获取bean的流程

1. org.springframework.context.support.AbstractApplicationContext#getBean(java.lang.String)
2. org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean
3. org.springframework.beans.factory.support.AbstractBeanFactory#transformedBeanName
   如果是别名，通过这个方法找到真正的bean的唯一id
4. org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#getSingleton(java.lang.String)
   从缓存中获取bean，获取顺序是一级缓存->二级缓存->三级缓存。如果一级缓存和二级缓存都没取到，从三级缓存中取出beanFactory,调用beanFactory的get(beanName)方法获取bean，并放入二级缓存，然后从三级缓存中删除。

/** Cache of singleton objects: bean name to bean instance.  */
/** 一级缓存：存放bean实例，先从这里获取  */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

/** Cache of early singleton objects: bean name to bean instance. */
/** 二级缓存：早期bean对象，半成品  */
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);

/** Cache of singleton factories: bean name to ObjectFactory. */
/** 三级缓存：创建bean的工厂对象 */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

1. 如果从缓存中取到了就调用getObjectForBeanInstance方法
   org.springframework.beans.factory.support.AbstractBeanFactory#getObjectForBeanInstance

2. 如果beanName是&开头的则beanInstance(从缓存中获取的)为beanFactory类型的，直接返回

3. 如果beanInstance不是beanFactory类型的，直接返回

4. beanName不是&开头且beanInstance是beanFactory类型的，调用工厂方法获取bean对象

5. 调用getParentBeanFactory，优先通过父类的工厂对象创建bean

6. 检测是否有循环依赖,先获取依赖的bean

7. 如果是单例，先从一级缓存获取一下，获取不到就走**创建bean**的流程
   如果是原型,直接走创建bean的流程
   org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#createBean()

创建bean

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#createBean()

生命周期

Spring Bean的生命周期（非常详细）

源码解析

调用InstantiationAwareBeanPostProcessor接口的方法

createBean

/**
 * Central method of this class: creates a bean instance,
 * populates the bean instance, applies post-processors, etc.
 * @see #doCreateBean
 */
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
		throws BeanCreationException {
	if (logger.isTraceEnabled()) {
		logger.trace("Creating instance of bean '" + beanName + "'");
	}
	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.
        // 解析bean的类型
	Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
	if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
		mbdToUse = new RootBeanDefinition(mbd);
		mbdToUse.setBeanClass(resolvedClass);
	}
	// Prepare method overrides.
	try {
                // 如果有重载的方法，进行校验，并将标记为不重载，避免参数检查导致性能的开销
		mbdToUse.prepareMethodOverrides();
	}
	catch (BeanDefinitionValidationException ex) {
		throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
				beanName, "Validation of method overrides failed", ex);
	}
	try {
		// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
		// 给了一个提供代理创建bean的机会，实现AOP的关键所在
                // 如果是InstantiationAwareBeanPostProcessor类型的，先调用postProcessBeforeInstantiation（）方法创建一个代理对象
                // 然后再调用postProcessAfterInitialization方法，再给你一个改变bean的机会
		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 {
                // 如果你没有把握机会，那现在去正式创建bean
		Object beanInstance = doCreateBean(beanName, mbdToUse, args);
		if (logger.isTraceEnabled()) {
			logger.trace("Finished creating instance of bean '" + beanName + "'");
		}
		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);
	}
}

resolveBeforeInstantiation

/**
 * Apply before-instantiation post-processors, resolving whether there is a
 * before-instantiation shortcut for the specified bean.
 * @param beanName the name of the bean
 * @param mbd the bean definition for the bean
 * @return the shortcut-determined bean instance, or {@code null} if none
 */
@Nullable
protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
	Object bean = null;
	if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
		// Make sure bean class is actually resolved at this point.
		if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
			Class<?> targetType = determineTargetType(beanName, mbd);
			if (targetType != null) {
                                // 一个手动创建bean的机会，比如代理
				bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
				if (bean != null) {
                                        // 再给一个改变bean的机会
					bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
				}
			}
		}
		mbd.beforeInstantiationResolved = (bean != null);
	}
	return bean;
}

protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) {
        // 实现了 InstantiationAwareBeanPostProcessor 接口的bean
	for (BeanPostProcessor bp : getBeanPostProcessors()) {
		if (bp instanceof InstantiationAwareBeanPostProcessor) {
			InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
			Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName);
			if (result != null) {
				return result;
			}
		}
	}
	return null;
}

@Override
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
		throws BeansException {
	Object result = existingBean;
        // 实现了BeanPostProcessor接口的bean
	for (BeanPostProcessor processor : getBeanPostProcessors()) {
		Object current = processor.postProcessAfterInitialization(result, beanName);
		if (current == null) {
			return result;
		}
		result = current;
	}
	return result;
}

