Spring Ioc 之 Bean的加载（四）：createBean()

createBean()

代码：

//AbstractAutowireCapableBeanFactory.java

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

	if (logger.isDebugEnabled()) {
		logger.debug("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.
	//判断需要创建的Bean是否可以实例化，即是否可以通过当前的类加载器加载
	Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
	if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
		mbdToUse = new RootBeanDefinition(mbd);
		mbdToUse.setBeanClass(resolvedClass);
	}

	// Prepare method overrides.
	//校验和准备Bean中的方法覆盖
	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.
		//如果Bean配置了初始化前和初始化后的处理器，则试图返回一个需要创建Bean的代理对象
		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 {
		//创建Bean的入口
		Object beanInstance = doCreateBean(beanName, mbdToUse, args);
		if (logger.isDebugEnabled()) {
			logger.debug("Finished creating instance of bean '" + beanName + "'");
		}
		return beanInstance;
	}
	catch (BeanCreationException ex) {
		// A previously detected exception with proper bean creation context already...
		throw ex;
	}
	catch (ImplicitlyAppearedSingletonException ex) {
		// An IllegalStateException to be communicated up to DefaultSingletonBeanRegistry...
		throw ex;
	}
	catch (Throwable ex) {
		throw new BeanCreationException(
				mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
	}
	}

这段代码分为以下几个步骤：

判断需要创建的Bean是否可以实例化，即是否可以通过当前的类加载器加载
校验和准备Bean中的方法注入
如果Bean配置了初始化前和初始化后的处理器，则试图返回一个需要创建Bean的代理对象
创建Bean

第1步

主要是获取bean的class，并设置到BeanDefinition中

第2步

主要是处理方法注入
代码：

public void prepareMethodOverrides() throws BeanDefinitionValidationException {
	// Check that lookup methods exists.
	//检测是否存在方法注入,并循环预处理方法注入
	if (hasMethodOverrides()) {
		Set<MethodOverride> overrides = getMethodOverrides().getOverrides();
		synchronized (overrides) {
		        //遍历处理
			for (MethodOverride mo : overrides) {
				prepareMethodOverride(mo);
			}
		}
	}
	}

prepareMethodOverride(mo)：

protected void prepareMethodOverride(MethodOverride mo) throws BeanDefinitionValidationException {
        // 统计注入的方法个数
	int count = ClassUtils.getMethodCountForName(getBeanClass(), mo.getMethodName());
	if (count == 0) {
		throw new BeanDefinitionValidationException(
				"Invalid method override: no method with name '" + mo.getMethodName() +
				"' on class [" + getBeanClassName() + "]");
	}
        // 如果为1,则将注入方法标记为未重载
	// 注意:当有多个重载方法时,为了确定调用哪个具体的方法,Spring对重载方法的参数解析是很复杂的
	// 所以,如果注入方法没有被重载这里就将其标记,省去了对方法参数的解析过程,直接调用即可
	else if (count == 1) {
		// Mark override as not overloaded, to avoid the overhead of arg type checking.
		mo.setOverloaded(false);
	}
	}

代码读到这里，大家可能有疑问，从代码上看明明是处理的方法重载，但是为什么处理的是方法注入呢？而且如果我们在bean里设置几个方法重载的话，hasMethodOverrides()方法返回的是false。如果我们打开 AbstractBeanDefinition 类的 hasMethodOverrides() 方法，就能打消我们之前的疑问。

public boolean hasMethodOverrides() {
		return (this.methodOverrides != null && !this.methodOverrides.isEmpty());
	}

其中methodOverrides是做什么的呢？通过类名AbstractBeanDefinition我们可以发现，该类是BeanDefinition的一个子类，那么它保存的应该是我们解析到的beanDefinition，spring在解析配置文件的时候，如果发现配置了replace-method或者lookup-method那么，就会对应的标签解析，并存入到 AbstractBeanDefinition 的 methodOverrides 属性中，那么当bean实例化的时候，如果检测到了methodOverrides属性不为空，则动态的为当前bean生成代理并使用相应的拦截器对bean做处理，这里大家只要把概念搞清楚即可。

第3步

主要是对bean前置后置处理器的处理，给 BeanPostProcessors 后置处理器一个返回代理对象的机会
详细代码：

protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
	Object bean = null;
	if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
		// Make sure bean class is actually resolved at this point.
		if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
			Class<?> targetType = determineTargetType(beanName, mbd);
			if (targetType != null) {
				bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
				if (bean != null) {
					bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
				}
			}
		}
		mbd.beforeInstantiationResolved = (bean != null);
	}
	return bean;
	}

