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Spring依赖注入和循环依赖的源码分析

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简介

循环依赖并不是一个好的设置，现实代码中很多离奇问题都由循环依赖导致（如死锁等）,但是Spring是如何解决这部分问题的呢？答案是利用了Java的特性，同一个对象的应用指向的是同一个类，这样就能在类还在实例化阶段，还没对值进行赋值阶段就创建一个早期实例化对象，依赖对象去持有这个临时对象的引用，在依赖对象完全初始化后，由于持有对象是拥有者依赖对象的同一个引用，这样就能完成循环依赖的操作，现实中循环依赖并不是一个好的模式，我也会在现实中去避免去使用，就如之前介绍的六边形架构一样，好的架构要层次分明，循环依赖会导致系统的复杂化和系统架构的边界的不清晰，从而导致大泥球式的代码。

源码分析
入口（AbstractApplicationContext）

在分析源码的时候如果能找到程序入口分析起来会简单很多，我们知道Spring启动时调用了容器的refresh方法于是我们进入查看

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@Override
	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);

				// 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();
			}
		}
	}

我们要查找的是处理依赖注入的逻辑，根据Spring源码的注释我们看到finishBeanFactoryInitialization(beanFactory);方法上面的注释写着初始化所有单例对象，那么八九不离十我们查看的代码就在里面。

继续进入方法finishBeanFactoryInitialization其是由AbstractApplicationContext去实现的

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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(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.
		beanFactory.preInstantiateSingletons();
	}
初始化 DefaultListableBeanFactory

同样根据代码注释进入beanFactory.preInstantiateSingletons();方法，其只有DefaultListableBeanFactory类实现了这个方法

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@Override
	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.
		//获取BeanDefinition中所有扫描到的Bean对象
		List<String> beanNames = new ArrayList<>(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)) {
					Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
					if (bean instanceof FactoryBean) {
						final 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 {
				//非factoryBean对象的入口
					getBean(beanName);
				}
			}
		}

		// Trigger post-initialization callback for all applicable beans...
		for (String beanName : beanNames) {
			Object singletonInstance = getSingleton(beanName);
			if (singletonInstance instanceof SmartInitializingSingleton) {
				final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
				if (System.getSecurityManager() != null) {
					AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
						smartSingleton.afterSingletonsInstantiated();
						return null;
					}, getAccessControlContext());
				}
				else {
					smartSingleton.afterSingletonsInstantiated();
				}
			}
		}
	}

这里我们可以看见这边的逻辑，从Spring中遍历所有已经或者扫描到的Bean对象，然后遍历判断是不是FactoryBean，如果不是的话调用

getBean(beanName);方法

AbstractBeanFactory获取bean对象

根据上文的线索进入DefaultListableBeanFactory 的getBean(beanName);方法其是由AbstractBeanFactory去实现的，其又调用了doGetBean方法

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@Override
	public Object getBean(String name) throws BeansException {
		return doGetBean(name, null, null, false);
	}
DefaultSingletonBeanRegistry循环依赖和三重缓存

doGetBean是一段很长的代码，且不好懂，在我进行debug后重点锁定以下重点方法

在doGetBean的入口起始就调用了该方法，该方法如下

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protected Object getSingleton(String beanName, boolean allowEarlyReference) {
		//先从singletonObjects对象中获取该对象是否已经实例化，singletonObjects为实例化对象的缓存，所有完成实例化的单例对象         //都会放在这里
		Object singletonObject = this.singletonObjects.get(beanName);
		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
			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;
	}

我们重点关注以下一下对象属性

  • singletonObjects
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private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

其是一个Map，所有已经初始化过的对象都会放在这个Map中

  • singletonsCurrentlyInCreation

    其是一个Set

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    private final Set<String> singletonsCurrentlyInCreation =
    			Collections.newSetFromMap(new ConcurrentHashMap<>(16));

在isSingletonCurrentlyInCreation(beanName)方法中操作了该对象

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public boolean isSingletonCurrentlyInCreation(String beanName) {
		return this.singletonsCurrentlyInCreation.contains(beanName);
	}

其是保存正在初始化的对象，为了解决循环依赖的问题，其处理的逻辑就是当对象正在初始化时会往里面插入数据，代表其未完全初始化，如果出现循环依赖，依赖的对象去这个Set中查询其依赖的对象是否包含还未初始化，如果还未初始化走循环依赖的逻辑

