Spring擴展點之Aware接口族

引言

Spring中提供了各類Aware接口，方便從上下文中獲取當前的運行環境，比較常見的幾個子接口有：BeanFactoryAware,BeanNameAware,ApplicationContextAware,EnvironmentAware，BeanClassLoaderAware等，這些Aware的做用均可以從命名得知

Aware處理

其中BeanNameAware、BeanClassLoaderAware和BeanFactoryAware這三個是直接在bean的初始化以前就處理了的，具體代碼在AbstractAutowireCapableBeanFactory.initializeBean方法中：

protected Object initializeBean(String beanName, Object bean, RootBeanDefinition mbd) {
    // 判斷對象實現的接口類型，處理特定的三種接口類型：BeanNameAware、BeanClassLoaderAware和BeanFactoryAware。
    if (bean instanceof BeanNameAware) {
        ((BeanNameAware) bean).setBeanName(beanName);
    }
 
    if (bean instanceof BeanClassLoaderAware) {
        ((BeanClassLoaderAware) bean).setBeanClassLoader(getBeanClassLoader());
    }
 
    if (bean instanceof BeanFactoryAware) {
        ((BeanFactoryAware) bean).setBeanFactory(this);
    }
    // 開始Bean初始化前處理、初始化、初始化後處理
    Object wrappedBean = bean;
    if (mbd == null || !mbd.isSynthetic()) {
        wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
    }
 
    try {
        invokeInitMethods(beanName, wrappedBean, mbd);
    }
    catch (Throwable ex) {
        throw new BeanCreationException(
                (mbd != null ? mbd.getResourceDescription() : null),
                beanName, "Invocation of init method failed", ex);
    }
 
    if (mbd == null || !mbd.isSynthetic()) {
        wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
    }
    return wrappedBean;
}

除了這三種以外的那些Aware接口的實現就不太同樣了，它們都是利用BeanPostProcessor接口完成的，關於BeanPostProcessor接口的原理能夠這篇文章：Spring擴展點之BeanPostProcessor

如ApplicationContextAware就是利用ApplicationContextAwareProcessor實現的：

public Object postProcessBeforeInitialization(final Object bean, String beanName) throws BeansException {
        AccessControlContext acc = null;

        if (System.getSecurityManager() != null &&
                (bean instanceof EnvironmentAware || bean instanceof EmbeddedValueResolverAware ||
                        bean instanceof ResourceLoaderAware || bean instanceof ApplicationEventPublisherAware ||
                        bean instanceof MessageSourceAware || bean instanceof ApplicationContextAware)) {
            acc = this.applicationContext.getBeanFactory().getAccessControlContext();
        }

        if (acc != null) {
            AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
              //具體實現
                invokeAwareInterfaces(bean);
                return null;
            }, acc);
        }
        else {
                        //具體實現
            invokeAwareInterfaces(bean);
        }

        return bean;
    }
private void invokeAwareInterfaces(Object bean) {
        if (bean instanceof Aware) {
            if (bean instanceof EnvironmentAware) {
                ((EnvironmentAware) bean).setEnvironment(this.applicationContext.getEnvironment());
            }
            if (bean instanceof EmbeddedValueResolverAware) {
                ((EmbeddedValueResolverAware) bean).setEmbeddedValueResolver(this.embeddedValueResolver);
            }
            if (bean instanceof ResourceLoaderAware) {
                ((ResourceLoaderAware) bean).setResourceLoader(this.applicationContext);
            }
            if (bean instanceof ApplicationEventPublisherAware) {
                ((ApplicationEventPublisherAware) bean).setApplicationEventPublisher(this.applicationContext);
            }
            if (bean instanceof MessageSourceAware) {
                ((MessageSourceAware) bean).setMessageSource(this.applicationContext);
            }
            if (bean instanceof ApplicationContextAware) {
                ((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);
            }
        }
    }

而ApplicationContextAwareProcessor的註冊奧祕在AbstractApplicationContext.prepareBeanFactory方法中：

beanFactory.setBeanClassLoader(getClassLoader());
beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this));
 beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this)); 
beanFactory.ignoreDependencyInterface(ResourceLoaderAware.class);
beanFactory.ignoreDependencyInterface(ApplicationEventPublisherAware.class);
beanFactory.ignoreDependencyInterface(MessageSourceAware.class);
beanFactory.ignoreDependencyInterface(ApplicationContextAware.class);