揭密springboot自动装配（5）--ioc及@Autowired注解

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@Autowired 注解的bean什么时候实例化给它？

我们直接从AbstractAutowireCapableBeanFactory.doCreateBean开始，这个方法从上一章内容可得知是创建实例化对象然后放入三级缓存的singletonFactories里面，我们接着这个方法继续深究

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @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);}final 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.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 {populateBean(beanName, mbd, instanceWrapper);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 " +"'getBeanNamesOfType' 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;}

这里可以看到bean实例化之后放入三级缓存接着就会进入populateBean方法进行判断该实例化对象是否有@Autowired注解，有的话会进行@Autowired的对应的bean的获取或创建，废话不多说，直接看代码：下面populateBean部分关键代码

AbstractAutowireCapableBeanFactory.populateBean方法我们看如图，遍历所有beanPostProcessors

这里的beanPostProcessors是在AbstractApplicationContext.refresh方法里registerBeanPostProcessors(beanFactory);放进去的，里面有对应的AutowiredAnnotationBeanPostProcessor，这个就是专门处理@Autowired注解的

接着调用PropertyValues pvsToUse = （AutowiredAnnotationBeanPostProcessor）ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);

这个方法里面可看到findAutowiringMetadata进行查询所有注解信息，可以看到拿到了usera这个属性

private com.example.demo.service.UserA com.example.demo.service.UserB.usera

我们进入AutowiredAnnotationBeanPostProcessor.findAutowiringMetadata

接着AutowiredAnnotationBeanPostProcessor.buildAutowiringMetadata

AutowiredAnnotationBeanPostProcessor.findAutowiredAnnotation

this.autowiredAnnotationTypes这里面放的就是在AbstractApplicationContext.refresh方法里registerBeanPostProcessors(beanFactory)构造AutowiredAnnotationBeanPostProcessor时放入的@Autowired