入口方法为SpringApplication#run()

1.SpringApplication#run()

(资料图)

/**
	 * Run the Spring application, creating and refreshing a new
	 * {@link ApplicationContext}.
	 * @param args the application arguments (usually passed from a Java main method)
	 * @return a running {@link ApplicationContext}
	 */
	public ConfigurableApplicationContext run(String... args) {
		long startTime = System.nanoTime();
		DefaultBootstrapContext bootstrapContext = createBootstrapContext();
		ConfigurableApplicationContext context = null;
		configureHeadlessProperty();
		SpringApplicationRunListeners listeners = getRunListeners(args);
		listeners.starting(bootstrapContext, this.mainApplicationClass);
		try {
			ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
			ConfigurableEnvironment environment = prepareEnvironment(listeners, bootstrapContext, applicationArguments);
			Banner printedBanner = printBanner(environment);
			context = createApplicationContext();
			context.setApplicationStartup(this.applicationStartup);
			prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);
			refreshContext(context);
			afterRefresh(context, applicationArguments);
			Duration timeTakenToStartup = Duration.ofNanos(System.nanoTime() - startTime);
			if (this.logStartupInfo) {
				new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), timeTakenToStartup);
			}
			listeners.started(context, timeTakenToStartup);
			callRunners(context, applicationArguments);
		}
		catch (Throwable ex) {
			if (ex instanceof AbandonedRunException) {
				throw ex;
			}
			handleRunFailure(context, ex, listeners);
			throw new IllegalStateException(ex);
		}
		try {
			if (context.isRunning()) {
				Duration timeTakenToReady = Duration.ofNanos(System.nanoTime() - startTime);
				listeners.ready(context, timeTakenToReady);
			}
		}
		catch (Throwable ex) {
			if (ex instanceof AbandonedRunException) {
				throw ex;
			}
			handleRunFailure(context, ex, null);
			throw new IllegalStateException(ex);
		}
		return context;
	}

2.SpringApplication#run()=> SpringApplication#refreshContext(context)=> SpringApplication#refresh(context)=>ConfigurableApplicationContext#refresh()=>AbstractApplicationContext#refresh()

@Override
	public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");

			// 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);

				StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
				// Invoke factory processors registered as beans in the context.
				invokeBeanFactoryPostProcessors(beanFactory);

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

				// 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();
				contextRefresh.end();
			}
		}
	}

3.SpringApplication#run()=> SpringApplication#refreshContext(context)=> SpringApplication#refresh(context)=>ConfigurableApplicationContext#refresh()=>AbstractApplicationContext#refresh()=>AbstractApplicationContext#finishBeanFactoryInitialization()=>ConfigurableListableBeanFactory#preInstantiateSingletons()=>DefaultListableBeanFactory#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 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 SmartFactoryBean smartFactoryBean && smartFactoryBean.isEagerInit()) {
						getBean(beanName);
					}
				}
				else {
					// 此处就是初始化bean的方法
					getBean(beanName);
				}
			}
		}

		// Trigger post-initialization callback for all applicable beans...
		for (String beanName : beanNames) {
			// 此处就是解决循环依赖的代码
			Object singletonInstance = getSingleton(beanName);
			if (singletonInstance instanceof SmartInitializingSingleton smartSingleton) {
				StartupStep smartInitialize = this.getApplicationStartup().start("spring.beans.smart-initialize")
						.tag("beanName", beanName);
				smartSingleton.afterSingletonsInstantiated();
				smartInitialize.end();
			}
		}
	}

