引言

这个部分提出问题，引发思考

  1. 单例bean和原型bean的区别以及创建过程的差别？

上文springboot源码之启动流程2中几乎已经走完spring启动流程，但是遗留了两处逻辑

todo 回顾启动流程！

  1. PostProcessorRegistrationDelegate#invokeBeanFactoryPostProcessors
  2. ServletWebServerApplicationContext#finishBeanFactoryInitialization

现在来看bean的创建流程：

ServletWebServerApplicationContext#finishBeanFactoryInitialization

完成bean factory的上下文初始化，初始化所有非懒加载的单例。

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/**
 * Finish the initialization of this context's bean factory,
 * initializing all remaining singleton beans.
 */
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
	// 初始化 上下文转换服务
	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));
	}

	// 如果没有BeanFactoryPostProcessor，则注入一个默认的值解析器
	// 主要用于解析注解注入的属性值
	if (!beanFactory.hasEmbeddedValueResolver()) {
		beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
	}

	// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
       // 初始化 LoadTimeWeaverAware 以提前注册 转化器
	String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
	for (String weaverAwareName : weaverAwareNames) {
		getBean(weaverAwareName);
	}

	// 不再使用临时的类加载器做类型匹配
	beanFactory.setTempClassLoader(null);

	// 禁止修改所有bean definition，不允许再修改
	beanFactory.freezeConfiguration();

	// 实例化所有非懒加载的单例
	beanFactory.preInstantiateSingletons();
}

DefaultListableBeanFactory#preInstantiateSingletons

确保所有(包含FactoryBeans)非懒加载的单例被初始化

1.
2.
3. 获取bean，针对beanfactory是否是SmartFactoryBean接口的实现类进行判断。最后会调用getBean方法。

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@Override
public void preInstantiateSingletons() throws BeansException {
	if (logger.isTraceEnabled()) {
		logger.trace("Pre-instantiating singletons in " + this);
	}

	// 对副本进行迭代以允许init方法注册新的bean定义
	// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
	List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

	// 调用 非懒加载的bean的初始化
	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);
			}
		}
	}

	// 触发后置初始化回调函数
	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();
			}
		}
	}
}

先来看比较常见的方法getMergedLocalBeanDefinition，bean定义的抽象类是AbstractBeanDefinition， 普通的 bean在spring加载bean定义的时候是GenericBeanDefinition，而spring上下文实例化所有bean使用的则是RootBeanDefinition，整个过程由getMergedLocalBeanDefinition完成，将非RootBeanDefinition转化为RootBeanDefinition以供后续操作

遍历所有bean，获取其beanDefinition，如果没有就找其父类的beanDefinition。

preInstantiateSingletons方法完成所有非懒加载的实例的创建。实例化bean过程需要满足如下三个条件：

  • 不是抽象类
  • 是单例对象
  • 非懒加载

后续逻辑，先判断一下该bean是否是FacctoryBean的实现，然后进一步判断是否是 SmartFactoryBean实现，其中SmartFactoryBean定义了isEagerInit方法，如果为True则立即实例化该bean。如果该bean不是BeanFactory的实现，也会实例化该bean。

AbstractBeanFactory#isFactoryBean
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@Override
public boolean isFactoryBean(String name) throws NoSuchBeanDefinitionException {
       // 获取bean的真实名
	String beanName = transformedBeanName(name);
       // 获取 工厂bean 单例
	Object beanInstance = getSingleton(beanName, false);
	if (beanInstance != null) {
		return (beanInstance instanceof FactoryBean);
	}
	// No singleton instance found -> check bean definition.
	if (!containsBeanDefinition(beanName) && getParentBeanFactory() instanceof ConfigurableBeanFactory) {
		// No bean definition found in this factory -> delegate to parent.
		return ((ConfigurableBeanFactory) getParentBeanFactory()).isFactoryBean(name);
	}
	return isFactoryBean(beanName, getMergedLocalBeanDefinition(beanName));
}
DefaultSingletonBeanRegistry#getSingleton

SingletonBeanRegistry是SingletonBeanRegistry接口定义的方法，如下是DefaultSingletonBeanRegistry的实现

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/**
 * Return the (raw) singleton object registered under the given name.
 * <p>Checks already instantiated singletons and also allows for an early
 * reference to a currently created singleton (resolving a circular reference).
 * @param beanName the name of the bean to look for
 * @param allowEarlyReference whether early references should be created or not
 * @return the registered singleton object, or {@code null} if none found
 */
@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
	// Quick check for existing instance without full singleton lock
	Object singletonObject = this.singletonObjects.get(beanName);
	if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
		singletonObject = this.earlySingletonObjects.get(beanName);
		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;
}

