ApplicationContextAware与ApplicationContext的关系原来是这样的

前言

今天看到一段代码

@Component
public class BaseApplicationContext implements ApplicationContextAware {

    private static ApplicationContext context;

    @Override
    public void setApplicationContext(@SuppressWarnings("NullableProblems")
                                              ApplicationContext applicationContext) throws BeansException {
        BaseApplicationContext.context = applicationContext;
    }

    public static ApplicationContext getContext(){
        return context;
    }
}

这个代码是用来干什么的呢？它的作用就是用来获取应用上下文（即ApplicationContext ）

ApplicationContext

首先，我们了解下什么是ApplicationContext
我们知道，Spring容器是生成Bean实例的工厂，并管理容器中的Bean。生活中我们一般会把生产产品的地方称为工厂，而在这里bean对象的地方官方取名为BeanFactory，直译Bean工厂（com.springframework.beans.factory.BeanFactory），我们一般称BeanFactory为IoC容器，而称ApplicationContext为应用上下文。

BeanFactory提供了OC容器最基本的形式，给具体的IOC容器的实现提供了规范，BeanFactory负责配置、创建、管理Bean等基本功能

ApplicationContext的中文意思是“应用前后关系”，它继承自BeanFactory接口，除了包含BeanFactory的所有功能之外，在国际化支持、资源访问（如URL和文件）、事件传播等方面进行了良好的支持，被推荐为Java EE应用之首选，可应用在Java APP与Java Web中。

ApplicationContextAware

ApplicationContextAware 接口是什么？

实现ApplicationContextAware接口之后,系统启动时就可以自动给我们的服务注入applicationContext对象（自动调用了接口中的setApplicationContext方法）,我们就可以获取到ApplicationContext里的所有信息了。

注意：实现ApplicationContextAware接口的类需要交给spring管理，使用@Component或者在配置文件声明bean

我们获取ApplicationContext，一般是用来获取bean的，AoolicationContext有一个getBean方法，此方法继承自BeanFactory

T getBean(Class requiredType) throws BeansException;

实现方法主要有以下几种

public Object getBean(String name) throws BeansException {
        this.assertBeanFactoryActive();
        return this.getBeanFactory().getBean(name);
    }

这个方法通过name来返回一个bean的实例，注意返回的是一个Object 的对象，要进行强制转换

public <T> T getBean(Class<T> requiredType) throws BeansException {
        this.assertBeanFactoryActive();
        return this.getBeanFactory().getBean(requiredType);
    }

这个方法通过Class类型来返回bean，返回的对象即是这个类型的bean对象，无需转换

原理

研究一下ApplicationContextAware 的原理
我们都知道spring的入口方法就在AbstractApplicationContext的refresh()方法.我们先去看看refresh().prepareBeanFactory()方法。

@Override
	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);
	.....



// Prepare the bean factory for use in this context.准备bean工厂，以便在此上下文中使用。
prepareBeanFactory(beanFactory);


//在prepareBeanFactory中，我们可以看到addBeanPostProcessor(new ApplicationContextAwareProcessor(this)),将一个ApplicationContextAwareProcessor添加到了BeanPostProcessor中
protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
		// Tell the internal bean factory to use the context's class loader etc.告诉内部bean工厂使用上下文的类加载器等等。
		beanFactory.setBeanClassLoader(getClassLoader());
		beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));
		beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));

		// Configure the bean factory with context callbacks.用上下文回调配置bean工厂。
		beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
		beanFactory.ignoreDependencyInterface(EnvironmentAware.class);
		beanFactory.ignoreDependencyInterface(EmbeddedValueResolverAware.class);
		beanFactory.ignoreDependencyInterface(ResourceLoaderAware.class);
		beanFactory.ignoreDependencyInterface(ApplicationEventPublisherAware.class);
		beanFactory.ignoreDependencyInterface(MessageSourceAware.class);
		beanFactory.ignoreDependencyInterface(ApplicationContextAware.class);
.....
}


ApplicationContextAwareProcessor的构造方法如下
	public ApplicationContextAwareProcessor(ConfigurableApplicationContext applicationContext) {
		this.applicationContext = applicationContext;
		this.embeddedValueResolver = new EmbeddedValueResolver(applicationContext.getBeanFactory());
	}



ApplicationContextAwareProcessor中有两个方法，一个是初始化回调，一个是实现

	/**
	 * Apply this {@code BeanPostProcessor} to the given new bean instance before any bean初始化回调
	 * initialization callbacks (like InitializingBean's {@code afterPropertiesSet}
	 * or a custom init-method). The bean will already be populated with property values.
	 * The returned bean instance may be a wrapper around the original.
	 * 
The default implementation returns the given {@code bean} as-is.
	 * @param bean the new bean instance
	 * @param beanName the name of the bean
	 * @return the bean instance to use, either the original or a wrapped one;
	 * if {@code null}, no subsequent BeanPostProcessors will be invoked
	 * @throws org.springframework.beans.BeansException in case of errors
	 * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet
	 */
	@Override
	@Nullable
	public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
		if (!(bean instanceof EnvironmentAware || bean instanceof EmbeddedValueResolverAware ||
				bean instanceof ResourceLoaderAware || bean instanceof ApplicationEventPublisherAware ||
				bean instanceof MessageSourceAware || bean instanceof ApplicationContextAware)){
			return bean;
		}

		AccessControlContext acc = null;

		if (System.getSecurityManager() != null) {
			acc = this.applicationContext.getBeanFactory().getAccessControlContext();
		}

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

		return bean;
	}

	private void invokeAwareInterfaces(Object bean) {
		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);
		}
        		  //针对实现了ApplicationContextAware的接口,spring都将调用其setApplicationContext,将applicationContext注入到当前bean对象。
		if (bean instanceof ApplicationContextAware) {
			((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);
		}
	}