调用getApplicationContext（）时调用的代码在哪里？

Question

I have looked into Android sources just out of interest. What I found is that Context is an abstract class with abstract method:

public abstract Context getApplicationContext();

The ContextWrapper.java extends Context.java which led to implementation of getApplicationContext() method:

 @Override
    public Context getApplicationContext() {
        return mBase.getApplicationContext();
    }

But mBase is reference to an object of type Context which is initialized in ContextWrapper 's constructor:

public ContextWrapper(Context base) {
    mBase = base;
}

So this mBase reference refers to abstract class? Well, I just don't understand where is the code which is executed when you call getApplicationContext() from your Activity .

Answer 1

This is an interesting example of polymorphism.

As long as your base extends Context , it has to provide an implementation of getApplicationContext() , which in the case of ContextWrapper is the code you provided here. There's this implementation, and the one in ContextImpl .

It's important to note a couple of things when reading the code you provided: ContextWrapper itself extends Context , but it also takes a Context as an input (which could be a ContextWrapper , or a Service , or an Application , or an Activity ). ContextWrapper doesn't care which kind it is; it just knows that they have a method getApplicationContext and it wants to call that method when asked. (For example, it could be passed another ContextWrapper , but because said ContextWrapper would also require a Context in its constructor, that would just add another level of nesting.)

The Application extends ContextWrapper class calls super(null) , which would mean that getApplicationContext() would throw a NullPointerException if it were left that way--however, in ContextWrapper it's also settable by attachBaseContext(Context) , and this is where it gets interesting.

Both Activity and Application have methods attach(Context [...other stuff]) . Each of them calls attachBaseContext() with the passed-in Context .

• In the Instrumentation class, you'll find android.app.Instrumentation.newApplication() , where a ContextImpl is created, and passed into an Application .
• In the ActivityThread class, you'll find handleBindApplication which creates a ContextImpl that gets passed to the Activity as its root Context .
• In the LoadedApk class, you'll find makeApplication which creates a ContextImpl that gets passed to an Application . Here are the places it's called.

So, at the end of the day, mBase typically ends up as a ContextImpl .

Potentially useful links I looked at while finding all of this out:

• Usages of attach
• Usages of attachBaseContext