AbstractBeanFactory#doGetBean

doGetBean方法由AbstractBeanFactory定义，有不同的实现，用以获取bean，其代码如下：

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/**
	 * Return an instance, which may be shared or independent, of the specified bean.
	 * @param name the name of the bean to retrieve
	 * @param requiredType the required type of the bean to retrieve
	 * @param args arguments to use when creating a bean instance using explicit arguments
	 * (only applied when creating a new instance as opposed to retrieving an existing one)
	 * @param typeCheckOnly whether the instance is obtained for a type check,
	 * not for actual use
	 * @return an instance of the bean
	 * @throws BeansException if the bean could not be created
	 */
	@SuppressWarnings("unchecked")
	protected <T> T doGetBean(
			String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
			throws BeansException {

		String beanName = transformedBeanName(name);
		Object bean;

		// Eagerly check singleton cache for manually registered singletons.
		Object sharedInstance = getSingleton(beanName);
		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 + "'");
				}
			}
			bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
		}

		else {
			// Fail if we're already creating this bean instance:
			// We're assumably within a circular reference.
            // 如果已经创建过这个bean实例，那假设处在循环引用 
            // 如果 beanName 正在被通过 原型模式创建，则抛出异常，无法应对循环依赖问题
			if (isPrototypeCurrentlyInCreation(beanName)) {
				throw new BeanCurrentlyInCreationException(beanName);
			}

			// 检查beanfactory中是否已经存在该 bean definition， 如果该 bean factory中没有该bean definition ，
            // 则查找其parent bean factory中是否有该 bean definition
			BeanFactory parentBeanFactory = getParentBeanFactory();
			if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
				// Not found -> check parent.
				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.
					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) {
				markBeanAsCreated(beanName);
			}

			try {
				RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                // 再次检查该 bean definition 是否是 抽象类，如果是，则抛出异常
				checkMergedBeanDefinition(mbd, beanName, args);

                // 确保 实例化该bean对象所依赖的bean 都已经被注册
				String[] dependsOn = mbd.getDependsOn();
				if (dependsOn != null) {
					for (String dep : dependsOn) {
                        // 确认指定的依赖bean是否已经注册给该bean，或者任何其可传递项
						if (isDependent(beanName, dep)) {
							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
						}
                        // 如果没有注册过，则注册依赖的bean
						registerDependentBean(dep, beanName);
						try {
							getBean(dep);
						}
						catch (NoSuchBeanDefinitionException ex) {
							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
						}
					}
				}

				// 创建 bean 实例， 有两种典型模式
				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);
				}

				else if (mbd.isPrototype()) {
					// It's a prototype -> create a new instance.
					Object prototypeInstance = null;
					try {
						beforePrototypeCreation(beanName);
						prototypeInstance = createBean(beanName, mbd, args);
					}
					finally {
						afterPrototypeCreation(beanName);
					}
					bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
				}
				// 其他场景，目前没有实现
				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) {
                // 在bean创建失败后，对缓存的元数据执行适当的清理。
				cleanupAfterBeanCreationFailure(beanName);
				throw ex;
			}
		}

        // 检查 期望的类型 和 当前bean实例的类型是否匹配
		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;
	}

第9行中通过调用getSingleton检查本地的单例缓存是否已经加载过bean，没有则检查earlySingletonObjects中是否已经加载过bean。如果该bean对象不为null则调用getObjectForBeanInstance方法获取bean。否则执行如下逻辑：

  1. 调用 mbd.getDependsOn()保证当前bean 依赖的bean会优先于当前bean被加载。
  2. 根据beandefinition的定义，按照单例/原型模式来创建bean。单例bean和原型bean的创建过程如下。

单例bean的创建过程

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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);
}

抽象类AbstractBeanFactory中定义了抽象方法createBean，由子类AbstractAutowireCapableBeanFactory进行重写。

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/**
	 * Central method of this class: creates a bean instance,
	 * populates the bean instance, applies post-processors, etc.
	 * @see #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 {
			// 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);
		}
	}

其中会再次调用doCreateBean的重载方法：

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/**
	 * Actually create the specified bean. Pre-creation processing has already happened
	 * at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.
	 * <p>Differentiates between default bean instantiation, use of a
	 * factory method, and autowiring a constructor.
	 * @param beanName the name of the bean
	 * @param mbd the merged bean definition for the bean
	 * @param args explicit arguments to use for constructor or factory method invocation
	 * @return a new instance of the bean
	 * @throws BeanCreationException if the bean could not be created
	 * @see #instantiateBean
	 * @see #instantiateUsingFactoryMethod
	 * @see #autowireConstructor
	 */
	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) {
			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 {
                    // 允许后置处理器修改合并后的bean definition
					applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
				}
				catch (Throwable ex) {
					throw new BeanCreationException(mbd.getResourceDescription(), beanName,
							"Post-processing of merged bean definition failed", ex);
				}
				mbd.postProcessed = true;
			}
		}

