最近把@Autowired注解的解析过程粗略的跟读了一下，记录一下解析流程。

BeanPostProcessor接口是Spring的Bean后置处理核心接口，在Bean初始化前后进行一些处理工作。很多解析器都实现了该接口，该接口有两个重要方法，即Bean初始化之前的postProcessBeforeInitialization和Bean初始化之后的postProcessAfterInitialization。

public interface BeanPostProcessor

{

Object postProcessBeforeInitialization(Object arg0, String arg1) throws BeansException;

Object postProcessAfterInitialization(Object arg0, String arg1) throws BeansException;

}

BeanPostProcessor的实现类：

1、定位注解解析器

既然BeanPostProcessor有那么多实现类，那具体是哪个处理器在解析@Autowired注解呢？写了一个单元测试类，自动注入一个car属性，故意没有通过任何方式初始化car的bean，所以解析器执行到自动注入@Autowired标记的属性时报错了，如下：

这样就能定位到是AutowiredAnnotationBeanPostProcessor在解析@Autowired标签：

Caused by: org.springframework.beans.factory.BeanCreationException: Could not autowire field: private com.bill99.bean.Car com.bill99.route.service.MyAutowiredTest.car; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [com.bill99.bean.Car] found for dependency: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}

at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:531)

at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:88)

at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:295)

... 26 more

Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [com.bill99.bean.Car] found for dependency: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}

at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoSuchBeanDefinitionException(DefaultListableBeanFactory.java:997)

at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:867)

at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:779)

at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:503)

2、解读注解解析器

这个序列图是通过调用栈梳理出来的，作为参考，下面对每一步进行解读：

从测试方法入口SpringJUnit4ClassRunner开始，调用栈先调用AbstractApplicationContext.refresh()//读取配置刷新容器，源码如下：

public void refresh() throws BeansException, IllegalStateException { synchronized (this.startupShutdownMonitor) { // 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);? // Invoke factory processors registered as beans in the context. invokeBeanFactoryPostProcessors(beanFactory);? // Register bean processors that intercept bean creation. registerBeanPostProcessors(beanFactory);? // 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(); } } }

重点看registerBeanPostProcessors//注册拦截bean创建的bean处理器，

可以看到有6类Bean处理器被加载，其中就有AutowiredAnnotationBeanPostProcessor。

再看finishBeanFactoryInitialization(beanFactory);//实例化所有剩余的非延迟加载的单实例，跟着序列图进入下一步：

beanFactory.preInstantiateSingletons();//实例化所有剩余的非延迟加载的单实例。

在这里debug一下，可以看到配置了car的bean和没配置car的bean，容器加载的区别，没配置car的情况下，除了自身的6个Bean，加上我的测试配置有12个bean：

配置了car之后，显示13个bean：

一层层调用到AbstractAutowireCapableBeanFactory.doCreateBean();//创建Bean

这个方法核心逻辑有两处：

1、populateBean(beanName,mbd, instanceWrapper)//设置Bean属性

上面报错的地方就是在populateBean流程中调用AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject方法中抛的错。因为测试类TxnRouteServiceTest有一个自动注入的属性car，而我没有以任何方式配置car的bean，所以初始化TxnRouteServiceTest的时候就会报NoSuchBeanDefinitionException：

org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [com.bill99.bean.Car] found for dependency: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}

从AutowiredAnnotationBeanPostProcessor的构造函数可以看出，这个解析器主要解析Autowired.class和Value.class的注解：

public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter

implements MergedBeanDefinitionPostProcessor, PriorityOrdered, BeanFactoryAware {

@SuppressWarnings("unchecked")

public AutowiredAnnotationBeanPostProcessor() {

this.autowiredAnnotationTypes.add(Autowired.class);

this.autowiredAnnotationTypes.add(Value.class);

try {

this.autowiredAnnotationTypes.add((Class<? extends Annotation>)

ClassUtils.forName("javax.inject.Inject", AutowiredAnnotationBeanPostProcessor.class.getClassLoader()));

logger.info("JSR-330 'javax.inject.Inject' annotation found and supported for autowiring");

}

catch (ClassNotFoundException ex) {

// JSR-330 API not available - simply skip.

}

}

}

debug调用栈信息如下：

2、initializeBean()//初始化Bean

initializeBean包含以下三部分：

{

applyBeanPostProcessorsBeforeInitialization//执行Bean初始化之前的操作，比如@PostConstruct注解标记的初始化方法

invokeInitMethods//执行Bean初始化

applyBeanPostProcessorsAfterInitialization//执行Bean初始化之后的操作

}

现在猜测一下@PostConstruct和@PreDestroy是哪个处理器在处理？因为这个跟初始化和销毁有关，我们顺着这个猜测，可以先看看是不是InitDestroyAnnotationBeanPostProcessor解析，发现不是它解析，看它的子类，最后定位到是CommonAnnotationBeanPostProcessor在处理：

public CommonAnnotationBeanPostProcessor() {

this.setOrder(2147483644);

this.setInitAnnotationType(PostConstruct.class);

this.setDestroyAnnotationType(PreDestroy.class);

this.ignoreResourceType("javax.xml.ws.WebServiceContext");

}

以上就是对@Autowired注解的解析过程粗略的跟读，按此方法同样的可以分析Spring对其他注解的解析。

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