解决循环依赖的代码如下：

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
// 尝试从缓存中获取成品的目标对象，如果存在，则直接返回
  Object singletonObject = this.singletonObjects.get(beanName);
  // 如果缓存中不存在目标对象，则判断当前对象是否已经处于创建过程中，在前面的讲解中，第一次尝试获取A对象
  // 的实例之后，就会将A对象标记为正在创建中，因而最后再尝试获取A对象的时候，这里的if判断就会为true
  if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
    synchronized (this.singletonObjects) {
      singletonObject = this.earlySingletonObjects.get(beanName);
      if (singletonObject == null && allowEarlyReference) {
        // 这里的singletonFactories是一个Map，其key是bean的名称，而值是一个ObjectFactory类型的
        // 对象，这里对于A和B而言，调用图其getObject()方法返回的就是A和B对象的实例，无论是否是半成品
        ObjectFactory singletonFactory = this.singletonFactories.get(beanName);
        if (singletonFactory != null) {
          // 获取目标对象的实例
          singletonObject = singletonFactory.getObject();
          this.earlySingletonObjects.put(beanName, singletonObject);
          this.singletonFactories.remove(beanName);
        }
      }
    }
  }
  return singletonObject;
}

一级缓存，singletonObjects 单例缓存，存储已经实例化的单例bean。二级缓存，earlySingletonObjects 提前暴露的单例缓存，这里存储的bean是刚刚构造完成，但还会通过属性注入bean。三级缓存，singletonFactories 生产单例的工厂缓存，存储工厂。

解决原理如下：

在第一层中，先去获取 A 的 Bean，发现没有就准备去创建一个，然后将 A 的代理工厂放入“三级缓存”（这个 A 其实是一个半成品，还没有对里面的属性进行注入），但是 A 依赖 B 的创建，就必须先去创建 B；在第二层中，准备创建 B，发现 B 又依赖 A，需要先去创建 A，去创建 A，因为第一层已经创建了 A 的代理工厂，直接从“三级缓存”中拿到 A 的代理工厂，获取 A 的代理对象，放入“二级缓存”，并清除“三级缓存”；有了 A 的代理对象，对 A 的依赖完美解决（这里的 A 仍然是个半成品），B 初始化成功。在 B 初始化成功，完成 A 对象的属性注入，然后再填充 A 的其它属性，以及 A 的其它步骤（包括 AOP），完成对 A 完整的初始化功能（这里的 A 才是完整的 Bean）。将 A 放入“一级缓存”。

4.SpringApplication#run()=> SpringApplication#refreshContext(context)=> SpringApplication#refresh(context)=>ConfigurableApplicationContext#refresh()=>AbstractApplicationContext#refresh()=>AbstractApplicationContext#finishBeanFactoryInitialization()=>ConfigurableListableBeanFactory#preInstantiateSingletons()=>DefaultListableBeanFactory#preInstantiateSingletons()=>AbstractBeanFactory#getBean() => AbstractBeanFactory#doGetBean()=>AbstractBeanFactory#createBean()=>AbstractAutowireCapableBeanFactory#createBean()=>AbstractAutowireCapableBeanFactory#doCreateBean()

bean的生命周期：

1.调用InstantiationAwareBeanPostProcessor# postProcessBeforeInstantiation
跟进doCreateBean()

@Override  
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)  
      throws BeanCreationException {  
  
   if (logger.isTraceEnabled()) {  
      logger.trace("Creating instance of bean "" + beanName + """);  
   }  
   RootBeanDefinition mbdToUse = mbd;  
  
   // Make sure bean class is actually resolved at this point, and  
   // clone the bean definition in case of a dynamically resolved Class   // which cannot be stored in the shared merged bean definition.   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 {  
      // 1.调用InstantiationAwareBeanPostProcessor# postProcessBeforeInstantiation
      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 {  
	  // 跟进doCreateBean()
      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);  
   }  
}

2.创建bean实例

跟进populateBean()
跟进initializeBean()

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) { 
	  // 2.创建bean实例 
      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.markAsPostProcessed();  
      }  
   }  
  
   // Eagerly cache singletons to be able to resolve circular references  
   // even when triggered by lifecycle interfaces like BeanFactoryAware.   boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&  
         isSingletonCurrentlyInCreation(beanName));  
   if (earlySingletonExposure) {  
      if (logger.isTraceEnabled()) {  
         logger.trace("Eagerly caching bean "" + beanName +  
               "" to allow for resolving potential circular references");  
      }  
      addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));  
   }  
  
