spring的bean加载方式-如何解决循环依赖-什么是三级缓存-源码走读

spring的bean加载方式-如何解决循环依赖-什么是三级缓存-源码走读

八月 09, 2025

spring的bean加载方式-如何解决循环依赖-什么是三级缓存-源码走读

文章的目的，通过阅读源码理清

  1. 三级缓存的作用
  2. 如何解决循环依赖
  3. 为什么需要三级缓存
  4. FactoryBean和BeanFactory是什么
  5. bean的初始化流程

创建项目

maven只需要这一个。太高版本的是并行创建bean了，而且循环依赖会报错，需要加额外配置，还是用低版本跟踪源码吧。

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<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-context</artifactId>
    <version>4.3.30.RELEASE</version>
</dependency>

主函数

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@Configuration
@ComponentScan(basePackages = "org.example.test")
public class TestApplication {

    public static void main(String[] args) {
        ConfigurableApplicationContext context = new AnnotationConfigApplicationContext(TestApplication.class);
        BeanA beanA = context.getBean("beanA", BeanA.class);
        System.out.println(beanA.getBeanB() == null);
    }
}

两个循环依赖类

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@Component
public class BeanA {
    @Autowired
    private BeanB beanB;
    public String getName() {
        return "BeanA";
    }
    public BeanB getBeanB() {return beanB;}
}

@Component
public class BeanB {
    @Autowired
    private BeanA beanA;
    public String getName() {
        return "BeanB";
    }
}

jdk最好也用低版本的，我最开始使用jdk23，根本启动不起来，换成11才可以。

三级缓存分别是什么

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private final Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>(256); // 一级，存放完全初始化完的对象

/** Cache of singleton factories: bean name --> ObjectFactory */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<String, ObjectFactory<?>>(16); // 三级，存放的是 ObjectFactory 也就是一个工厂，通过工厂的getObject可以拿到对象

/** Cache of early singleton objects: bean name --> bean instance */
private final Map<String, Object> earlySingletonObjects = new HashMap<String, Object>(16); // 二级，存放的是从工厂里面拿出来的对象

下面正式开始debug。

beanA的创建

入口函数在这里, AbstractApplicationContext.java

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public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			// Prepare this context for refreshing.
			prepareRefresh();

			// Tell the subclass to refresh the internal bean factory.
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// Prepare the bean factory for use in this context.
			prepareBeanFactory(beanFactory);

			try {
				// Allows post-processing of the bean factory in context subclasses.
				postProcessBeanFactory(beanFactory);

				// Invoke factory processors registered as beans in the context.
				invokeBeanFactoryPostProcessors(beanFactory);

				// Register bean processors that intercept bean creation.
				registerBeanPostProcessors(beanFactory);

				// Initialize message source for this context.
				initMessageSource();

				// Initialize event multicaster for this context.
				initApplicationEventMulticaster();

				// Initialize other special beans in specific context subclasses.
				onRefresh();

				// Check for listener beans and register them.
				registerListeners();

				// Instantiate all remaining (non-lazy-init) singletons.
				finishBeanFactoryInitialization(beanFactory); // 这里创建bean

				// Last step: publish corresponding event.
				finishRefresh();
			}

			catch (BeansException ex) {
				if (logger.isWarnEnabled()) {
					logger.warn("Exception encountered during context initialization - " +
							"cancelling refresh attempt: " + ex);
				}

				// Destroy already created singletons to avoid dangling resources.
				destroyBeans();

				// Reset 'active' flag.
				cancelRefresh(ex);

				// Propagate exception to caller.
				throw ex;
			}

			finally {
				// Reset common introspection caches in Spring's core, since we
				// might not ever need metadata for singleton beans anymore...
				resetCommonCaches();
			}
		}
	}

在 finishBeanFactoryInitialization 里面

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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(new StringValueResolver() {
			@Override
			public String resolveStringValue(String strVal) {
				return getEnvironment().resolvePlaceholders(strVal);
			}
		});
	}

	// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
       // 这部分是处理代理的
	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.
	beanFactory.preInstantiateSingletons(); // 这里才真正创建
}

