Spring源码解析之八finishBeanFactoryInitialization方法即初始化单例bean

Spring源码解析之八finishBeanFactoryInitialization方法即初始化单例bean

七千字长文深刻解读,Spirng中是如何初始化单例bean的,和面试中最常问的Spring是如何解决循环依赖?

今天解读Spring核心方法refresh()中最最重要的一个方法finishBeanFactoryInitialization()方法,该方法负责初始化所有的单例bean。

finishBeanFactoryInitialization()方法位于refresh()中下标为8的位置。

到目前为止,应该说 BeanFactory 已经创建完成,并且所有的实现了 BeanFactoryPostProcessor 接口的 Bean 都已经初始化并且其中的 postProcessBeanFactory(factory) 方法已经得到回调执行了。而且 Spring 已经“手动”注册了一些特殊的 Bean,如 environmentsystemProperties 等。

剩下的就是初始化 singleton beans 了,大都数我们的业务中都是单例bean,就像我们写的@Controller、@Service的类(没有设置懒加载的)都是在这个地方初始化,以供我们使用,如果没有设置懒加载,那么 Spring 会在接下来初始化所有的 singleton beans。

我们先看一下refresh()的源码,大概看下finishBeanFactoryInitialization(beanFactory)所处的位置。

@Override
	public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			// Prepare this context for refreshing.
			//1、刷新前的准备
			prepareRefresh();

			// Tell the subclass to refresh the internal bean factory.
			//2、将会初始化 BeanFactory、加载 Bean、注册 Bean
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// Prepare the bean factory for use in this context.
			//3、设置 BeanFactory 的类加载器,添加几个 BeanPostProcessor,手动注册几个特殊的 bean
			prepareBeanFactory(beanFactory);

			try {
				//4、模板方法
				// Allows post-processing of the bean factory in context subclasses.
				postProcessBeanFactory(beanFactory);

				// Invoke factory processors registered as beans in the context.
				//执行BeanFactory后置处理器
				invokeBeanFactoryPostProcessors(beanFactory);

				// 5、Register bean processors that intercept bean creation.
				//注册bean后置处理器
				registerBeanPostProcessors(beanFactory);

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

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

				// Initialize other special beans in specific context subclasses.
				//6、模板方法--springboot实现了这个方法
				onRefresh();

				// Check for listener beans and register them.
				//7、注册监听器
				registerListeners();

				// Instantiate all remaining (non-lazy-init) singletons.
				//8、完成bean工厂的初始化**方法重要**********************************************
				finishBeanFactoryInitialization(beanFactory);

				//9、 Last step: publish corresponding event.
				finishRefresh();
			}

我们深入finishBeanFactoryInitialization(beanFactory)中,里面的调用线路错综复杂,还望读者可以做好心理准备。

/**
	 * 负责单例bean的初始化
	 * Finish the initialization of this context's bean factory,
	 * initializing all remaining singleton beans.
	 */
	protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
		// Initialize conversion service for this context.
		//最先初始化名字为 conversionService的类,conversionService类 它用来将前端传过来的参数和后端的 controller 方法上的参数进行绑定的时候用
		//尤其是用于非基础类型的转换
		if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
				beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
			beanFactory.setConversionService(
					//初始化在getBean()方法中实现
					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.
		// 先初始化 LoadTimeWeaverAware 类型的 Bean aop相关注:大概有个印象,以后解析aop会和它串起来。
		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.
		//freeze的单词意思是冻结,这个时候已经开始预初始化, bean 定义解析、加载、注册先停止
		beanFactory.freezeConfiguration();

		// Instantiate all remaining (non-lazy-init) singletons.
		//开始初始化
		beanFactory.preInstantiateSingletons();
	}

该方法是判断bean的一系列是不是属于某个类型的bean,如果是就调用getBean()方法,如果不是,就调用beanFactory.preInstantiateSingletons()进行初始化,我们先把getBean()放一放,重点看一看beanFactory.preInstantiateSingletons()方法。