  • earlySingletonObjects
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private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);

其是一个Map，在出现循环依赖的情况下

  • singletonFactories
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private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

其是一个Map它的作用于是用循环依赖的核心，当Spring中发现singletonsCurrentlyInCreation中发现查找的Bean对象正在创建的话，那么就会生成一个FactoryBean来延迟加载Bean对象，再后续中通过FactoryBean来获取对象，达到获取未完全初始化的依赖Bean对象

从代码中这边有3个if，所以网上说这是解决循环依赖的3重缓存，那singletonFactories,earlySingletonObjects,singletonsCurrentlyInCreation又是在哪里被赋值的呢，我们后面接着看

AbstractBeanFactory 对象的创建

调用完DefaultSingletonBeanRegistry 的三重缓存方法后接着回到AbstractBeanFactory类的doGetBean方法，在该方法下找到以下代码 如果Spring对象是单例模式的就执行这个if逻辑

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// Create bean instance.
				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;
						}
					});
					bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
				}
DefaultSingletonBeanRegistry 对象初始化前制和后置工作

这里我们重点看getSingleton方法

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public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
		Assert.notNull(beanName, "Bean name must not be null");
		synchronized (this.singletonObjects) {
		    //从缓存中查询对象是否已经被初始化
			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 + "'");
				}
				//标记singletonsCurrentlyInCreation，表明对象正在初始化
				beforeSingletonCreation(beanName);
				boolean newSingleton = false;
				boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
				if (recordSuppressedExceptions) {
					this.suppressedExceptions = new LinkedHashSet<>();
				}
				try {
					//从factoryBean中延迟获取对象
					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);
				}
				//如果是新创建的对象
				if (newSingleton) {
					addSingleton(beanName, singletonObject);
				}
			}
			return singletonObject;
		}
	}

主要的逻辑我已经在上面代码中注释出来了我们重点看下一下方法

  • beforeSingletonCreation

    我们很清楚的看到这里执行了singletonsCurrentlyInCreation.add(beanName)方法将正在初始化的对象存入上文说的缓存，代表该对象正常初始化来应对循环依赖

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    protected void beforeSingletonCreation(String beanName) {
    		if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
    			throw new BeanCurrentlyInCreationException(beanName);
    		}
    	}

  • addSingleton这里对应的是对象初始化完成的操作

    singletonObjects,singletonFactories,earlySingletonObjects分别被清空

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protected void addSingleton(String beanName, Object singletonObject) {
		synchronized (this.singletonObjects) {
		    //初始化完成，将对象存储单例对象缓存
			this.singletonObjects.put(beanName, singletonObject);
			//将对象factory移除，因为已经完成初始化，循环依赖中不需要再去创建未完全初始化的应用
			this.singletonFactories.remove(beanName);
			//删除对象创建早期缓存
			this.earlySingletonObjects.remove(beanName);
			this.registeredSingletons.add(beanName);
		}
	}
  • 代码 singletonFactory.getObject()

主要初始化逻辑是由factoryBean这个对象加载而来的，那么我们要看到主要初始化逻辑就在这里

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//从factoryBean中延迟获取对象
					singletonObject = singletonFactory.getObject();
AbstractAutowireCapableBeanFactory 对象的实例化

查看完DefaultSingletonBeanRegistry->getSingleton的方法我们查看他的初始化bean对象的函数表达式，来到了 AbstractAutowireCapableBeanFactory

于是我们进入传入这个对方法的函数表达式参数createBean(beanName, mbd, args)的代码逻辑,其有且只有被AbstractAutowireCapableBeanFactory实现

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@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.
		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 {
			// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
			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 {
			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);
		}
	}

重点看下Object beanInstance = doCreateBean(beanName, mbdToUse, args);其是具体初始化逻辑的代码

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protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable 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.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 {
					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.
		//判断注入对象是否已经正在创建，实际就是判断是否循环依赖
		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 {
			populateBean(beanName, mbd, instanceWrapper);
			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 " +
								"'getBeanNamesOfType' 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;
	}

上面代码我写了注释其中重点看以下代码,这里是初始化对象的地方

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protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable 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());
		}

		Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
		if (instanceSupplier != null) {
			return obtainFromSupplier(instanceSupplier, beanName);
		}

