SpringBoot中@Autowired生效方式详解-Java开发

@Autowired注解可以用在类属性，构造函数，setter方法和函数参数上，该注解可以准确地控制bean在何处如何自动装配的过程。在默认情况下，该注解是类型驱动的注入

前言

@Component
public class SimpleBean3 {
    @Autowired
    private SimpleBean simpleBean;
}

@Autowired修饰的字段会被容器自动注入.那么Spring Boot中使如何实现这一功能呢? AutowiredAnnotationBeanPostProcessor!

BeanPostProcessor implementation that autowires annotated fields, setter methods, and arbitrary config methods. Such members to be injected are detected through annotations: by default, Spring's @Autowired and @Value annotations.
Also supports JSR-330's @Inject annotation, if available, as a direct alternative to Spring's own @Autowired.

AutowiredAnnotationBeanPostProcessor(以下简称AutowiredProcessor)间接实现了InstantiationAwareBeanPostProcessor接口.通过postProcessProperties(...)完成@Autowired的注入,本文将按照下图流程梳理AutowiredProcessor的生效逻辑.

SpringBoot-autowired.png

正文

注册AutowiredProcessor的BeanDefinition

SpringApplication#createApplicationContext默认会创建 AnnotationConfigApplicationContext,而AnnotationConfigApplicationContext又会创建AnnotatedBeanDefinitionReader

    public AnnotationConfigApplicationContext() {
        this.reader = new AnnotatedBeanDefinitionReader(this);
        this.scanner = new ClassPathBeanDefinitionScanner(this);
    }

AnnotatedBeanDefinitionReader构造时会调用AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry),将AutowiredProcessor的BeanDefinition注册到容器

    public static Set<BeanDefinitionHolder> registerAnnotationConfigProcessors(
            BeanDefinitionRegistry registry, @Nullable Object source) {
        //忽略部分代码...
        if (!registry.containsBeanDefinition(AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME)) {
            RootBeanDefinition def = new RootBeanDefinition(AutowiredAnnotationBeanPostProcessor.class);
            def.setSource(source);
            beanDefs.add(registerPostProcessor(registry, def, AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME));
        }
        //忽略部分代码...
        return beanDefs;
    }

实例化AutowiredProcessor

在AbstractApplicationContext的refresh阶段,会注册并实例化所有的BeanPostProcessor

public void refresh() throws BeansException, IllegalStateException {
            //...忽略部分代码
                // Allows post-processing of the bean factory in context subclasses.
                postProcessBeanFactory(beanFactory);
                // Invoke factory processors registered as beans in the context.
                invokeBeanFactoryPostProcessors(beanFactory);
                // ########### 这里注册所有的BeanPostProcessor ##########  
                // 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()
            //...忽略部分代码
    }

实际的注册逻辑交给了PostProcessorRegistrationDelegate

    protected void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory) {
        PostProcessorRegistrationDelegate.registerBeanPostProcessors(beanFactory, this);
    }

在PostProcessorRegistrationDelegate中,获取到所有的BeanPostProcessor(基于BeanDefinition),并将其分为几种类型,并按照不同的优先级进行处理化,这块不是这篇文章的重点,我们只需要知道在这里AutowiredProcessor被注册就可以了.

创建bean时进行注入

以SimpleBean3的注入为例, 它是单例的,在AbstractApplicationContext.refresh()的finishBeanFactoryInitialization(beanFactory)时创建.

    protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
        //...忽略部分代码
        // Instantiate all remaining (non-lazy-init) singletons.
        beanFactory.preInstantiateSingletons();
    }

调用到了BeanFactory.preInstantiateSingletons(),走到getBean()逻辑

public void preInstantiateSingletons() throws BeansException {
        //...忽略部分代码
        List<String> 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 FactoryBean) {
                        final 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);
                }
            }
        }
        //...忽略部分代码
    }

经过一连串的辗转,最终调用到AbstractAutowireCapableBeanFactory#populateBean

附上调用链路

image.png

在populateBean中,会将所有的BeanPostProcessor应用在这个bean上,包括AutowiredProcessor

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
        //...忽略部分代码
        PropertyDescriptor[] filteredPds = null;
        if (hasInstAwareBpps) {
            if (pvs == null) {
                pvs = mbd.getPropertyValues();
            }
            for (BeanPostProcessor bp : getBeanPostProcessors()) {
                if (bp instanceof InstantiationAwareBeanPostProcessor) {
                    InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
          //###### 调用到postProcessProperties #####
                    PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
                    if (pvsToUse == null) {
                        if (filteredPds == null) {
                            filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
                        }
                        pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
                        if (pvsToUse == null) {
                            return;
                        }
                    }
                    pvs = pvsToUse;
                }
            }
        }
    //...忽略部分代码
    }

AutowiredProcessor的postProcessProperties()会进行注入操作,这需要找到注入的元数据(InjectionMetadata)

public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {     
        //### 找到AutowringMetadata #####
        InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
        try {
      // #### 注入 ###
            metadata.inject(bean, beanName, pvs);
        }
        catch (BeanCreationException ex) {
            throw ex;
        }
        catch (Throwable ex) {
            throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
        }
        return pvs;
    }

findAutowiringMetadata()又调用到buildAutowiringMetadata(),生成代表可注入元素的InjectMetadata

private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
        List<InjectionMetadata.InjectedElement> elements = new ArrayList<>();
        Class<?> targetClass = clazz;
        do {
            final List<InjectionMetadata.InjectedElement> currElements = new ArrayList<>();
            ReflectionUtils.doWithLocalFields(targetClass, field -> {
        //###### 找到带有可注入注解的字段
                AnnotationAttributes ann = findAutowiredAnnotation(field);
                if (ann != null) {
                    if (Modifier.isStatic(field.getModifiers())) {
                        if (logger.isInfoEnabled()) {
                            logger.info("Autowired annotation is not supported on static fields: " + field);
                        }
                        return;
                    }
                    boolean required = determineRequiredStatus(ann);
                    currElements.add(new AutowiredFieldElement(field, required));
                }
            });
            //...忽略部分代码
    }

findAutowiredAnnotation()根据AutowiredProcessor的实例字段autowiredAnnotationTypes,去查看是否匹配,这个字段是在AutowiredProcessor创建时初始化,可以看到支持@Autowired,@Value,@Inject三种类型的注入标识.最终据此完成注入

public AutowiredAnnotationBeanPostProcessor() {
        this.autowiredAnnotationTypes.add(Autowired.class);
        this.autowiredAnnotationTypes.add(Value.class);
        try {
            this.autowiredAnnotationTypes.add((Class&lt;? extends Annotation&gt;)
                    ClassUtils.forName("javax.inject.Inject", AutowiredAnnotationBeanPostProcessor.class.getClassLoader()));
            logger.trace("JSR-330 'javax.inject.Inject' annotation found and supported for autowiring");
        }
        catch (ClassNotFoundException ex) {
            // JSR-330 API not available - simply skip.
        }
    }

后记

最后再来梳理一下整个流程.

首先是AutowiredPorcessor的BeanDefinition的注册

=> 创建ApplicationContext

=> 创建AnnotatedBeanDefinitionReader

=> 注册BeanDefinition registerAnnotationConfigProcessors

然后是AutowiredProcessor注册为bean

=> registerBeanPostProcessors

最后是注入

=> 获取bean getBean()

=> 创建bean doCreateBean()

=>生成bean populateBean()

=> 应用AutowiredProcessor ibp.postProcessProperties()

=> 找到可注入的字段 buildAutowiringMetadata

=> 注入 metadata.inject

至此,@Autowired生效逻辑梳理完成

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