@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.
		// this.beanDefinitionNames 保存了所有的 beanNames

		List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

		// Trigger initialization of all non-lazy singleton beans...
		//// 下面这个循环,触发所有的非懒加载的 singleton beans 的初始化操作
		for (String beanName : beanNames) {
			RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
			// 非抽象、非懒加载的 singletons。如果配置了 'abstract = true',那是不需要初始化的
			if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
				// 处理 FactoryBean (负责初始化工厂的bean)
				if (isFactoryBean(beanName)) {
					// FactoryBean 的话,在 beanName 前面加上 ‘&’ 符号
					//此处调用getBean()方法
					Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
					if (bean instanceof FactoryBean) {
						FactoryBean<?> factory = (FactoryBean<?>) bean;
						// 判断当前 FactoryBean 是否是 SmartFactoryBean 的实现
						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 {
					// 对于普通的 Bean,只要调用 getBean(beanName) 这个方法就可以进行初始化了
					getBean(beanName);
				}
			}
		}

		// Trigger post-initialization callback for all applicable beans...
		// 到这里说明所有的非懒加载的 singleton beans 已经完成了初始化
		// 如果我们定义的 bean 是实现了 SmartInitializingSingleton 接口的,那么在这里得到回调
		//如果你想在单例bean初始化后做一些事 那就实现该接口
		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();
				}
			}
		}
	}

preInstantiateSingletons()方法的主要任务是进行初始化的,在初始化前同样是一系列判断,如,是否是懒加载的,是否是一个factorybean(一个特别的bean,负责工厂创建的bean),最后调用getBean()方法。

其中有个插曲是否实现了SmartInitializingSingleton接口,将接口让你可以在bean初始化后做一些事,我们写一个简单的实例测试一下。

image-20220309095756363

其他地方读者看注释了解一下即可,我们开始继续深入getBean()方法。

getBean()方法内部调用了doGetBean()我们直接看doGetBean方法。

	
	// 我们在剖析初始化 Bean 的过程,但是 getBean 方法我们经常是用来从容器中获取 Bean 用的,注意切换思路,
	// 已经初始化过了就从容器中直接返回,否则就先初始化再返回
	protected <T> T doGetBean(
			String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
			throws BeansException {

		// 获取一个 “正统的” beanName,处理两种情况,一个是前面说的 FactoryBean(前面带 ‘&’),
		// 一个是别名问题,因为这个方法是 getBean,获取 Bean 用的,你要是传一个别名进来,是完全可以的
		String beanName = transformedBeanName(name);
		// 返回值
		Object bean;

		// Eagerly check singleton cache for manually registered singletons.
		// 检查下是不是已经创建过了
		Object sharedInstance = getSingleton(beanName);
		// 这里说下 args ,虽然看上去一点不重要。前面我们一路进来的时候都是 getBean(beanName),
		// 所以 args 传参其实是 null 的,但是如果 args 不为空的时候,那么意味着调用方不是希望获取 Bean,而是创建 Bean
		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 + "'");
				}
			}
			// 下面这个方法:如果是普通 Bean 的话,直接返回 sharedInstance,
			// 如果是 FactoryBean 的话,返回它创建的那个实例对象
			bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
		}

		else {
			// Fail if we're already creating this bean instance:
			// We're assumably within a circular reference.
			// 创建过了此 beanName 的 prototype 类型的 bean,那么抛异常,
			// 往往是因为陷入了循环引用 哦,原来之前的循环依赖都是在这抛的异常,再有问题就不是无头苍蝇了
			if (isPrototypeCurrentlyInCreation(beanName)) {
				throw new BeanCurrentlyInCreationException(beanName);
			}

