spring IOC

1.为什么使用IOC?什么是IOC?什么是DI?

IOC 中文为控制反转。

IOC是一种设计、设计模式。DI是注入的一种方式。

一般依赖使用：

Student student = new Student();
Score score = new Score();
student.setScore(score);

降低了程序里对象与对象之间的耦合性。

2.spring ioc 的源码分析及实现

首先先明确几个概念：

Environment 环境，用于获取系统环境及系统配置的对象

post processor 称后置处理器或增强器，就是对类进制扩展，用反射，beanFactoryPostProcessor是对bean工厂进行增强，beanPostProcessor是对bean进行增强。

aware 意识、察觉 继承aware后可以获取对应的aware的属性值，spring 初始化时会调用设置。

BeanDefinitionReader 根据xml 或 注解解析成 beanDefinition的抽象接口

beanDefinition是xml 或 注解 里定义的bean结构的对象

listener 监听器

envent 事件

Multicast 多播器

spring ioc创建对象流程（对象生命周期）

1.创建项目时会指定使用啥方式配置spring, 比如 xml 或 注解

2.加载配置文件

3.生成beanDefinition并注册到容器

4.BeanFactoryPostProcessor的执行

5.根据反射实例化bean对象

6.初始化bean对象

6.1填充属性

6.2执行aware接口需要实现的方法

6.3执行before的beanPostProcessor

6.4执行bean指定的init-method

6.5执行after的beanPostProcessor

这时bean对象是完整的

7.点击 程序停止 -> 容器销毁 -> 执行destory-method

源码

public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			// Prepare this context for refreshing.
			/**
			 * 做容器刷新前的准备工作
			 * 1、设置容器的启动时间
			 * 2、设置活跃状态为true
			 * 3、设置关闭状态为false
			 * 4、获取Environment对象，并加载当前系统的属性值到Environment对象中
			 * 5、准备监听器和事件的集合对象，默认为空的集合
			 */

			prepareRefresh();

			// Tell the subclass to refresh the internal bean factory.
			// 创建容器对象：DefaultListableBeanFactory
			// 加载xml配置文件的属性值到当前工厂中，最重要的就是BeanDefinition
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// Prepare the bean factory for use in this context.
			// beanFactory的准备工作，对各种属性进行填充
			prepareBeanFactory(beanFactory);

			try {
				// Allows post-processing of the bean factory in context subclasses.
				// 子类覆盖方法做额外的处理，此处我们自己一般不做任何扩展工作，但是可以查看web中的代码，是有具体实现的
				postProcessBeanFactory(beanFactory);

				// Invoke factory processors registered as beans in the context.
				// 调用各种beanFactory处理器
				invokeBeanFactoryPostProcessors(beanFactory);

				// Register bean processors that intercept bean creation.
				// 注册bean处理器，这里只是注册功能，真正调用的是getBean方法
				registerBeanPostProcessors(beanFactory);

				// Initialize message source for this context.
				// 为上下文初始化message源，即不同语言的消息体，国际化处理,在springmvc的时候通过国际化的代码重点讲
				initMessageSource();

				// Initialize event multicaster for this context.
				// 初始化事件监听多路广播器
				initApplicationEventMulticaster();

				// Initialize other special beans in specific context subclasses.
				// 留给子类来初始化其他的bean
				onRefresh();

				// Check for listener beans and register them.
				// 在所有注册的bean中查找listener bean,注册到消息广播器中
				registerListeners();

				// Instantiate all remaining (non-lazy-init) singletons.
				// 初始化剩下的单实例（非懒加载的）
				finishBeanFactoryInitialization(beanFactory);

				// Last step: publish corresponding event.
				// 完成刷新过程，通知生命周期处理器lifecycleProcessor刷新过程，同时发出ContextRefreshEvent通知别人
				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.
				// 为防止bean资源占用，在异常处理中，销毁已经在前面过程中生成的单件bean
				destroyBeans();

				// Reset 'active' flag.
				// 重置active标志
				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();
			}
		}
	}