进入 beanFactory.preInstantiateSingletons

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   public void preInstantiateSingletons() throws BeansException {
	if (logger.isDebugEnabled()) {
		logger.debug("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<String>(this.beanDefinitionNames);

	// Trigger initialization of all non-lazy singleton beans...
	for (String beanName : beanNames) {
		RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
		if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
			if (isFactoryBean(beanName)) {
				final FactoryBean<?> factory = (FactoryBean<?>) getBean(FACTORY_BEAN_PREFIX + beanName);
				boolean isEagerInit;
				if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
					isEagerInit = AccessController.doPrivileged(new PrivilegedAction<Boolean>() {
						@Override
						public Boolean run() {
							return ((SmartFactoryBean<?>) factory).isEagerInit();
						}
					}, getAccessControlContext());
				}
				else {
					isEagerInit = (factory instanceof SmartFactoryBean &&
							((SmartFactoryBean<?>) factory).isEagerInit());
				}
				if (isEagerInit) {
					getBean(beanName); 
				}
			}
			else {
				getBean(beanName); // 普通的bean都会执行到这里
			}
		}
	}

	// 如果bean实现了SmartInitializingSingleton 接口，在这里回调
	for (String beanName : beanNames) {
		Object singletonInstance = getSingleton(beanName);
		if (singletonInstance instanceof SmartInitializingSingleton) {
			final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
			if (System.getSecurityManager() != null) {
				AccessController.doPrivileged(new PrivilegedAction<Object>() {
					@Override
					public Object run() {
						smartSingleton.afterSingletonsInstantiated();
						return null;
					}
				}, getAccessControlContext());
			}
			else {
				smartSingleton.afterSingletonsInstantiated();
			}
		}
	}
}

getBean会进入doGetBean

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

doGetBean 有很长一部分

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protected <T> T doGetBean(
    final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
    throws BeansException {
        final String beanName = transformedBeanName(name);
        Object bean;

        // Eagerly check singleton cache for manually registered singletons.
        Object sharedInstance = getSingleton(beanName); // 这里会检查三级缓存，一会要说
        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 + "'");
				}
			}
			bean = getObjectForBeanInstance(sharedInstance, name, beanName, null); // 这里主要处理FactoryBean的情况，一会说
		}
        ......
        // Create bean instance.
        if (mbd.isSingleton()) { // getSingleton函数中会利用第二个参数，也就是factory.getObject得到bean，getObject在这里实现就是createBean
            sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() { // 一会要说
                @Override
                public Object getObject() throws BeansException {
                    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;
                    }
                }
            });
            // 就是上面那个函数，一会说
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
        }
        ...
        return (T) bean;
    }

先看getSingleton，如果bean 没有存在于一级缓存（singletonObjects）并且也没有正在被创建，那就说明真的没有当前bean，需要自己创建。

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protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    Object singletonObject = this.singletonObjects.get(beanName);
    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) { // 这个if不会进入，后面这个函数判断的是一个set，在bean开始创建的时候，会把beanNamne添加进去
        synchronized (this.singletonObjects) {
            singletonObject = this.earlySingletonObjects.get(beanName);
            if (singletonObject == null && allowEarlyReference) {
                ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                if (singletonFactory != null) {
                    singletonObject = singletonFactory.getObject();
                    this.earlySingletonObjects.put(beanName, singletonObject);
                    this.singletonFactories.remove(beanName);
                }
            }
        }
    }
    return (singletonObject != NULL_OBJECT ? singletonObject : null);
}

然后是 getObjectForBeanInstance，处理FactoryBean。但是这里我们没用从一级缓存中获取到值，所以这段逻辑不会执行，但是在第二节处理循环依赖的时候，一级缓存是可以获取到值的，这部分代码就会执行。

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protected Object getObjectForBeanInstance(
			Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {

		// Don't let calling code try to dereference the factory if the bean isn't a factory.
		if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
			throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
		}

		// Now we have the bean instance, which may be a normal bean or a FactoryBean.
		// If it's a FactoryBean, we use it to create a bean instance, unless the
		// caller actually wants a reference to the factory.
		if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) { // 如果不是factoryBean，这里直接返回了
			return beanInstance;
		}