如果代理对象不为空，则直接返回代理对象，这一步骤有非常重要的作用，Spring 后续实现 AOP 就是基于这个地方判断的。
这个方法核心就在于 applyBeanPostProcessorsBeforeInstantiation()和 applyBeanPostProcessorsAfterInitialization() 两个方法，before 为实例化前的后处理器应用，after 为实例化后的后处理器应用。

第4步

doCreateBean()
创建Bean

//真正创建Bean的方法
	protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
		throws BeanCreationException {

	// Instantiate the bean.
	//BeanWrapper是对Bean的包装，其接口中所定义的功能很简单包括设置获取被包装的对象，获取被包装bean的属性描述器
	BeanWrapper instanceWrapper = null;
	if (mbd.isSingleton()) {
	        //单例模式，删除factoryBean缓存
		instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
	}
	if (instanceWrapper == null) {
	        //使用合适的实例化策略来创建Bean：工厂方法、构造函数自动注入、简单初始化
		instanceWrapper = createBeanInstance(beanName, mbd, args);
	}
	//从包装类中获取实例化的Bean
	final Object bean = instanceWrapper.getWrappedInstance();
	//获取实例化对象的类型
	Class<?> beanType = instanceWrapper.getWrappedClass();
	if (beanType != NullBean.class) {
		mbd.resolvedTargetType = beanType;
	}

	// Allow post-processors to modify the merged bean definition.
	//检查是否有后置处理
	synchronized (mbd.postProcessingLock) {
		if (!mbd.postProcessed) {
			try {
			        //调用PostProcessor后置处理器，修改 BeanDefinition
				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.
	// 解决单例模式的循环依赖
	boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
			isSingletonCurrentlyInCreation(beanName));
	if (earlySingletonExposure) {
		if (logger.isDebugEnabled()) {
			logger.debug("Eagerly caching bean '" + beanName +
					"' to allow for resolving potential circular references");
		}
		//这里是一个匿名内部类，为了防止循环引用，尽早持有对象的引用
		addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
	}

	// Initialize the bean instance.
	//Bean对象的初始化，依赖注入在此触发
	//这个exposedObject在初始化完成之后返回作为依赖注入完成后的Bean
	Object exposedObject = bean;
	try {
		//将Bean实例对象封装，并且Bean定义中配置的属性值赋值给实例对象
		populateBean(beanName, mbd, instanceWrapper);
		//初始化Bean对象
		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) {
		//获取指定名称的已注册的单例模式Bean对象
		Object earlySingletonReference = getSingleton(beanName, false);
		if (earlySingletonReference != null) {
			//根据名称获取的已注册的Bean和正在实例化的Bean是同一个
			if (exposedObject == bean) {
				//当前实例化的Bean初始化完成
				exposedObject = earlySingletonReference;
			}
			//当前Bean依赖其他Bean，并且当发生循环引用时不允许新创建实例对象
			else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
				String[] dependentBeans = getDependentBeans(beanName);
				Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
				//获取当前Bean所依赖的其他Bean
				for (String dependentBean : dependentBeans) {
					//对依赖Bean进行类型检查
					if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
						actualDependentBeans.add(dependentBean);
					}
				}
				if (!actualDependentBeans.isEmpty()) {
					throw new BeanCurrentlyInCreationException(beanName,
							"Bean with name '" + beanName + "' has been injected into other beans [" +
							StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
							"] in its raw version as part of a circular reference, but has eventually been " +
							"wrapped. This means that said other beans do not use the final version of the " +
							"bean. This is often the result of over-eager type matching - consider using " +
							"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
				}
			}
		}
	}

	// Register bean as disposable.
	//注册完成依赖注入的Bean
	try {
		registerDisposableBeanIfNecessary(beanName, bean, mbd);
	}
	catch (BeanDefinitionValidationException ex) {
		throw new BeanCreationException(
				mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
	}

	return exposedObject;
	}

代码很长，不过别慌，我们来按步骤分析一波

如果是单例模式，从factoryBeanInstanceCache 缓存中获取BeanWrapper 实例对象并删除缓存
调用 createBeanInstance() 实例化 bean
（主要是将 BeanDefinition 转换为 BeanWrapper）
后置处理
单例模式的循环依赖处理
初始化 bean 实例对象
（属性填充）
依赖检查
注册 DisposableBean

doCreateBean() 完成 bean 的创建和初始化工作，内容太多，比较复杂，这里只列出大致流程，接下来我们将分几篇文章来分别阐述相关内容。

星爵22

Spring Ioc 之 Bean的加载（四）：createBean()

createBean()

第1步

第2步

第3步

第4步