		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?
		//如果有参数的构造则调用Spring的自动注入机制去寻找Bean依赖，如果找不到则报错
		Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
		if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
				mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
			return autowireConstructor(beanName, mbd, ctors, args);
		}

		// Preferred constructors for default construction?
		ctors = mbd.getPreferredConstructors();
		if (ctors != null) {
			return autowireConstructor(beanName, mbd, ctors, null);
		}

		// No special handling: simply use no-arg constructor.
		//如果没有构造方法
		return instantiateBean(beanName, mbd);
	}

我们进入instantiateBean方法，它是默认只有无参的初始化方法

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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((PrivilegedAction<Object>) () ->
						getInstantiationStrategy().instantiate(mbd, beanName, parent),
						getAccessControlContext());
			}
			else {
				beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
			}
			BeanWrapper bw = new BeanWrapperImpl(beanInstance);
			initBeanWrapper(bw);
			return bw;
		}
		catch (Throwable ex) {
			throw new BeanCreationException(
					mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
		}
	}

我们再继续看

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beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
SimpleInstantiationStrategy 底层实例化具体代码

上文调用了SimpleInstantiationStrategy的instantiate方法来进行初始化，那么看到这里就很清楚了这里就是普通的反射，我们我们这里很清楚的看到我们得到了一个未初始化的实例对象

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@Override
	public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
		// Don't override the class with CGLIB if no overrides.
		if (!bd.hasMethodOverrides()) {
			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(
									(PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);
						}
						else {
						   //获取构造方法
							constructorToUse = clazz.getDeclaredConstructor();
						}
						bd.resolvedConstructorOrFactoryMethod = constructorToUse;
					}
					catch (Throwable ex) {
						throw new BeanInstantiationException(clazz, "No default constructor found", ex);
					}
				}
			}
			return BeanUtils.instantiateClass(constructorToUse);
		}
		else {
			// Must generate CGLIB subclass.
			return instantiateWithMethodInjection(bd, beanName, owner);
		}
	}
AbstractAutowireCapableBeanFactory 对象的初始化赋值

上文我们已经完成了对象的实例化，我们接着往下看AbstractAutowireCapableBeanFactory下的doCreateBean方法的以下代码，

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// Eagerly cache singletons to be able to resolve circular references
		// even when triggered by lifecycle interfaces like BeanFactoryAware.
		//判断singletonsCurrentlyInCreation是否有值，代表是否是循环依赖进来查找对象的
		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));
		}

进来第一步如上面的注释，判断是否是循环依赖进来的，如果是的话执行addSingletonFactory方法

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protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
		Assert.notNull(singletonFactory, "Singleton factory must not be null");
		synchronized (this.singletonObjects) {
			if (!this.singletonObjects.containsKey(beanName)) {
			    //单例工程把正在循环依赖的对象放入缓存
				this.singletonFactories.put(beanName, singletonFactory);
				this.earlySingletonObjects.remove(beanName);
				this.registeredSingletons.add(beanName);
			}
		}
	}

接下来执行populateBean(beanName, mbd, instanceWrapper);，里面包含一段代码，是注入Bean的属性逻辑，其原理就是遍历BeanPostProcessor的实现类InstantiationAwareBeanPostProcessor,调用postProcessProperties完成依赖注入

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//@Autowired 依赖注入代码在这部分实现
for (BeanPostProcessor bp : getBeanPostProcessors()) {
				if (bp instanceof InstantiationAwareBeanPostProcessor) {
					InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
					PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
					if (pvsToUse == null) {
						if (filteredPds == null) {
							filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
						}
						pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
						if (pvsToUse == null) {
							return;
						}
					}
					pvs = pvsToUse;
				}
			}
AutowiredAnnotationBeanPostProcessor 完成@Autowired 标注属性元数据查找

接上面,依赖注入其执行了postProcessProperties代码，其中findAutowiringMetadata的作用是查询依赖注入的元数据，如会去查询对象的@Autowired 和@Value注解

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public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
		//查询依赖注入的元数据，如会去查询对象的@Autowired 和@Value注解
		InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
		try {
			metadata.inject(bean, beanName, pvs);
		}
		catch (BeanCreationException ex) {
			throw ex;
		}
		catch (Throwable ex) {
			throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
		}
		return pvs;
	}