   // Initialize the bean instance.  
   Object exposedObject = bean;  
   try {
	  // 跟进populateBean()
      populateBean(beanName, mbd, instanceWrapper); 
      // 跟进initializeBean() 
      exposedObject = initializeBean(beanName, exposedObject, mbd);  
   }  
   catch (Throwable ex) {  
      if (ex instanceof BeanCreationException bce && beanName.equals(bce.getBeanName())) {  
         throw bce;  
      }  
      else {  
         throw new BeanCreationException(mbd.getResourceDescription(), beanName, ex.getMessage(), 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 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;  
}

3.调用InstantiationAwareBeanPostProcessor#postProcessAfterInstantiation

4.注入bean属性

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;  
      }  
   }  
  
   if (bw.getWrappedClass().isRecord()) {  
      if (mbd.hasPropertyValues()) {  
         throw new BeanCreationException(  
               mbd.getResourceDescription(), beanName, "Cannot apply property values to a record");  
      }  
      else {  
         // Skip property population phase for records since they are immutable.  
         return;  
      }  
   }  
  
      // 3.调用InstantiationAwareBeanPostProcessor# postProcessAfterInstantiation
      for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {  
         if (!bp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {  
            return;  
         }  
      }  
   }  
  
   PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);  
  
   int resolvedAutowireMode = mbd.getResolvedAutowireMode();  
   if (resolvedAutowireMode == AUTOWIRE_BY_NAME || resolvedAutowireMode == AUTOWIRE_BY_TYPE) {  
      MutablePropertyValues newPvs = new MutablePropertyValues(pvs);  
      // Add property values based on autowire by name if applicable.  
      if (resolvedAutowireMode == AUTOWIRE_BY_NAME) {  
         autowireByName(beanName, mbd, bw, newPvs);  
      }  
      // Add property values based on autowire by type if applicable.  
      if (resolvedAutowireMode == AUTOWIRE_BY_TYPE) {  
         autowireByType(beanName, mbd, bw, newPvs);  
      }  
      pvs = newPvs;  
   }  
   if (hasInstantiationAwareBeanPostProcessors()) {  
      if (pvs == null) { 
         pvs = mbd.getPropertyValues();  
      }  
      for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {  
         PropertyValues pvsToUse = bp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);  
         if (pvsToUse == null) {  
            return;  
         }  
         pvs = pvsToUse;  
      }  
   }  
  
   boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);  
   if (needsDepCheck) {  
      PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);  
      checkDependencies(beanName, mbd, filteredPds, pvs);  
   }  
  
   if (pvs != null) {
	  // 4.注入属性  
      applyPropertyValues(beanName, mbd, bw, pvs);  
   }  
}

5.设置Aware接口的属性

6.调用BeanPostProcessor的初始化前置方法

7.先((InitializingBean) bean).afterPropertiesSet(),后调用init-method方法，进行初始化操作

8.调用BeanPostProcessor的初始化后置方法

protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
    // 5.设置Aware接口的属性
   invokeAwareMethods(beanName, bean);  
  
   Object wrappedBean = bean;  
   if (mbd == null || !mbd.isSynthetic()) {
	  // 5.调用BeanPostProcessor的初始化前置方法  
      wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);  
   }  
  
   try {
	  // 6.调用init-method方法，进行初始化操作  
      invokeInitMethods(beanName, wrappedBean, mbd);  
   }  
   catch (Throwable ex) {  
      throw new BeanCreationException(  
            (mbd != null ? mbd.getResourceDescription() : null), beanName, ex.getMessage(), ex);  
   }  
   if (mbd == null || !mbd.isSynthetic()) { 
      // 7. 调用BeanPostProcessor的初始化后置方法
      wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);  
   }  
  
   return wrappedBean;  
}

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