			// Check if bean definition exists in this factory.
			// 检查一下这个 BeanDefinition 在容器中是否存在 BeanDefinition既是包含了bean的一系列信息
			BeanFactory parentBeanFactory = getParentBeanFactory();
			if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
				// Not found -> check parent.
				// 如果当前容器不存在这个 BeanDefinition,试试父容器中有没有
				String nameToLookup = originalBeanName(name);
				if (parentBeanFactory instanceof AbstractBeanFactory) {
					return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
							nameToLookup, requiredType, args, typeCheckOnly);
				}
				else if (args != null) {
					// Delegation to parent with explicit args.
					// 返回父容器的查询结果
					return (T) parentBeanFactory.getBean(nameToLookup, args);
				}
				else if (requiredType != null) {
					// No args -> delegate to standard getBean method.
					return parentBeanFactory.getBean(nameToLookup, requiredType);
				}
				else {
					return (T) parentBeanFactory.getBean(nameToLookup);
				}
			}

			if (!typeCheckOnly) {
				// typeCheckOnly 为 false,将当前 beanName 放入一个 alreadyCreated 的 Set 集合中。
				markBeanAsCreated(beanName);
			}

			/*
			 * 稍稍总结一下:
			 * 到这里的话,要准备创建 Bean 了,对于 singleton 的 Bean 来说,容器中还没创建过此 Bean;
			 * 对于 prototype 的 Bean 来说,本来就是要创建一个新的 Bean。
			 */
			try {
				RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
				checkMergedBeanDefinition(mbd, beanName, args);

				// Guarantee initialization of beans that the current bean depends on.
				// 先初始化依赖的所有 Bean,这个很好理解。
				// 注意,这里的依赖指的是 depends-on 中定义的依赖
				String[] dependsOn = mbd.getDependsOn();
				if (dependsOn != null) {
					for (String dep : dependsOn) {
						// 检查是不是有循环依赖,这里的循环依赖和我们前面说的循环依赖又不一样
						if (isDependent(beanName, dep)) {
							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
						}
						// 注册一下依赖关系
						registerDependentBean(dep, beanName);
						try {
							// 先初始化被依赖项
							getBean(dep);
						}
						catch (NoSuchBeanDefinitionException ex) {
							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
						}
					}
				}

				// Create bean instance.
				// 如果是 singleton scope 的,创建 singleton 的实例
				if (mbd.isSingleton()) {
					sharedInstance = getSingleton(beanName, () -> {
						try {
							// 执行创建 Bean,详情继续深入
							// 第三个参数 args 数组代表创建实例需要的参数,不就是给构造方法用的参数,或者是工厂 Bean 的参数嘛,不过要注意,在我们的初始化阶段,args 是 null。
							// 这回我们要到一个新的类了 AbstractAutowireCapableBeanFactory,看类名,AutowireCapable?类名是不是也说明了点问题了。
							// 主要是为了以下场景,采用 @Autowired 注解注入属性值:
							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);
				}
				// 如果是 prototype scope 的,创建 prototype 的实例
				else if (mbd.isPrototype()) {
					// It's a prototype -> create a new instance.
					Object prototypeInstance = null;
					try {
						beforePrototypeCreation(beanName);
						// 执行创建 Bean
						prototypeInstance = createBean(beanName, mbd, args);
					}
					finally {
						afterPrototypeCreation(beanName);
					}
					bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
				}

				else {
					String scopeName = mbd.getScope();
					if (!StringUtils.hasLength(scopeName)) {
						throw new IllegalStateException("No scope name defined for bean ´" + beanName + "'");
					}
					Scope scope = this.scopes.get(scopeName);
					if (scope == null) {
						throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
					}
					try {
						Object scopedInstance = scope.get(beanName, () -> {
							beforePrototypeCreation(beanName);
							try {
								return createBean(beanName, mbd, args);
							}
							finally {
								afterPrototypeCreation(beanName);
							}
						});
						bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
					}
					catch (IllegalStateException ex) {
						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) {
				cleanupAfterBeanCreationFailure(beanName);
				throw ex;
			}
		}