		Object object = null;
		if (mbd == null) {
			object = getCachedObjectForFactoryBean(beanName);
		}
		if (object == null) {
			// Return bean instance from factory.
			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());
			object = getObjectFromFactoryBean(factory, beanName, !synthetic); // 对于FactoryBean，要通过 FactoryBean的getObject获取对象。FactoryBean是一个接口，可以通过实现这个接口，实现复杂的bean初始化过程。
		}
		return object;
	}

先进入这部分代码的getSingleton函数

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sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
    @Override
    public Object getObject() throws BeansException {
        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;
        }
    }
});
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public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
		Assert.notNull(beanName, "'beanName' 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<Exception>();
				}
				try {
					singletonObject = singletonFactory.getObject(); // 这里就是上面传的lambda表达式
					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) { // 创建成功之后要加进缓存，这个第二节会说
					addSingleton(beanName, singletonObject);
				}
			}
			return (singletonObject != NULL_OBJECT ? singletonObject : null);
		}
	}

进入 lambda表达式的 createBean

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protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
    ......
    Object beanInstance = doCreateBean(beanName, mbdToUse, args); // 这里
    if (logger.isDebugEnabled()) {
        logger.debug("Finished creating instance of bean '" + beanName + "'");
    }
    return beanInstance;
}
``
然后才是真正创建
```java
    protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
			throws BeanCreationException {

		// Instantiate the bean.
		BeanWrapper instanceWrapper = null;
		if (mbd.isSingleton()) {
			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
		}
		if (instanceWrapper == null) {
			instanceWrapper = createBeanInstance(beanName, mbd, args); // 创建bean
		}
		final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
		Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
		mbd.resolvedTargetType = beanType;

		// 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.isDebugEnabled()) {
				logger.debug("Eagerly caching bean '" + beanName +
						"' to allow for resolving potential circular references");
			}
			addSingletonFactory(beanName, new ObjectFactory<Object>() {
				@Override
				public Object getObject() throws BeansException {
					return getEarlyBeanReference(beanName, mbd, bean);
				}
			});
		}

		// Initialize the bean instance.
		Object exposedObject = bean;
		try { // 属性赋值
			populateBean(beanName, mbd, instanceWrapper);
			if (exposedObject != null) {
				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<String>(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;
	}

先进入 createBeanInstance 在这里实例化了 bean ,流程大概是先找到对应的class，然后找到对应的构造函数

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protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
		// Make sure bean class is actually resolved at this point.
		Class<?> beanClass = resolveBeanClass(mbd, beanName);

		if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
			throw new BeanCreationException(mbd.getResourceDescription(), beanName,
					"Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
		}

		if (mbd.getFactoryMethodName() != null) {
			return instantiateUsingFactoryMethod(beanName, mbd, args);
		}

		// Shortcut when re-creating the same bean...
		boolean resolved = false;
		boolean autowireNecessary = false;
		if (args == null) {
			synchronized (mbd.constructorArgumentLock) {
				if (mbd.resolvedConstructorOrFactoryMethod != null) {
					resolved = true;
					autowireNecessary = mbd.constructorArgumentsResolved;
				}
			}
		}
		if (resolved) {
			if (autowireNecessary) {
				return autowireConstructor(beanName, mbd, null, null);
			}
			else {
				return instantiateBean(beanName, mbd);
			}
		}

		// Candidate constructors for autowiring?
		Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
		if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
				mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
			return autowireConstructor(beanName, mbd, ctors, args);
		}

		// No special handling: simply use no-arg constructor.
		return instantiateBean(beanName, mbd); // 实际进入这里
	}

进入instantiateBean

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protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
    try {
        Object beanInstance;
        final BeanFactory parent = this;
        if (System.getSecurityManager() != null) {
            beanInstance = AccessController.doPrivileged(new PrivilegedAction<Object>() {
                @Override
                public Object run() {