调用MergedBeanDefinitionPostProcessor接口的方法

doCreateBean（）

/**
 * Actually create the specified bean. Pre-creation processing has already happened
 * at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.
 * <p>Differentiates between default bean instantiation, use of a
 * factory method, and autowiring a constructor.
 * @param beanName the name of the bean
 * @param mbd the merged bean definition for the bean
 * @param args explicit arguments to use for constructor or factory method invocation
 * @return a new instance of the bean
 * @throws BeanCreationException if the bean could not be created
 * @see #instantiateBean
 * @see #instantiateUsingFactoryMethod
 * @see #autowireConstructor
 */
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bean实例
                // 通过factory-method、有参构造、默认无参构造创建一个BeanWrapper
		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 {
                                // 所有实现了MergedBeanDefinitionPostProcessor接口的bean,调用postProcessMergedBeanDefinition()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;
		}
	}
	// 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");
		}
                // 如果允许暴露早起的bean（单例且允许循环引用且正在创建），添加到三级缓存中。
                // 当从三级缓存中获取到，调用getObject()方法时，调用getEarlyBeanReference(),
                // 创建代理对象 AOP有用到
		addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
	}
	// Initialize the bean instance.  
	Object exposedObject = bean;
	try {
                // 填充bean的属性值
                // 根据名称注入，根据类型注入，
                // 在赋值之前可以通过实现InstantiationAwareBeanPostProcessor接口来对值做一些操作
		populateBean(beanName, mbd, instanceWrapper);
                // 初始化bean实例
		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) {
		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;
}

applyMergedBeanDefinitionPostProcessors

protected void applyMergedBeanDefinitionPostProcessors(RootBeanDefinition mbd, Class<?> beanType, String beanName) {
	for (BeanPostProcessor bp : getBeanPostProcessors()) {
		if (bp instanceof MergedBeanDefinitionPostProcessor) {
			MergedBeanDefinitionPostProcessor bdp = (MergedBeanDefinitionPostProcessor) bp;
			bdp.postProcessMergedBeanDefinition(mbd, beanType, beanName);
		}
	}
}

getEarlyBeanReference()

/**
 * Obtain a reference for early access to the specified bean,
 * typically for the purpose of resolving a circular reference.
 * @param beanName the name of the bean (for error handling purposes)
 * @param mbd the merged bean definition for the bean
 * @param bean the raw bean instance
 * @return the object to expose as bean reference
 */
protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
	Object exposedObject = bean;
	if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
		for (BeanPostProcessor bp : getBeanPostProcessors()) {
			if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
				SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
				exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
			}
		}
	}
	return exposedObject;
}

调用BeanNameAware接口的方法

调用BeanClassLoaderAware接口的方法

调用BeanFactoryAware接口的方法

调用BeanPostProcessor接口的postProcessBeforeInitialization方法

调用ApplicationContextAware、EnvironmentAware等接口的方法

如果bean实现了ApplicationContextAware、EnvironmentAware等接口
参见：org.springframework.context.support.ApplicationContextAwareProcessor#invokeAwareInterfaces

调用InitializingBean接口的方法

调用BeanPostProcessor接口的postProcessAfterInitialization方法

initializeBean()

调用BeanNameAware、BeanClassLoaderAware、BeanFactoryAware、InitializingBean 等接口的方法
通过org.springframework.context.support.ApplicationContextAwareProcessor#invokeAwareInterfaces方法调用ApplicationContextAware等xxxAware接口的方法。

/**
 * Initialize the given bean instance, applying factory callbacks
 * as well as init methods and bean post processors.
 * <p>Called from {@link #createBean} for traditionally defined beans,
 * and from {@link #initializeBean} for existing bean instances.
 * @param beanName the bean name in the factory (for debugging purposes)
 * @param bean the new bean instance we may need to initialize
 * @param mbd the bean definition that the bean was created with
 * (can also be {@code null}, if given an existing bean instance)
 * @return the initialized bean instance (potentially wrapped)
 * @see BeanNameAware
 * @see BeanClassLoaderAware
 * @see BeanFactoryAware
 * @see #applyBeanPostProcessorsBeforeInitialization
 * @see #invokeInitMethods
 * @see #applyBeanPostProcessorsAfterInitialization
 */
protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
	if (System.getSecurityManager() != null) {
		AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
			invokeAwareMethods(beanName, bean);
			return null;
		}, getAccessControlContext());
	}
	else {
                // 调用实现了BeanNameAware、BeanClassLoaderAware、BeanFactoryAware的bean的接口方法
		invokeAwareMethods(beanName, bean);
	}
	Object wrappedBean = bean;
	if (mbd == null || !mbd.isSynthetic()) {
                // 初始化之前，实现了 BeanPostProcessor的bean，调用postProcessBeforeInitialization() 方法
                // ApplicationContextAware等xxxAware接口方法调用的入口，参考org.springframework.context.support.ApplicationContextAwareProcessor#invokeAwareInterfaces 
		wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
	}
	try {
                // 给bean一个机会，如果bean实现了InitializingBean接口，在赋值之后调用afterPropertiesSet()方法
		invokeInitMethods(beanName, wrappedBean, mbd);
	}
	catch (Throwable ex) {
		throw new BeanCreationException(
				(mbd != null ? mbd.getResourceDescription() : null),
				beanName, "Invocation of init method failed", ex);
	}
	if (mbd == null || !mbd.isSynthetic()) {
                // 初始化之后，实现了 BeanPostProcessor的bean，调用postProcessAfterInitialization() 方法
		wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
	}
	return wrappedBean;
}