		// Check if required type matches the type of the actual bean instance.
		// 最后,检查一下类型对不对,不对的话就抛异常,对的话就返回了
		if (requiredType != null && !requiredType.isInstance(bean)) {
			try {
				T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
				if (convertedBean == null) {
					throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
				}
				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());
			}
		}
		return (T) bean;
	}

具体的实例化过程在createBean()方法中,我们继续深入createBean()方法。

@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.
		// 确保 BeanDefinition 中的 Class 被加载
		Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
		if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
			mbdToUse = new RootBeanDefinition(mbd);
			mbdToUse.setBeanClass(resolvedClass);
		}

		// Prepare method overrides.
		// 准备方法覆写,这里又涉及到一个概念:MethodOverrides,它来自于 bean 定义中的 <lookup-method />
		// 和 <replaced-method />,如果读者感兴趣,回到 bean 解析的地方看看对这两个标签的解析。
		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.
			// 让 InstantiationAwareBeanPostProcessor 在这一步有机会返回代理,
			// 在 《Spring AOP 源码分析》那篇文章中有解释,这里先跳过
			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 {
			// 重头戏,创建 bean
			Object beanInstance = doCreateBean(beanName, mbdToUse, args);
			if (logger.isTraceEnabled()) {
				logger.trace("Finished creating instance of bean '" + beanName + "'");
			}
			return beanInstance;
		}
		catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
			// A previously detected exception with proper bean creation context already,
			// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
			throw ex;
		}
		catch (Throwable ex) {
			throw new BeanCreationException(
					mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
		}
	}

我们继续往里看 doCreateBean 这个方法,这个调用过程是真的深。

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

		// Instantiate the bean.
		BeanWrapper instanceWrapper = null;
		if (mbd.isSingleton()) {
			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
		}
		if (instanceWrapper == null) {
			// 说明不是 FactoryBean,这里实例化 Bean,这里非常关键,细节之后再说**********
			instanceWrapper = createBeanInstance(beanName, mbd, args);
		}
		Object bean = instanceWrapper.getWrappedInstance();
		Class<?> beanType = instanceWrapper.getWrappedClass();
		if (beanType != NullBean.class) {
			mbd.resolvedTargetType = beanType;
		}

		// Allow post-processors to modify the merged bean definition.
		synchronized (mbd.postProcessingLock) {
			if (!mbd.postProcessed) {
				try {
					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.
		// 下面这块代码是为了解决循环依赖的问题,这是个重头戏,解决循环依赖问题
		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 {
			// 这一步也是非常关键的,这一步负责属性装配,因为前面的实例只是实例化了,并没有设值,这里就是设值***************
			populateBean(beanName, mbd, instanceWrapper);
			// 还记得 init-method 吗?还有 InitializingBean 接口?还有 BeanPostProcessor 接口?
			// 这里就是处理 bean 初始化完成后的各种回调**************
			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);
			}
		}
	// 下面这块代码是为了解决循环依赖的问题,这是个重头戏,解决循环依赖问题	
		if (earlySingletonExposure) {
				//循环依赖的核心方法调用
			Object earlySingletonReference = getSingleton(beanName, false);
			if (earlySingletonReference != null) {
				if (exposedObject == bean) {
					exposedObject = earlySingletonReference;
				}
				else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
					String[] dependentBeans = getDependentBeans(beanName);
					Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
					for (String dependentBean : dependentBeans) {
						if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
							actualDependentBeans.add(dependentBean);
						}
					}
					if (!actualDependentBeans.isEmpty()) {
						throw new BeanCurrentlyInCreationException(beanName,
								"Bean with name '" + beanName + "' has been injected into other beans [" +
								StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
								"] in its raw version as part of a circular reference, but has eventually been " +
								"wrapped. This means that said other beans do not use the final version of the " +
								"bean. This is often the result of over-eager type matching - consider using " +
								"'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");
					}
				}
			}
		}