                    return getInstantiationStrategy().instantiate(mbd, beanName, parent);
                }
            }, getAccessControlContext());
        }
        else {
            beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent); // 在这一行实例化
        }
        BeanWrapper bw = new BeanWrapperImpl(beanInstance);
        initBeanWrapper(bw);
        return bw;
    }
    catch (Throwable ex) {
        throw new BeanCreationException(
                mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
    }
}

这里会选择一个策略执行，当前会进入到SimpleInstantiationStrategy

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public Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner) {
		// Don't override the class with CGLIB if no overrides.
		if (bd.getMethodOverrides().isEmpty()) {
			Constructor<?> constructorToUse;
			synchronized (bd.constructorArgumentLock) {
				constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
				if (constructorToUse == null) {
					final Class<?> clazz = bd.getBeanClass();
					if (clazz.isInterface()) {
						throw new BeanInstantiationException(clazz, "Specified class is an interface");
					}
					try {
						if (System.getSecurityManager() != null) {
							constructorToUse = AccessController.doPrivileged(new PrivilegedExceptionAction<Constructor<?>>() {
								@Override
								public Constructor<?> run() throws Exception {
									return clazz.getDeclaredConstructor((Class[]) null);
								}
							});
						}
						else {
							constructorToUse =	clazz.getDeclaredConstructor((Class[]) null);
						}
						bd.resolvedConstructorOrFactoryMethod = constructorToUse;
					}
					catch (Throwable ex) {
						throw new BeanInstantiationException(clazz, "No default constructor found", ex);
					}
				}
			}
			return BeanUtils.instantiateClass(constructorToUse); // 里面会调用  constructor.newInstance创建实例
		}
		else {
			// Must generate CGLIB subclass.
			return instantiateWithMethodInjection(bd, beanName, owner);
		}
	}

BeanUtils.instantiateClass里面会通过newInstance创建实例
这里还没结束，当前的bean在创建之后，在doCreateBean里面有这一段

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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, new ObjectFactory<Object>() {
        @Override
        public Object getObject() throws BeansException {
            return getEarlyBeanReference(beanName, mbd, bean);
        }
    });
}

earlySingletonExposure 是true，所以需要把自己的bean放入三级缓存中。
ObjectFactory 里面的getObject调用的是getEarlyBeanReference，看一下getEarlyBeanReference函数

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protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
    Object exposedObject = bean;
    if (bean != null && !mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
        for (BeanPostProcessor bp : getBeanPostProcessors()) {
            if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
                SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
                exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName); // 这里的 SmartInstantiationAwareBeanPostProcessor.getEarlyBeanReference 有两个实现类，分别对应返回代理对象，或者直接返回 exposedObject，也就是原本对象。这里就是工厂这个缓存存在的意义，也是为什么要三级缓存的原因。因为有代理的情况，要由调用方决定要不要生成代理，而不是当前bean生成的时候，直接生成代理。
                if (exposedObject == null) {
                    return null;
                }
            }
        }
    }
    return exposedObject;
}

属性赋值

回到创建bean的函数中

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   protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
		throws BeanCreationException {

	// Instantiate the bean.
	BeanWrapper instanceWrapper = null;
	if (mbd.isSingleton()) {
		instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
	}
	if (instanceWrapper == null) {
		instanceWrapper = createBeanInstance(beanName, mbd, args);
	}
	final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
	Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
	mbd.resolvedTargetType = beanType;

	// 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.isDebugEnabled()) {
			logger.debug("Eagerly caching bean '" + beanName +
					"' to allow for resolving potential circular references");
		}
		addSingletonFactory(beanName, new ObjectFactory<Object>() {
			@Override
			public Object getObject() throws BeansException {
				return getEarlyBeanReference(beanName, mbd, bean);
			}
		});
	}

	// Initialize the bean instance.
	Object exposedObject = bean;
	try { // 属性赋值
		populateBean(beanName, mbd, instanceWrapper);
		if (exposedObject != null) {
			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<String>(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;
}

populateBean 函数进行属性赋值。现在我们的类是A依赖B，B依赖A。预期情况是，当前是A类，要去把B赋值，但是B还没有实例化，所以应该要先实例化B。

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   protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {
	PropertyValues pvs = mbd.getPropertyValues();

	if (bw == null) {
		if (!pvs.isEmpty()) {
			throw new BeanCreationException(
					mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
		}
		else {
			// Skip property population phase for null instance.
			return;
		}
	}

	// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
	// state of the bean before properties are set. This can be used, for example,
	// to support styles of field injection.
	if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
		for (BeanPostProcessor bp : getBeanPostProcessors()) {
			if (bp instanceof InstantiationAwareBeanPostProcessor) {
				InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
				if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
					return;
				}
			}
		}
	}

	int resolvedAutowireMode = mbd.getResolvedAutowireMode();
	if (resolvedAutowireMode == AUTOWIRE_BY_NAME || resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
		MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
		// Add property values based on autowire by name if applicable.
		if (resolvedAutowireMode == AUTOWIRE_BY_NAME) {
			autowireByName(beanName, mbd, bw, newPvs);
		}
		// Add property values based on autowire by type if applicable.
		if (resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
			autowireByType(beanName, mbd, bw, newPvs);
		}
		pvs = newPvs;
	}

	boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
	boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);

	if (hasInstAwareBpps || needsDepCheck) {
		PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
		if (hasInstAwareBpps) {
			for (BeanPostProcessor bp : getBeanPostProcessors()) { // 这里会进行属性赋值，for里面有很多，但是 AutowiredAnnotationBeanPostProcessor 才是赋值b的
				if (bp instanceof InstantiationAwareBeanPostProcessor) {
					InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
					pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
					if (pvs == null) {
						return;
					}
				}
			}
		}
		if (needsDepCheck) {
			checkDependencies(beanName, mbd, filteredPds, pvs);
		}
	}

	applyPropertyValues(beanName, mbd, bw, pvs);
}

postProcessPropertyValues 函数点进去是 AutowiredAnnotationBeanPostProcessor 的函数

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public PropertyValues postProcessPropertyValues(
        PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeanCreationException {

    InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
    try {
        metadata.inject(bean, beanName, pvs); //注入
    }
    catch (BeanCreationException ex) {
        throw ex;
    }
    catch (Throwable ex) {
        throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
    }
    return pvs;
}

进入inject

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public void inject(Object target, String beanName, PropertyValues pvs) throws Throwable {
    Collection<InjectedElement> elementsToIterate =
            (this.checkedElements != null ? this.checkedElements : this.injectedElements);
    if (!elementsToIterate.isEmpty()) { // elementsToIterate 里面只有一个元素，因为只写了一个autowired
        for (InjectedElement element : elementsToIterate) {
            if (logger.isDebugEnabled()) {
                logger.debug("Processing injected element of bean '" + beanName + "': " + element);
            }
            element.inject(target, beanName, pvs);
        }
    }
}

进入 element.inject

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 protected void inject(Object bean, String beanName, PropertyValues pvs) throws Throwable {
Field field = (Field) this.member;
Object value;
if (this.cached) { // 第一次创建，没有cached
	value = resolvedCachedArgument(beanName, this.cachedFieldValue);
}
else {
	DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
	desc.setContainingClass(bean.getClass());
	Set<String> autowiredBeanNames = new LinkedHashSet<String>(1);
	TypeConverter typeConverter = beanFactory.getTypeConverter();
	try {
		value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter); // 创建bean
	}
	catch (BeansException ex) {
		throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);
	}
	synchronized (this) {
		if (!this.cached) {
			if (value != null || this.required) {
				this.cachedFieldValue = desc;
				registerDependentBeans(beanName, autowiredBeanNames);
				if (autowiredBeanNames.size() == 1) {
					String autowiredBeanName = autowiredBeanNames.iterator().next();
					if (beanFactory.containsBean(autowiredBeanName) &&
							beanFactory.isTypeMatch(autowiredBeanName, field.getType())) {
						this.cachedFieldValue = new ShortcutDependencyDescriptor(
								desc, autowiredBeanName, field.getType());
					}
				}
			}
			else {
				this.cachedFieldValue = null;
			}
