Spring ApplicationContext 是如何被注入的

//ERROR No qualifying bean of type 'org.springframework.context.ApplicationContext' available
applicationContext.getBean(ApplicationContext.class);

//SUCCESS
@Component
public class SimpleBean3 {
    @Autowired
    private ApplicationContext applicationContext;  
    @Autowired
    private SimpleBean2 simpleBean2;
}

ApplicationContext是Spring中的重要组件,它不是bean,因此无法通过getBean获取它,但是可以通过Autowired注入获得,其中必定有特殊的处理。

普通Bean的元数据存放在DefaultListableBeanFactory的beanDefinitionNames和beanDefinitionMap,普通Bean通过遵照Spring提供的机制自动注册添加,这是Spring提供的功能。

private volatile List<String> beanDefinitionNames = new ArrayList<>(256);
private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>(256);

ApplicationContext和BeanFactory存储在DefaultListableBeanFactory的resolvableDependencies,它们需要手动注册添加,这是Spring的框架内部逻辑

private final Map<Class<?>, Object> resolvableDependencies = new ConcurrentHashMap<>(16);

在查找依赖时,会同时搜寻beanDefinitionNames和resolvableDependencies,因此ApplicationContext也能被查找到。

而getBean时只会查找上面的BeanDefinitionMap,因此找不到ApplicationContext。

注入流程
注册 ApplicationContext 为 resolvableDependencies

在 AbstractApplicationContext.prepareBeanFactory() 中, ApplicationContext 被注册到 resolvableDependencies 中。

protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
        //...忽略部分代码
    
        // BeanFactory interface not registered as resolvable type in a plain factory.
        // MessageSource registered (and found for autowiring) as a bean.
        beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);
        beanFactory.registerResolvableDependency(ResourceLoader.class, this);
        beanFactory.registerResolvableDependency(ApplicationEventPublisher.class, this);
        beanFactory.registerResolvableDependency(ApplicationContext.class, this);
        //...忽略部分代码
    }

生成Bean时查找依赖

带有 @Autowired 字段的在 AutowiredAnnotationPostProcessor.postProcessProperties() 中完成注入,查找依赖的入口就在 metadata.inject(bean, beanName, pvs)

    public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
        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;
    }

和注入相关的流程如下

    AutowiredAnnotationBeanPostProcesspr.postProcessProperties()

    =>InjectionMetadata.inject()

    ==>AutowiredFieldElement.inject()

    ===>DefaultListableBeanFactory.resolveDependency()

    ====>DefaultListableBeanFactory.doResolveDependency()

    =====>DefaultListableBeanFactory.findAutowireCandidates()

我们直接跳到 DefaultListableBeanFactory.findAutowireCandidates(), 可以看到是同时从 BeanDefinitionNames和resolvableDependencies两个地方,因此可以找到ApplicationContext.

    protected Map<String, Object> findAutowireCandidates(
            @Nullable String beanName, Class<?> requiredType, DependencyDescriptor descriptor) {
        String[] candidateNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
                this, requiredType, true, descriptor.isEager());
        Map<String, Object> result = new LinkedHashMap<>(candidateNames.length);
        //从 resolvableDependencies 中寻找
        for (Map.Entry<Class<?>, Object> classObjectEntry : this.resolvableDependencies.entrySet()) {
            //...
        }
        //从 BeanDefinitionNames 中寻找
        for (String candidate : candidateNames) {
            //...
        }
        return result;
    }

REF

ApplicationContext是如何被注入的

https://www.jianshu.com/p/64a25883b836

@Autowired可以注入ApplicationContext

https://zhuanlan.zhihu.com/p/124249445