		// Register bean as disposable.
		try {
			registerDisposableBeanIfNecessary(beanName, bean, mbd);
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanCreationException(
					mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
		}

		return exposedObject;
	}

到这里,我们已经分析完了 doCreateBean 方法,总的来说,我们已经说完了整个初始化流程。

在实例化bean后有一个特别重要的知识点,也是面试中最常问的,Spring怎么解决循环依赖问题?核心代码就在这个方法里面。

循环依赖其实就是循环引用,也就是两个或则两个以上的bean互相持有对方,最终形成闭环。比如A依赖于B,B依赖于C,C又依赖于A。如下图:

image-20220309103101026

doCreateBean 方法有三个核心流程。

image-20220309103238497

(1)createBeanInstance:实例化,其实也就是调用对象的构造方法实例化对象

(2)populateBean:填充属性,这一步主要是多bean的依赖属性进行填充

(3)initializeBean:调用spring xml中的init 方法。

从上面讲述的单例bean初始化步骤我们可以知道,循环依赖主要发生在第一、第二步。也就是构造器循环依赖和field循环依赖。

那么我们要解决循环引用也应该从初始化过程着手,对于单例来说,在Spring容器整个生命周期内,有且只有一个对象,所以很容易想到这个对象应该存在Cache中,Spring为了解决单例的循环依赖问题,使用了三级缓存。

我们看一下getSingleton方法。

该方法还依赖于三个map,这三个map就是三级缓存。

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

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

/** Cache of early singleton objects: bean name to bean instance. */
//提前曝光的单例对象的Cache
private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);
@Nullable
	protected Object getSingleton(String beanName, boolean allowEarlyReference) {
		// Quick check for existing instance without full singleton lock
		Object singletonObject = this.singletonObjects.get(beanName);
		
		//判断当前单例bean是否正在创建中,也就是没有初始化完成(比如A的构造器依赖了B对象所以得先去创建B对象
		// 或则在A的populateBean过程中依赖了B对象,得先去创建B对象,这时的A就是处于创建中的状态。
		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
			singletonObject = this.earlySingletonObjects.get(beanName);
			
			// 是否允许从singletonFactories中通过getObject拿到对象
			if (singletonObject == null && allowEarlyReference) {
				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) {
							ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
							if (singletonFactory != null) {
								singletonObject = singletonFactory.getObject();
								this.earlySingletonObjects.put(beanName, singletonObject);
								this.singletonFactories.remove(beanName);
							}
						}
					}
				}
			}
		}
		return singletonObject;
	}

分析getSingleton()的整个过程,Spring首先从一级缓存singletonObjects中获取。如果获取不到,并且对象正在创建中,就再从二级缓存earlySingletonObjects中获取。

如果还是获取不到且允许singletonFactories通过getObject()获取,就从三级缓存singletonFactory.getObject()(三级缓存)获取,如果获取到了则:

this.earlySingletonObjects.put(beanName, singletonObject);
                        this.singletonFactories.remove(beanName);
                        

从singletonFactories中移除,并放入earlySingletonObjects中。其实也就是从三级缓存移动到了二级缓存。

从上面三级缓存的分析,我们可以知道,Spring解决循环依赖的诀窍就在于singletonFactories这个三级cache。

里就是解决循环依赖的关键,这段代码发生在createBeanInstance之后,也就是说单例对象此时已经被创建出来(调用了构造器)。这个对象已经被生产出来了,虽然还不完美(还没有进行初始化的第二步和第三步),但是已经能被人认出来了(根据对象引用能定位到堆中的对象),所以Spring此时将这个对象提前曝光出来让大家认识,让大家使用。

这样做有什么好处呢?

