ApplicationContext是如何被注入的

SpringBoot.jpeg

前言

//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,本文将简述ApplicationContext及类似的BeanFactory是如何实现注入逻辑的.最后稍微介绍一下Aware接口.

TLDR

普通Bean依赖Spring的容器功能进行注入,而ApplicationContext通过Spring的内建机制进行注入.

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.

正文

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

SpringBoot-inject-stack.png

创建SimpleBean3时,Spring检测到有applicationContext和simpleBean2两个依赖,便会进行查找.

在 SpringBoot中@Autowired是如何生效的那篇文章中,讲到了带有@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;
    }

Aware接口

SpringBoot-aware.png

Aware是一个标记接口,要求子接口需要提供一个回调setter方法.使用者可以在恰当的时机被回调,举个例子,实现了ApplicationContextAware的实例会在某个时间触发setApplicationContext获得applicationContext的实例.

public interface ApplicationContextAware extends Aware {
    void setApplicationContext(ApplicationContext applicationContext) throws BeansException;

}

那么直接@Autowired注入和使用ApplicationContextAware注入有什么区别呢?

触发时机不同,@Autowired方式会在InstantiationAwareBeanPostProcessor.postProcessProperties()阶段触发,ApplicationContextAware方式会在BeanPostProcessor.postProcessBeforeInitialization()阶段触发. @Autowired方式早于ApplicationContextAware
ApplicationContextAware有更大的自由度,可以做更多自定义逻辑

后记

ApplicationContext不是bean,不能通过bean的方式进行注入,为了方面使用Spring通过特殊方式满足用户的注入需求,同时又提供了Aware系列接口,给与用户更大的自由度.