			this.cached = true;
		}
	}
}
if (value != null) {
	ReflectionUtils.makeAccessible(field);
	field.set(bean, value); // 这里进行属性赋值
}
 }

resolveDependency 里面只进入最后一个else

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   public Object resolveDependency(DependencyDescriptor descriptor, String requestingBeanName,
		Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException {

	descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());
	if (javaUtilOptionalClass == descriptor.getDependencyType()) {
		return new OptionalDependencyFactory().createOptionalDependency(descriptor, requestingBeanName);
	}
	else if (ObjectFactory.class == descriptor.getDependencyType() ||
			ObjectProvider.class == descriptor.getDependencyType()) {
		return new DependencyObjectProvider(descriptor, requestingBeanName);
	}
	else if (javaxInjectProviderClass == descriptor.getDependencyType()) {
		return new Jsr330ProviderFactory().createDependencyProvider(descriptor, requestingBeanName);
	}
	else {
		Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(
				descriptor, requestingBeanName); // 判断是不是lazy加载
		if (result == null) {
			result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter); // 进入 doResolveDependency
		}
		return result;
	}
}

这里面先判断了对应的属性是不是lazy加载，如果是lazy的话返回代理类

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public Object getLazyResolutionProxyIfNecessary(DependencyDescriptor descriptor, String beanName) {
		return (isLazy(descriptor) ? buildLazyResolutionProxy(descriptor, beanName) : null);
	}

不是lazy，进入 doResolveDependency

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   public Object doResolveDependency(DependencyDescriptor descriptor, String beanName,
		Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException {

	InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
	try { // 这下面很多if都不会进入，直接看最后一个else
		Object shortcut = descriptor.resolveShortcut(this);
		if (shortcut != null) {
			return shortcut;
		}

		Class<?> type = descriptor.getDependencyType();
		Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
		if (value != null) {
			if (value instanceof String) {
				String strVal = resolveEmbeddedValue((String) value);
				BeanDefinition bd = (beanName != null && containsBean(beanName) ? getMergedBeanDefinition(beanName) : null);
				value = evaluateBeanDefinitionString(strVal, bd);
			}
			TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
			return (descriptor.getField() != null ?
					converter.convertIfNecessary(value, type, descriptor.getField()) :
					converter.convertIfNecessary(value, type, descriptor.getMethodParameter()));
		}

		Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);
		if (multipleBeans != null) {
			return multipleBeans;
		}

		Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
		if (matchingBeans.isEmpty()) {
			if (isRequired(descriptor)) {
				raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
			}
			return null;
		}

		String autowiredBeanName;
		Object instanceCandidate;

		if (matchingBeans.size() > 1) {
			autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);
			if (autowiredBeanName == null) {
				if (isRequired(descriptor) || !indicatesMultipleBeans(type)) {
					return descriptor.resolveNotUnique(type, matchingBeans);
				}
				else {
					// In case of an optional Collection/Map, silently ignore a non-unique case:
					// possibly it was meant to be an empty collection of multiple regular beans
					// (before 4.3 in particular when we didn't even look for collection beans).
					return null;
				}
			}
			instanceCandidate = matchingBeans.get(autowiredBeanName);
		}
		else {
			// We have exactly one match.
			Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();
			autowiredBeanName = entry.getKey(); // bean的名字，此示例是beanB
			instanceCandidate = entry.getValue(); // bean的类型
		}

		if (autowiredBeanNames != null) {
			autowiredBeanNames.add(autowiredBeanName);
		}
		return (instanceCandidate instanceof Class ?
				descriptor.resolveCandidate(autowiredBeanName, type, this) : instanceCandidate); // 进入 resolveCandidate
	}
	finally {
		ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);
	}
}

resolveCandidate直接调用factory的getBean

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   public Object resolveCandidate(String beanName, Class<?> requiredType, BeanFactory beanFactory)
		throws BeansException {

	return beanFactory.getBean(beanName, requiredType);
}

当前是AbstractBeanFactory的getBean，所以会调用doGetBean

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public <T> T getBean(String name, Class<T> requiredType) throws BeansException {
    return doGetBean(name, requiredType, null, false);
}

doGetBean基本逻辑和第一节相同。

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protected <T> T doGetBean(