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
		// Initialize conversion service for this context.
		// 为上下文初始化类型转换器
		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.
		// 如果beanFactory之前没有注册嵌入值解析器，则注册默认的嵌入值解析器，主要用于注解属性值的解析
		if (!beanFactory.hasEmbeddedValueResolver()) {
			beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
		}

		// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
		// 尽早初始化loadTimeWeaverAware bean,以便尽早注册它们的转换器
		String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
		for (String weaverAwareName : weaverAwareNames) {
			getBean(weaverAwareName);
		}

		// Stop using the temporary ClassLoader for type matching.
		// 禁止使用临时类加载器进行类型匹配
		beanFactory.setTempClassLoader(null);

		// Allow for caching all bean definition metadata, not expecting further changes.
		// 冻结所有的bean定义，说明注册的bean定义将不被修改或任何进一步的处理
		beanFactory.freezeConfiguration();

		// Instantiate all remaining (non-lazy-init) singletons.
		// 实例化剩下的单例对象
		beanFactory.preInstantiateSingletons();
	}

@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.
		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) {
						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 {
					getBean(beanName);
				}
			}
		}

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

@SuppressWarnings("unchecked")
	protected <T> T doGetBean(
			String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
			throws BeansException {

		/**
		 * 提取对应的beanName，很多同学可能会认为此处直接使用即可，为什么还要进行转换呢，原因在于当bean对象实现FactoryBean接口之后就会变成&beanName，同时如果存在别名，也需要把别名进行转换*/
		String beanName = transformedBeanName(name);
		Object bean;

		// Eagerly check singleton cache for manually registered singletons.
		/**提前检查单例缓存中是否有手动注册的单例对象，跟循环依赖有关联*/
		Object sharedInstance = getSingleton(beanName);
		// 如果bean的单例对象找到了，且没有创建bean实例时要使用的参数
		if (sharedInstance != null && args == null) {
			if (logger.isTraceEnabled()) {
				if (isSingletonCurrentlyInCreation(beanName)) {
					logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
							"' that is not fully initialized yet - a consequence of a circular reference");
				}
				else {
					logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
				}
			}
			// 返回对象的实例，很多同学会理解不了这句话存在的意义，当你实现了FactoryBean接口的对象，需要获取具体的对象的时候就需要此方法来进行获取了
			bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
		}

		else {
			// Fail if we're already creating this bean instance:
			// We're assumably within a circular reference.
			// 当对象都是单例的时候会尝试解决循环依赖的问题，但是原型模式下如果存在循环依赖的情况，那么直接抛出异常
			if (isPrototypeCurrentlyInCreation(beanName)) {
				throw new BeanCurrentlyInCreationException(beanName);
			}

			// Check if bean definition exists in this factory.
			// 如果bean定义不存在，就检查父工厂是否有
			BeanFactory parentBeanFactory = getParentBeanFactory();
			// 如果beanDefinitionMap中也就是在所有已经加载的类中不包含beanName，那么就尝试从父容器中获取
			if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
				// Not found -> check parent.
				// 获取name对应的规范名称【全类名】，如果name前面有'&'，则会返回'&'+规范名称【全类名】
				String nameToLookup = originalBeanName(name);
				// 如果父工厂是AbstractBeanFactory的实例
				if (parentBeanFactory instanceof AbstractBeanFactory) {
					// 调用父工厂的doGetBean方法，就是该方法。【递归】
					return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
							nameToLookup, requiredType, args, typeCheckOnly);
				}
				else if (args != null) {
					// Delegation to parent with explicit args.
					// 如果有创建bean实例时要使用的参数
					// Delegation to parent with explicit args. 使用显示参数委派给父工厂
					// 使用父工厂获取该bean对象,通bean全类名和创建bean实例时要使用的参数
					return (T) parentBeanFactory.getBean(nameToLookup, args);
				}
				else if (requiredType != null) {
					// No args -> delegate to standard getBean method.
					// 没有创建bean实例时要使用的参数 -> 委托给标准的getBean方法。
					// 使用父工厂获取该bean对象,通bean全类名和所需的bean类型
					return parentBeanFactory.getBean(nameToLookup, requiredType);
				}
				else {
					// 使用父工厂获取bean，通过bean全类名
					return (T) parentBeanFactory.getBean(nameToLookup);
				}
			}
			// 如果不是做类型检查，那么表示要创建bean，此处在集合中做一个记录
			if (!typeCheckOnly) {
				// 为beanName标记为已经创建（或将要创建）
				markBeanAsCreated(beanName);
			}