让我们来分析一下“A的某个field或者setter依赖了B的实例对象,同时B的某个field或者setter依赖了A的实例对象”这种循环依赖的情况。

A首先完成了初始化的第一步,并且将自己提前曝光到singletonFactories中,此时进行初始化的第二步,发现自己依赖对象B,此时就尝试去get(B),发现B还没有被create,所以走create流程,B在初始化第一步的时候发现自己依赖了对象A,于是尝试get(A),尝试一级缓存singletonObjects(肯定没有,因为A还没初始化完全),尝试二级缓存earlySingletonObjects(也没有),尝试三级缓存singletonFactories,由于A通过ObjectFactory将自己提前曝光了,所以B能够通过ObjectFactory.getObject拿到A对象(虽然A还没有初始化完全,但是总比没有好呀),B拿到A对象后顺利完成了初始化阶段1、2、3,完全初始化之后将自己放入到一级缓存singletonObjects中。

此时返回A中,A此时能拿到B的对象顺利完成自己的初始化阶段2、3,最终A也完成了初始化,进去了一级缓存singletonObjects中,而且更加幸运的是,由于B拿到了A的对象引用,所以B现在hold住的A对象完成了初始化。

知道了这个原理时候,肯定就知道为啥Spring不能解决“A的构造方法中依赖了B的实例对象,同时B的构造方法中依赖了A的实例对象”这类问题了!因为加入singletonFactories三级缓存的前提是执行了构造器,所以构造器的循环依赖没法解决。

接下来我们挑 doCreateBean 中的三个细节出来说说。一个是创建 Bean 实例的 createBeanInstance 方法,一个是依赖注入的 populateBean 方法,还有就是回调方法 initializeBean。

这三个方法也是极其复杂的,读者有兴趣可以继续的深入进去。

1、 createBeanInstance 方法

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
		// Make sure bean class is actually resolved at this point.
		// 确保已经加载了此 class
		Class<?> beanClass = resolveBeanClass(mbd, beanName);

		// 校验一下这个类的访问权限
		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) {
			// 采用工厂方法实例化,不熟悉这个概念的读者请看附录,注意,不是 FactoryBean
			return instantiateUsingFactoryMethod(beanName, mbd, args);
		}

		// Shortcut when re-creating the same bean...
		// 如果不是第一次创建,比如第二次创建 prototype bean。
		// 这种情况下,我们可以从第一次创建知道,采用无参构造函数,还是构造函数依赖注入 来完成实例化
		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) {
				// 构造函数依赖注入
				return autowireConstructor(beanName, mbd, null, null);
			}
			else {
				// 无参构造函数
				return instantiateBean(beanName, mbd);
			}
		}

		// Candidate constructors for autowiring?
		// 判断是否采用有参构造函数
		Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
		if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
				mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
			// 构造函数依赖注入
			return autowireConstructor(beanName, mbd, ctors, args);
		}

		// Preferred constructors for default construction?
		ctors = mbd.getPreferredConstructors();
		if (ctors != null) {
			// 构造函数依赖注入
			return autowireConstructor(beanName, mbd, ctors, null);
		}

		// No special handling: simply use no-arg constructor.
		// 调用无参构造函数
		return instantiateBean(beanName, mbd);
	}

看一下instantiateBean方法是怎么做的。

protected BeanWrapper instantiateBean(String beanName, RootBeanDefinition mbd) {
		try {
			Object beanInstance;
			if (System.getSecurityManager() != null) {
				beanInstance = AccessController.doPrivileged(
						(PrivilegedAction<Object>) () -> getInstantiationStrategy().instantiate(mbd, beanName, this),
						getAccessControlContext());
			}
			else {
				// 实例化
				beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, this);
			}
			// 包装一下,返回
			BeanWrapper bw = new BeanWrapperImpl(beanInstance);
			initBeanWrapper(bw);
			return bw;
		}
		catch (Throwable ex) {
			throw new BeanCreationException(
					mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
		}
	}

我们可以看到,关键的地方在于:beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);