    final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
    throws BeansException {
        final String beanName = transformedBeanName(name);
        Object bean;

        // Eagerly check singleton cache for manually registered singletons.
        Object sharedInstance = getSingleton(beanName); // 这里会检查三级缓存
        ...
        // Create bean instance.
        if (mbd.isSingleton()) { // getSingleton函数中会利用第二个参数，也就是factory.getObject得到bean，getObject在这里实现就是createBean
            sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
                @Override
                public Object getObject() throws BeansException {
                    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);
        }
        ...
        return (T) bean;
    }

getSingleton会调用getObject，getObject调用createBean
然后进入doCreateBean，完成实例化，这部分和之前一样，跳过去了。
注意这部分，现在工厂缓存里面有两个工厂了，分别是beanA和beanB

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addSingletonFactory(beanName, new ObjectFactory<Object>() {
    @Override
    public Object getObject() throws BeansException {
        return getEarlyBeanReference(beanName, mbd, bean);
    }
});

然后b也要完成属性赋值。

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populateBean(beanName, mbd, instanceWrapper);

属性赋值中执行到这里，也是之前的代码

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if (hasInstAwareBpps || needsDepCheck) {
    PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
    if (hasInstAwareBpps) {
        for (BeanPostProcessor bp : getBeanPostProcessors()) {
            if (bp instanceof InstantiationAwareBeanPostProcessor) {
                InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName); // 在Autowired进入这里
                if (pvs == null) {
                    return;
                }
            }
        }
    }
    if (needsDepCheck) {
        checkDependencies(beanName, mbd, filteredPds, pvs);
    }
}

然后分别进入

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metadata.inject(bean, beanName, pvs);

element.inject(target, beanName, pvs);

value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);

result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);

descriptor.resolveCandidate(autowiredBeanName, type, this);

beanFactory.getBean(beanName, requiredType);

doGetBean(name, requiredType, null, false);

当前是BeanB，这里doGetBean就是要获取beanA了

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protected <T> T doGetBean(
			final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
			throws BeansException {

		final String beanName = transformedBeanName(name);
		Object bean;

		// Eagerly check singleton cache for manually registered singletons.
		Object sharedInstance = getSingleton(beanName); // 这里可以获取到beanA
        ......
            }

getSingleton现在可以获取到A了

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protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    Object singletonObject = this.singletonObjects.get(beanName);
    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) { // 一级缓存没有，但是现在在创建A
        synchronized (this.singletonObjects) {
            singletonObject = this.earlySingletonObjects.get(beanName); // 二级缓存里面没有
            if (singletonObject == null && allowEarlyReference) {
                ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName); // 从三级缓存里面拿工厂
                if (singletonFactory != null) { // 通过工厂的getObject获取值，这里的工厂就是在第一节末尾放入的，getObject 通过 getEarlyBeanReference 获取对象
                    singletonObject = singletonFactory.getObject();
                    this.earlySingletonObjects.put(beanName, singletonObject); // 放入二级缓存
                    this.singletonFactories.remove(beanName); // 移除三级缓存的工厂，下次所有要获取这个对象的，都不用工厂了，直接从二级缓存拿
                }
            }
        }
    }
    return (singletonObject != NULL_OBJECT ? singletonObject : null);
}

再次回到 doCreateBean

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try {
    populateBean(beanName, mbd, instanceWrapper); // 属性赋值结束
    if (exposedObject != null) {
        exposedObject = initializeBean(beanName, exposedObject, mbd);
    }
}

进入初始化阶段，这里面会执行 postProcessBeforeInitialization 回调函数、 afterPropertiesSet 回调函数，init-method函数， postProcessAfterInitialization 回调

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protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) {
    if (System.getSecurityManager() != null) {
        AccessController.doPrivileged(new PrivilegedAction<Object>() {
            @Override
            public Object run() {
                invokeAwareMethods(beanName, bean);
                return null;
            }
        }, getAccessControlContext());
    }
    else {
        invokeAwareMethods(beanName, bean);
    }

    Object wrappedBean = bean;