			try {
				// 此处做了BeanDefinition对象的转换，当我们从xml文件中加载beandefinition对象的时候，封装的对象是GenericBeanDefinition,
				// 此处要做类型转换，如果是子类bean的话，会合并父类的相关属性
				RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
				// 检查mbd的合法性，不合格会引发验证异常
				checkMergedBeanDefinition(mbd, beanName, args);

				// Guarantee initialization of beans that the current bean depends on.
				// 如果存在依赖的bean的话，那么则优先实例化依赖的bean
				String[] dependsOn = mbd.getDependsOn();
				if (dependsOn != null) {
					// 如果存在依赖，则需要递归实例化依赖的bean
					for (String dep : dependsOn) {
						// 如果beanName已注册依赖于dependentBeanName的关系
						if (isDependent(beanName, dep)) {
							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
						}
						// 注册各个bean的依赖关系，方便进行销毁
						registerDependentBean(dep, beanName);
						try {
							// 递归优先实例化被依赖的Bean
							getBean(dep);
						}
						// 捕捉为找到BeanDefinition异常：'beanName'依赖于缺少的bean'dep'
						catch (NoSuchBeanDefinitionException ex) {
							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
						}
					}
				}

				// Create bean instance.
				// 创建bean的实例对象
				if (mbd.isSingleton()) {
					// 返回以beanName的(原始)单例对象，如果尚未注册，则使用singletonFactory创建并注册一个对象:
					sharedInstance = getSingleton(beanName, () -> {
						try {
							// 为给定的合并后BeanDefinition(和参数)创建一个bean实例
							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.
							// 显示地从单例缓存中删除实例：它可能是由创建过程急切地放在那里，以允许循环引用解析。还要删除
							// 接收到该Bean临时引用的任何Bean
							// 销毁给定的bean。如果找到相应的一次性Bean实例，则委托给destoryBean
							destroySingleton(beanName);
							// 重新抛出ex
							throw ex;
						}
					});
					// 从beanInstance中获取公开的Bean对象，主要处理beanInstance是FactoryBean对象的情况，如果不是
					// FactoryBean会直接返回beanInstance实例
					bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
				}
				// 原型模式的bean对象创建
				else if (mbd.isPrototype()) {
					// It's a prototype -> create a new instance.
					// 它是一个原型 -> 创建一个新实例
					// 定义prototype实例
					Object prototypeInstance = null;
					try {
						// 创建Prototype对象前的准备工作，默认实现将beanName添加到prototypesCurrentlyInCreation中
						beforePrototypeCreation(beanName);
						// 为mbd(和参数)创建一个bean实例
						prototypeInstance = createBean(beanName, mbd, args);
					}
					finally {
						// 创建完prototype实例后的回调，默认是将beanName从prototypesCurrentlyInCreation移除
						afterPrototypeCreation(beanName);
					}
					// 从beanInstance中获取公开的Bean对象，主要处理beanInstance是FactoryBean对象的情况，如果不是
					// FactoryBean会直接返回beanInstance实例
					bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
				}

