简单来说就是Spring中的高级容器，可以获取容器中的各种bean组件，注册监听事件，加载资源文件等功能。
具体定义可以参考官网：https://spring.io/understanding/application-context
Understanding Application Context
The ApplicationContext is the central interface within a Spring application for providing configuration information to the application. It is read-only at run time, but can be reloaded if necessary and supported by the application. A number of classes implement the ApplicationContext interface, allowing for a variety of configuration options and types of applications.
The ApplicationContext provides:
Bean factory methods for accessing application components.
The ability to load file resources in a generic fashion.
The ability to publish events to registered listeners.
The ability to resolve messages to support internationalization.
Inheritance from a parent context.
Application Context获取的几种方式
1、直接注入
@Resource private ApplicationContext ctx;
2、实现ApplicationContextAware接口（推荐）