invokeAwareMethods

private void invokeAwareMethods(final String beanName, final Object bean) {
	if (bean instanceof Aware) {
		if (bean instanceof BeanNameAware) {
			((BeanNameAware) bean).setBeanName(beanName);
		}
		if (bean instanceof BeanClassLoaderAware) {
			ClassLoader bcl = getBeanClassLoader();
			if (bcl != null) {
				((BeanClassLoaderAware) bean).setBeanClassLoader(bcl);
			}
		}
		if (bean instanceof BeanFactoryAware) {
			((BeanFactoryAware) bean).setBeanFactory(AbstractAutowireCapableBeanFactory.this);
		}
	}
}

applyBeanPostProcessorsBeforeInitialization

@Override
public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)
		throws BeansException {
	Object result = existingBean;
	for (BeanPostProcessor processor : getBeanPostProcessors()) {
		Object current = processor.postProcessBeforeInitialization(result, beanName);
		if (current == null) {
			return result;
		}
		result = current;
	}
	return result;
}

invokeAwareInterfaces

@Override
@Nullable
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
	if (!(bean instanceof EnvironmentAware || bean instanceof EmbeddedValueResolverAware ||
			bean instanceof ResourceLoaderAware || bean instanceof ApplicationEventPublisherAware ||
			bean instanceof MessageSourceAware || bean instanceof ApplicationContextAware ||
			bean instanceof ApplicationStartupAware)) {
		return bean;
	}
	AccessControlContext acc = null;
	if (System.getSecurityManager() != null) {
		acc = this.applicationContext.getBeanFactory().getAccessControlContext();
	}
	if (acc != null) {
		AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
			invokeAwareInterfaces(bean);
			return null;
		}, acc);
	}
	else {
		invokeAwareInterfaces(bean);
	}
	return bean;
}
private void invokeAwareInterfaces(Object bean) {
	if (bean instanceof EnvironmentAware) {
		((EnvironmentAware) bean).setEnvironment(this.applicationContext.getEnvironment());
	}
	if (bean instanceof EmbeddedValueResolverAware) {
		((EmbeddedValueResolverAware) bean).setEmbeddedValueResolver(this.embeddedValueResolver);
	}
	if (bean instanceof ResourceLoaderAware) {
		((ResourceLoaderAware) bean).setResourceLoader(this.applicationContext);
	}
	if (bean instanceof ApplicationEventPublisherAware) {
		((ApplicationEventPublisherAware) bean).setApplicationEventPublisher(this.applicationContext);
	}
	if (bean instanceof MessageSourceAware) {
		((MessageSourceAware) bean).setMessageSource(this.applicationContext);
	}
	if (bean instanceof ApplicationStartupAware) {
		((ApplicationStartupAware) bean).setApplicationStartup(this.applicationContext.getApplicationStartup());
	}
	if (bean instanceof ApplicationContextAware) {
		((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);
	}
}

invokeInitMethods

protected void invokeInitMethods(String beanName, final Object bean, @Nullable RootBeanDefinition mbd)
		throws Throwable {
	boolean isInitializingBean = (bean instanceof InitializingBean);
	if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
		if (logger.isTraceEnabled()) {
			logger.trace("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
		}
		if (System.getSecurityManager() != null) {
			try {
				AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {
					((InitializingBean) bean).afterPropertiesSet();
					return null;
				}, getAccessControlContext());
			}
			catch (PrivilegedActionException pae) {
				throw pae.getException();
			}
		}
		else {
			((InitializingBean) bean).afterPropertiesSet();
		}
	}
	if (mbd != null && bean.getClass() != NullBean.class) {
		String initMethodName = mbd.getInitMethodName();
		if (StringUtils.hasLength(initMethodName) &&
				!(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
				!mbd.isExternallyManagedInitMethod(initMethodName)) {
			invokeCustomInitMethod(beanName, bean, mbd);
		}
	}
}

调用DestructionAwareBeanPostProcessor的方法

protected void registerDisposableBeanIfNecessary(String beanName, Object bean, RootBeanDefinition mbd) {
	AccessControlContext acc = (System.getSecurityManager() != null ? getAccessControlContext() : null);
	if (!mbd.isPrototype() && requiresDestruction(bean, mbd)) {
		if (mbd.isSingleton()) {
			// Register a DisposableBean implementation that performs all destruction
			// work for the given bean: DestructionAwareBeanPostProcessors,
			// DisposableBean interface, custom destroy method.
			registerDisposableBean(beanName,
					new DisposableBeanAdapter(bean, beanName, mbd, getBeanPostProcessors(), acc));
		}
		else {
			// A bean with a custom scope...
			Scope scope = this.scopes.get(mbd.getScope());
			if (scope == null) {
				throw new IllegalStateException("No Scope registered for scope name '" + mbd.getScope() + "'");
			}
			scope.registerDestructionCallback(beanName,
					new DisposableBeanAdapter(bean, beanName, mbd, getBeanPostProcessors(), acc));
		}
	}
}