        // 如果 bdf是单例且允许循环引用且当前该bean处于创建过程中，则使用三级缓存，预先将该bean创建所使用的beanfactory存入三级缓存中
		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 {
            // 填充field属性
			populateBean(beanName, mbd, instanceWrapper);
            // 初始化该bean，包含bean的注入等
			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 " +
								"'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;
	}

根据beandefinition是否是单例，如果是则从factoryBeanInstanceCache移除该instanceWrapper，如果instanceWrapper为空，则调用createBeanInstance创建bean实例并包装为beanwrapper。createBeanInstance函数代码如下：

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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?
	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.
	return instantiateBean(beanName, mbd);
}

继续往下看，instantiateBean函数通过调用instantiate函数根据beanName和beanDefinition创建bean实例并调用initBeanWrapper将该bean包装为为BeanWrapper返回。

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/**
	 * Instantiate the given bean using its default constructor.
	 * @param beanName the name of the bean
	 * @param mbd the bean definition for the bean
	 * @return a BeanWrapper for the new instance
	 */
	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 {
				beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, this);
			}
			BeanWrapper bw = new BeanWrapperImpl(beanInstance);
			initBeanWrapper(bw);
			return bw;
		}
		catch (Throwable ex) {
			throw new BeanCreationException(
					mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
		}
	}

SimpleInstantiationStrategy#instantiate 

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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 {
						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);
	}
}

如果该beandefinition没有被重写过，则在调用BeanUtils.instantiateClass创建bean实例之前选择一个构造方法；否则抛出异常。

BeanUtils#instantiateClass

该方法通过反射创建bean实例，通过调用ReflectionUtils.makeAccessible(ctor)，即使该bean的构造方法是私有的也可以用来创建实例。

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/**
 * Convenience method to instantiate a class using the given constructor.
 * <p>Note that this method tries to set the constructor accessible if given a
 * non-accessible (that is, non-public) constructor, and supports Kotlin classes
 * with optional parameters and default values.
 * @param ctor the constructor to instantiate
 * @param args the constructor arguments to apply (use {@code null} for an unspecified
 * parameter, Kotlin optional parameters and Java primitive types are supported)
 * @return the new instance
 * @throws BeanInstantiationException if the bean cannot be instantiated
 * @see Constructor#newInstance
 */
public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException {
	Assert.notNull(ctor, "Constructor must not be null");
	try {
		ReflectionUtils.makeAccessible(ctor);
		if (KotlinDetector.isKotlinReflectPresent() && KotlinDetector.isKotlinType(ctor.getDeclaringClass())) {
			return KotlinDelegate.instantiateClass(ctor, args);
		}
		else {
			Class<?>[] parameterTypes = ctor.getParameterTypes();
			Assert.isTrue(args.length <= parameterTypes.length, "Can't specify more arguments than constructor parameters");
			Object[] argsWithDefaultValues = new Object[args.length];
			for (int i = 0 ; i < args.length; i++) {
				if (args[i] == null) {
					Class<?> parameterType = parameterTypes[i];
					argsWithDefaultValues[i] = (parameterType.isPrimitive() ? DEFAULT_TYPE_VALUES.get(parameterType) : null);
				}
				else {
					argsWithDefaultValues[i] = args[i];
				}
			}
			return ctor.newInstance(argsWithDefaultValues);
		}
	}
	catch (InstantiationException ex) {
		throw new BeanInstantiationException(ctor, "Is it an abstract class?", ex);
	}
	catch (IllegalAccessException ex) {
		throw new BeanInstantiationException(ctor, "Is the constructor accessible?", ex);
	}
	catch (IllegalArgumentException ex) {
		throw new BeanInstantiationException(ctor, "Illegal arguments for constructor", ex);
	}
	catch (InvocationTargetException ex) {
		throw new BeanInstantiationException(ctor, "Constructor threw exception", ex.getTargetException());
	}
}

原型bean的创建

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else if (mbd.isPrototype()) {
	// It's a prototype -> create a new instance.
       // 如果是原型模式，则创建一个新对象
	Object prototypeInstance = null;
	try {
		beforePrototypeCreation(beanName);
		prototypeInstance = createBean(beanName, mbd, args);
	}
	finally {
		afterPrototypeCreation(beanName);
	}
	bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}

引用

1. SpringBoot深入学习（一）– refresh()

2. Spring中BeanFactory和FactoryBean的区别以及使用场景

3. Spring的单例bean与原型bean的区别和创建过程

4. 【Spring源码分析】非懒加载的单例Bean初始化过程（上篇）