继续进入findAutowiringMetadata

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private InjectionMetadata findAutowiringMetadata(String beanName, Class<?> clazz, @Nullable PropertyValues pvs) {
		// Fall back to class name as cache key, for backwards compatibility with custom callers.
		String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());
		// Quick check on the concurrent map first, with minimal locking.
		InjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);
		if (InjectionMetadata.needsRefresh(metadata, clazz)) {
			synchronized (this.injectionMetadataCache) {
				metadata = this.injectionMetadataCache.get(cacheKey);
				if (InjectionMetadata.needsRefresh(metadata, clazz)) {
					if (metadata != null) {
						metadata.clear(pvs);
					}
					//构建依赖元数据的方法
					metadata = buildAutowiringMetadata(clazz);
					this.injectionMetadataCache.put(cacheKey, metadata);
				}
			}
		}
		return metadata;
	}

继续进入buildAutowiringMetadata(clazz);

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private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
		List<InjectionMetadata.InjectedElement> elements = new ArrayList<>();
		Class<?> targetClass = clazz;

		do {
			final List<InjectionMetadata.InjectedElement> currElements = new ArrayList<>();
			//查询字段注入的元数据
			ReflectionUtils.doWithLocalFields(targetClass, field -> {
				AnnotationAttributes ann = findAutowiredAnnotation(field);
				if (ann != null) {
					if (Modifier.isStatic(field.getModifiers())) {
						if (logger.isInfoEnabled()) {
							logger.info("Autowired annotation is not supported on static fields: " + field);
						}
						return;
					}
					boolean required = determineRequiredStatus(ann);
					currElements.add(new AutowiredFieldElement(field, required));
				}
			});
			//查询方法注入的元数据
			ReflectionUtils.doWithLocalMethods(targetClass, method -> {
				Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);
				if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
					return;
				}
				AnnotationAttributes ann = findAutowiredAnnotation(bridgedMethod);
				if (ann != null && method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {
					if (Modifier.isStatic(method.getModifiers())) {
						if (logger.isInfoEnabled()) {
							logger.info("Autowired annotation is not supported on static methods: " + method);
						}
						return;
					}
					if (method.getParameterCount() == 0) {
						if (logger.isInfoEnabled()) {
							logger.info("Autowired annotation should only be used on methods with parameters: " +
									method);
						}
					}
					boolean required = determineRequiredStatus(ann);
					PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);
					currElements.add(new AutowiredMethodElement(method, required, pd));
				}
			});

			elements.addAll(0, currElements);
			targetClass = targetClass.getSuperclass();
		}
		while (targetClass != null && targetClass != Object.class);

		return new InjectionMetadata(clazz, elements);
	}

我们重点看下字段注入的这段代码

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//查询字段注入的元数据
			ReflectionUtils.doWithLocalFields(targetClass, field -> {
				AnnotationAttributes ann = findAutowiredAnnotation(field);
				if (ann != null) {
					if (Modifier.isStatic(field.getModifiers())) {
						if (logger.isInfoEnabled()) {
							logger.info("Autowired annotation is not supported on static fields: " + field);
						}
						return;
					}
					boolean required = determineRequiredStatus(ann);
					currElements.add(new AutowiredFieldElement(field, required));
				}
			});

进入doWithLocalFields方法，这里我们就看得很清楚了，它查询到了我要实例化的所有字段，然后再用FieldCallback去过滤需要注入的字段

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public static void doWithLocalFields(Class<?> clazz, FieldCallback fc) {
       //遍历需要类的所有字段，调用方法表达式的方法
		for (Field field : getDeclaredFields(clazz)) {
			try {
				fc.doWith(field);
			}
			catch (IllegalAccessException ex) {
				throw new IllegalStateException("Not allowed to access field '" + field.getName() + "': " + ex);
			}
		}
	}