里面是具体是实例化过程,我们进去看看。

@Override
	public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
		// Don't override the class with CGLIB if no overrides.
		// 如果不存在方法覆写,那就使用 java 反射进行实例化,否则使用 CGLIB,
		// 方法覆写 请参见附录"方法注入"中对 lookup-method 和 replaced-method 的介绍
		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 {
							constructorToUse = clazz.getDeclaredConstructor();
						}
						bd.resolvedConstructorOrFactoryMethod = constructorToUse;
					}
					catch (Throwable ex) {
						throw new BeanInstantiationException(clazz, "No default constructor found", ex);
					}
				}
			}
			// 利用构造方法进行实例化
			return BeanUtils.instantiateClass(constructorToUse);
		}
		else {
			// Must generate CGLIB subclass.
			// 存在方法覆写,利用 CGLIB 来完成实例化,需要依赖于 CGLIB 生成子类,这里就不展开了。
			// tips: 因为如果不使用 CGLIB 的话,存在 override 的情况 JDK 并没有提供相应的实例化支持
			return instantiateWithMethodInjection(bd, beanName, owner);
		}
	}

到这里,我们就算实例化完成了。我们开始说怎么进行属性注入。

2、populateBean 方法

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
		if (bw == null) {
			if (mbd.hasPropertyValues()) {
				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;
					}
				}
			}
		}

		// bean 实例的所有属性都在这里了
		PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);

		int resolvedAutowireMode = mbd.getResolvedAutowireMode();
		// 通过名字找到所有属性值,如果是 bean 依赖,先初始化依赖的 bean。记录依赖关系
		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);

		PropertyDescriptor[] filteredPds = null;
		if (hasInstAwareBpps) {
			if (pvs == null) {
				pvs = mbd.getPropertyValues();
			}
			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);
						}
						// 这里有个非常有用的 BeanPostProcessor 进到这里: AutowiredAnnotationBeanPostProcessor
						// 对采用 @Autowired、@Value 注解的依赖进行设值,这里的内容也是非常丰富的
						pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
						if (pvsToUse == null) {
							return;
						}
					}
					pvs = pvsToUse;
				}
			}
		}
		if (needsDepCheck) {
			if (filteredPds == null) {
				filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
			}
			checkDependencies(beanName, mbd, filteredPds, pvs);
		}

		if (pvs != null) {
			// 设置 bean 实例的属性值
			applyPropertyValues(beanName, mbd, bw, pvs);
		}
	}

属性注入完成后,这一步其实就是处理各种回调了,这块代码比较简单。

3、 initializeBean方法

protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
		if (System.getSecurityManager() != null) {
			AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
				invokeAwareMethods(beanName, bean);
				return null;
			}, getAccessControlContext());
		}
		else {
			// 如果 bean 实现了 BeanNameAware、BeanClassLoaderAware 或 BeanFactoryAware 接口,回调
			invokeAwareMethods(beanName, bean);
		}

		Object wrappedBean = bean;
		if (mbd == null || !mbd.isSynthetic()) {
			// BeanPostProcessor 的 postProcessBeforeInitialization 回调
			wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
		}

		try {
			// 处理 bean 中定义的 init-method,
			// 或者如果 bean 实现了 InitializingBean 接口,调用 afterPropertiesSet() 方法
			invokeInitMethods(beanName, wrappedBean, mbd);
		}
		catch (Throwable ex) {
			throw new BeanCreationException(
					(mbd != null ? mbd.getResourceDescription() : null),
					beanName, "Invocation of init method failed", ex);
		}
		if (mbd == null || !mbd.isSynthetic()) {
			// BeanPostProcessor 的 postProcessAfterInitialization 回调
			//BeanPostProcessor 的两个回调都发生在这边,只不过中间处理了 init-method
			wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
		}

		return wrappedBean;
	}

自此,Spring实例化单例非懒加载bean的过程也就完成了,这也是Spirng最最重要的方法了。在我们的日常使用Spring中,定义好各个类,然后在上面加上,@Controller,@Service,Autowired等注解,这些注解是怎么起作用的呢?

想必大部分同学都是知其然,不知其所以然,想必通过本文,读者心中能有一个清楚的认识。

posted @ 2022-03-09 11:10  程序员田同学  阅读(239)  评论(0编辑  收藏  举报