				else {
					// 指定的scope上实例化bean
					String scopeName = mbd.getScope();
					if (!StringUtils.hasLength(scopeName)) {
						throw new IllegalStateException("No scope name defined for bean ´" + beanName + "'");
					}
					// 从scopes中获取scopeName对于的Scope对象
					Scope scope = this.scopes.get(scopeName);
					// 如果scope为null
					if (scope == null) {
						// 抛出非法状态异常：没有名为'scopeName'的scope注册
						throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
					}
					try {
						// 从scope中获取beanName对应的实例对象
						Object scopedInstance = scope.get(beanName, () -> {
							// 创建Prototype对象前的准备工作，默认实现 将beanName添加到prototypesCurrentlyInCreation中
							beforePrototypeCreation(beanName);
							try {
								// 为mbd(和参数)创建一个bean实例
								return createBean(beanName, mbd, args);
							}
							finally {
								// 创建完prototype实例后的回调，默认是将beanName从prototypesCurrentlyInCreation移除
								afterPrototypeCreation(beanName);
							}
						});
						// 从beanInstance中获取公开的Bean对象，主要处理beanInstance是FactoryBean对象的情况，如果不是
						// FactoryBean会直接返回beanInstance实例
						bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
					}
					catch (IllegalStateException ex) {
						// 捕捉非法状态异常
						// 抛出Bean创建异常：作用域 'scopeName' 对于当前线程是不活动的；如果您打算从单个实例引用它，请考虑为此
						// beanDefinition一个作用域代理
						throw new BeanCreationException(beanName,
								"Scope '" + scopeName + "' is not active for the current thread; consider " +
								"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
								ex);
					}
				}
			}
			catch (BeansException ex) {
				// 捕捉获取Bean对象抛出的Bean异常
				// 在Bean创建失败后，对缓存的元数据执行适当的清理
				cleanupAfterBeanCreationFailure(beanName);
				// 重新抛出ex
				throw ex;
			}
		}

		// Check if required type matches the type of the actual bean instance.
		// 检查requiredType是否与实际Bean实例的类型匹配
		// 如果requiredType不为null&&bean不是requiredType的实例
		if (requiredType != null && !requiredType.isInstance(bean)) {
			try {
				// 获取此BeanFactory使用的类型转换器，将bean转换为requiredType
				T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
				// 如果convertedBean为null
				if (convertedBean == null) {
					// 抛出Bean不是必要类型的异常
					throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
				}
				// 返回convertedBean
				return convertedBean;
			}
			catch (TypeMismatchException ex) {
				if (logger.isTraceEnabled()) {
					logger.trace("Failed to convert bean '" + name + "' to required type '" +
							ClassUtils.getQualifiedName(requiredType) + "'", ex);
				}
				throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
			}
		}
		// 将bean返回出去
		return (T) bean;
	}

@Nullable
	protected Object getSingleton(String beanName, boolean allowEarlyReference) {
		// Quick check for existing instance without full singleton lock
		// 从单例对象缓存中获取beanName对应的单例对象
		Object singletonObject = this.singletonObjects.get(beanName);
		// 如果单例对象缓存中没有，并且该beanName对应的单例bean正在创建中
		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
			//从早期单例对象缓存中获取单例对象（之所称成为早期单例对象，是因为earlySingletonObjects里
			// 的对象的都是通过提前曝光的ObjectFactory创建出来的，还未进行属性填充等操作）
			singletonObject = this.earlySingletonObjects.get(beanName);
			// 如果在早期单例对象缓存中也没有，并且允许创建早期单例对象引用
			if (singletonObject == null && allowEarlyReference) {
				// 如果为空，则锁定全局变量并进行处理
				synchronized (this.singletonObjects) {
					// Consistent creation of early reference within full singleton lock
					singletonObject = this.singletonObjects.get(beanName);
					if (singletonObject == null) {
						singletonObject = this.earlySingletonObjects.get(beanName);
						if (singletonObject == null) {
							// 当某些方法需要提前初始化的时候则会调用addSingletonFactory方法将对应的ObjectFactory初始化策略存储在singletonFactories
							ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
							if (singletonFactory != null) {
								// 如果存在单例对象工厂，则通过工厂创建一个单例对象
								singletonObject = singletonFactory.getObject();
								// 记录在缓存中，二级缓存和三级缓存的对象不能同时存在
								this.earlySingletonObjects.put(beanName, singletonObject);
								// 从三级缓存中移除
								this.singletonFactories.remove(beanName);
							}
						}
					}
				}
			}
		}
		return singletonObject;
	}

循环依赖

三级缓存

/** Cache of singleton objects: bean name to bean instance. */
/** 单例对象的缓存：bean 名称到 bean 实例。 */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

/** Cache of singleton factories: bean name to ObjectFactory. */
/** 单例工厂的缓存：对象工厂的 bean 名称。 */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

