactory方法会将beanName加入到三级缓存singletonFactories中去
4. populateBean进行属性的赋值,@Autowired发生在这里
5. initializeBean初始化bean

protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)throws BeanCreationException {// Instantiate the bean.BeanWrapper instanceWrapper = null;if (mbd.isSingleton()) {//是否单例instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);}if (instanceWrapper == null) {// 创建Bean的实列,默认使用无参构造创建对象instanceWrapper = createBeanInstance(beanName, mbd, args);}Object bean = instanceWrapper.getWrappedInstance();Class<?> beanType = instanceWrapper.getWrappedClass();if (beanType != NullBean.class) {mbd.resolvedTargetType = beanType;}//允许MergedBeanDefinitionPostProcessor.postProcessMergedBeanDefinition再来修改下BeanDefinition 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 {// 给创建好的对象每个属性进行赋值,@Autowired发生在这里populateBean(beanName, mbd, instanceWrapper);// 初始化beanexposedObject = 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;}protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {Assert.notNull(singletonFactory, "Singleton factory must not be null");synchronized (this.singletonObjects) {if (!this.singletonObjects.containsKey(beanName)) {this.singletonFactories.put(beanName, singletonFactory);this.earlySingletonObjects.remove(beanName);this.registeredSingletons.add(beanName);}}}

⑧. getBean最详细流程图