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Context is the bridge between components. You use it to communicate between components, instantiate components, and access components.

我们使用context去初始化Activity, Content Provider, BroadcastReceiver等等，我们也是用它获取资源resource或者文件系统。

Context作为安卓系统的一个入口点，比如去获取系统的WifiManager, Vibrator, and PackageManager. context.getSystemService(Context.WIFI_SERVICE)

Let’s agree that everything in Android is a component. Context is the bridge between components. You use it to communicate between components, instantiate components, and access components. I hope the definition is now clear.

I divide context types into two categories: UI Context and Non-UI Context.

In reality, only the ContextThemeWrapper is UI Context — which means Context + Your theme.

UI Context:

- Activity
	- Instance of your activity (this)
- Fragment
	- getContext() in Fragment
- View
	- getContext() in View (if View was constructed using UI-Context)

Non-UI Context is allowed do almost everything UI-Context can do (bad design spotted). But as we pointed out above, you lose theming.

Non-UI Context

- Application instance as context
- Activity
	- getApplicationContext() in Activity
- Broadcast Receiver
	- Context received in broadcast receiver
- Service
	- Instance of your service (this)
	- getApplicationContext() in Service
- Context
	- getApplicationContext() in Context instance

Summary

• Do you need to access UI related stuff? Use UI-Context. Inflating Views and showing dialogue are the two use cases I can think of.
• Otherwise, Use Non UI Context.
• Make sure you do not pass short-living context to long-living objects.

Context类本身是一个纯abstract类，它有两个具体的实现子类：ContextImpl和ContextWrapper。其中ContextWrapper类，如其名所言，这只是一个包装而已，ContextWrapper构造函数中必须包含一个真正的Context引用，同时ContextWrapper中提供了attachBaseContext（）用于给ContextWrapper对象中指定真正的Context对象，调用ContextWrapper的方法都会被转向其所包含的真正的Context对象。

参考

Mastering Android context
Context都没弄明白，还怎么做Android开发？