/** Cache of early singleton objects: bean name to bean instance. */
/** 早期单例对象的缓存：bean 名称到 bean 实例。 */
private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);

0 = {ApplicationContextAwareProcessor@8586} 
1 = {WebApplicationContextServletContextAwareProcessor@8593} 
2 = {ConfigurationClassPostProcessor$ImportAwareBeanPostProcessor@8600} 
3 = {PostProcessorRegistrationDelegate$BeanPostProcessorChecker@8613} 
4 = {ConfigurationPropertiesBindingPostProcessor@8757} 
5 = {RabbitConnectionFactoryMetricsPostProcessor@8764} 
6 = {AnnotationAwareAspectJAutoProxyCreator@8771} "proxyTargetClass=true; optimize=false; opaque=false; exposeProxy=false; frozen=false"
7 = {MethodValidationPostProcessor@8796} "proxyTargetClass=true; optimize=false; opaque=false; exposeProxy=false; frozen=false"
8 = {RabbitListenerAnnotationBeanPostProcessor@8803} 
9 = {PersistenceExceptionTranslationPostProcessor@8810} "proxyTargetClass=true; optimize=false; opaque=false; exposeProxy=false; frozen=false"
10 = {WebServerFactoryCustomizerBeanPostProcessor@8817} 
11 = {ErrorPageRegistrarBeanPostProcessor@8824} 
12 = {HealthEndpointConfiguration$HealthEndpointGroupsBeanPostProcessor@8832} 
13 = {MessagingAnnotationPostProcessor@8839} 
14 = {StreamListenerAnnotationBeanPostProcessor@8847} 
15 = {MeterRegistryPostProcessor@8854} 
16 = {IntegrationManagementConfigurer@8861} 
17 = {ConfigurationPropertiesBeans@8868} 
18 = {TraceRestTemplateBeanPostProcessor@8875} 
19 = {FeignContextBeanPostProcessor@8882} 
20 = {ExecutorBeanPostProcessor@8889} 
21 = {SleuthRabbitBeanPostProcessor@8896} 
22 = {IntegrationFlowBeanPostProcessor@8903} 
23 = {BaseIntegrationFlowDefinition$ReplyProducerCleaner@8910} 
24 = {GlobalChannelInterceptorProcessor@8917} 
25 = {CommonAnnotationBeanPostProcessor@8924} 
26 = {AutowiredAnnotationBeanPostProcessor@8946} 
27 = {ApplicationListenerDetector@8966} 

AutowiredAnnotationBeanPostProcessor

@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;
}

@Override
		protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
			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 {
					value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
				}
				catch (BeansException ex) {
					throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);
				}
				synchronized (this) {
					if (!this.cached) {
						Object cachedFieldValue = null;
						if (value != null || this.required) {
							cachedFieldValue = desc;
							registerDependentBeans(beanName, autowiredBeanNames);
							if (autowiredBeanNames.size() == 1) {
								String autowiredBeanName = autowiredBeanNames.iterator().next();
								if (beanFactory.containsBean(autowiredBeanName) &&
										beanFactory.isTypeMatch(autowiredBeanName, field.getType())) {
									cachedFieldValue = new ShortcutDependencyDescriptor(
											desc, autowiredBeanName, field.getType());
								}
							}
						}
						this.cachedFieldValue = cachedFieldValue;
						this.cached = true;
					}
				}
			}
			if (value != null) {
				ReflectionUtils.makeAccessible(field);
				field.set(bean, value);
			}
		}
	}

@Override
	@Nullable
	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;
		}
	}

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

instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);

这里会调用beanFactory.getBean(“Bservice”)，创建Bservice的bean,中间还会调依赖注入Aservice，这时二级缓存（早期的缓存中）中会有Aservice，当Bservice创建完后，Bservice是完整的，回到Aservice的创建过程，Aservice从二级缓存中删除，放入一级缓存中。

不能解决循环依赖：

1.通过构造器注入的不能解决

2.非单例的不能解决

三级缓存有啥用

三级缓存是用来存储代理Bean,当目标对象需要代理工作创建时，放入三级缓存中，最终赋值好的Bean放入一级缓存中。