    if (mbd == null || !mbd.isSynthetic()) {
        wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
    }

    try {
        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()) {
        wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
    }
    return wrappedBean;
}

然后b就创建完了，再逐渐返回，先返回到这里，这里的上一层是，我们通过这里的lambda里面的createBean创建完成了beanB

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sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
    @Override
    public Object getObject() throws BeansException {
        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;
        }
    }
});

getSingleton末尾有一段函数，addSingleton

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public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
		Assert.notNull(beanName, "'beanName' 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<Exception>();
				}
				try {
					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 != NULL_OBJECT ? singletonObject : null);
		}
	}
protected void addSingleton(String beanName, Object singletonObject) {
		synchronized (this.singletonObjects) {
			this.singletonObjects.put(beanName, (singletonObject != null ? singletonObject : NULL_OBJECT));
			this.singletonFactories.remove(beanName);
			this.earlySingletonObjects.remove(beanName);
			this.registeredSingletons.add(beanName);
		}
	}

addSingleton 会把beanB加入到一级缓存，把二级缓存和三级缓存清空，beanB本来也没在二级缓存，只有beanA在二级缓存。beanA自然会在创建结束之后，再次调用这个函数，把beanA从二级缓存清空。

最后一步

一直回退到如下

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public void preInstantiateSingletons() throws BeansException {
    if (logger.isDebugEnabled()) {
        logger.debug("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<String>(this.beanDefinitionNames);

    // Trigger initialization of all non-lazy singleton beans...
    for (String beanName : beanNames) {
        RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
        if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
            if (isFactoryBean(beanName)) {
                final FactoryBean<?> factory = (FactoryBean<?>) getBean(FACTORY_BEAN_PREFIX + beanName);
                boolean isEagerInit;
                if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                    isEagerInit = AccessController.doPrivileged(new PrivilegedAction<Boolean>() {
                        @Override
                        public Boolean run() {
                            return ((SmartFactoryBean<?>) factory).isEagerInit();
                        }
                    }, getAccessControlContext());
                }
                else {
                    isEagerInit = (factory instanceof SmartFactoryBean &&
                            ((SmartFactoryBean<?>) factory).isEagerInit());
                }
                if (isEagerInit) {
                    getBean(beanName);
                }
            }
            else {
                getBean(beanName); // 我们之前在这里执行了bean的实例化和属性赋值
            }
        }
    }

    // Trigger post-initialization callback for all applicable beans...
    for (String beanName : beanNames) { // 这部分用来处理 SmartInitializingSingleton 接口
        Object singletonInstance = getSingleton(beanName);
        if (singletonInstance instanceof SmartInitializingSingleton) {
            final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
            if (System.getSecurityManager() != null) {
                AccessController.doPrivileged(new PrivilegedAction<Object>() {
                    @Override
                    public Object run() {
                        smartSingleton.afterSingletonsInstantiated();
                        return null;
                    }
                }, getAccessControlContext());
            }
            else {
                smartSingleton.afterSingletonsInstantiated();
            }
        }
    }
}

处理 SmartInitializingSingleton 的回调，这部分回调的执行是在所有的bean属性赋值成功之后的。比如 EnableScheduling 会调用这个实现 ScheduledAnnotationBeanPostProcessor。

文字总结-面试版

在preInstantiateSingletons函数里面通过getBean初始化单例的bean，getBean会调用doCreateBean
第一步是实例化，通过拿到bean的构造函数，调用newInstance获取bean实例，这一步是用BeanUtils.instantiateClass传递一个Constructor类型。
然后A的过程还没完，还要把自己的bean放到三级缓存里面，也就是工厂，工厂里面可以拿到当前这个bean或者这个bean的代理。
然后是属性赋值。在postProcessPropertyValues函数里面调用metadata.inject注入属性b。然后属性赋值的时候，检测是不是Lazy的，如果是Lazy的需要对属性先创建一个代理出来，而不是直接创建对象。假设我的A依赖B。
然后去一级缓存查b，查不到并且这个bean没有正在被创建，就开始创建b。进入b的doGetBean。b实例化之后，也是把自己放入三级缓存。然后属性赋值，在赋值A的时候，先去一级缓存中查找，发现一级缓存没有，并且这个bean在被创建，那就去二级缓存获取，二级缓存也没有，就去三级缓存把工厂拿出来，通过工厂的getObject方法获取A对象。这个时候再决定是获取代理还是原始对象。获取之后就把对象放入二级缓存，删掉三级缓存的工厂，之后所有要获取这个对象的人，都从二级缓存获取。
然后就属性赋值完成，可以执行初始化逻辑，就是postProcessBeforeInitialization、afterPropertiesSet回调，init-method函数，postProcessAfterInitialization回调。
然后A也可以从一级缓存拿到B的bean，A也初始化完成。
最终再把二级和三级缓存删除，把bean放到一级缓存，就大功告成了。