生成完依赖注入元数据后,代码执行回到postProcessProperties,又执行了InjectionMetadata的inject方法

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@Override
	public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
		InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
		try {
			metadata.inject(bean, beanName, pvs);
		}
		catch (BeanCreationException ex) {
			throw ex;
		}
		catch (Throwable ex) {
			throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
		}
		return pvs;
	}
InjectionMetadata 执行依赖注入
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public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
		Collection<InjectedElement> checkedElements = this.checkedElements;
		Collection<InjectedElement> elementsToIterate =
				(checkedElements != null ? checkedElements : this.injectedElements);
		if (!elementsToIterate.isEmpty()) {
		//遍历Bean对象中可注入的属性和方法
			for (InjectedElement element : elementsToIterate) {
				if (logger.isTraceEnabled()) {
					logger.trace("Processing injected element of bean '" + beanName + "': " + element);
				}
				element.inject(target, beanName, pvs);
			}
		}
	}
InjectedElement 依赖注入真正执行的地方

由于我这边的代码是属性注入所以跟踪到它的实现类AutowiredFieldElement

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protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {			 //获得需要注入的字段
			Field field = (Field) this.member;
			Object value;
			if (this.cached) {
				value = resolvedCachedArgument(beanName, this.cachedFieldValue);
			}
			else {
				DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
				desc.setContainingClass(bean.getClass());
				Set<String> autowiredBeanNames = new LinkedHashSet<>(1);
				Assert.state(beanFactory != null, "No BeanFactory available");
				TypeConverter typeConverter = beanFactory.getTypeConverter();
				try {
				 	//从BeanFactory中查找依赖属性
					value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
				}
				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);
			}
		}

其中以下代码这里又回到了beanFactory，说明本质还是依靠BeanFactory来完成依赖注入

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//从BeanFactory中查找依赖属性
value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames,
DefaultListableBeanFactory 依赖查找

看了一圈代码发现我们已经兜回了原点回到了DefaultListableBeanFactory

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

		descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());
		if (Optional.class == descriptor.getDependencyType()) {
			return 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 Jsr330Factory().createDependencyProvider(descriptor, requestingBeanName);
		}
		else {
			Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(
					descriptor, requestingBeanName);
			if (result == null) {
				result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
			}
			return result;
		}
	}

我们看下代码根据我们debug的代码我们查找的类明显属于最后一个else的内容且非懒加载所以进入doResolveDependency

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

		InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
		try {
			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(descriptor.getResolvableType(), 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();
				instanceCandidate = entry.getValue();
			}

			if (autowiredBeanNames != null) {
				autowiredBeanNames.add(autowiredBeanName);
			}
			if (instanceCandidate instanceof Class) {
				instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
			}
			Object result = instanceCandidate;
			if (result instanceof NullBean) {
				if (isRequired(descriptor)) {
					raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
				}
				result = null;
			}
			if (!ClassUtils.isAssignableValue(type, result)) {
				throw new BeanNotOfRequiredTypeException(autowiredBeanName, type, instanceCandidate.getClass());
			}
			return result;
		}
		finally {
			ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);
		}
	}

我们再次debug，我们查找的类型属于class所以进入以下代码

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if (instanceCandidate instanceof Class) {
				instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
			}

再次进入,发现我们现在已经彻底回到原点

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

		return beanFactory.getBean(beanName);
	}

再次进入顺着先前的逻辑到

getSingleton：跳转指定位置

由于我们之前已经设置过singletonFactories值 跳转指定位置

所以 singletonFactory就可以直接获取到值，而值为上面方法所初步实例化没有赋值的对象的对象,跳转指定位置

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protected Object getSingleton(String beanName, boolean allowEarlyReference) {
		//先从singletonObjects对象中获取该对象是否已经实例化，singletonObjects为实例化对象的缓存，所有完成实例化的单例对象         //都会放在这里
		Object singletonObject = this.singletonObjects.get(beanName);
		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
			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;
	}

到此循环依赖的过程就结束了。

总结

假设现在有2个对象

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public class A{

	@Autowired
	private B b;


}
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public class B{

	@Autowired
	private A a;


}

那么他的依赖查找顺序是这样的

假设先扫描到A

  1. 实例化A，发现其存在一个B的依赖，并且实例化A，将其缓存在singletonFactories中
  2. 实例化B，发现其有一个A的依赖
  3. 发现A正在创建，存在singletonsCurrentlyInCreation中，未完成初始化完成
  4. B调用singletonFactories缓存的ObjectFactory获取未初始化的A
  5. B依赖注入完成，A也依赖注入完成，由于java的同一引用代表一个对象的特性，A和B互相持有的未初始化完全的对象，在A和B初始化后仍然是同一个对象