Getting the Application Context

问题:

This might be a simple question but I just wanted to make sure I am right.

In my android application I have a constructor that uses:

activity.getApplicationContext()

The activity is passed into the constructor as a parameter.

The problem is that I am calling this class from a Service. If I make a second constructor which accepts the Service as a parameter and uses service.getApplicationContext? Will I get the same application context?

回答1:

Will I get the same application context?

Yes. You can check the android documentation, they have provided

 getApplicationContext()

Return the context of the single, global Application object of the current process.

So it should not be changed for the whole application process.

Please also take a note of this:

getApplicationContext() generally should only be used if you need a Context whose lifecycle is separate from the current context, that is tied to the lifetime of the process rather than the current component.

Correct me if I'm wrong.

Thanks

回答2:

The easiest way to get the application context is:

Create a class App that extends android.app.Application

public class App extends Application {
    public static Context context;

    @Override public void onCreate() {
        super.onCreate();
        context = getApplicationContext();
    }
}

Modify your AndroidManifest.xml 's <application> tag to have the attribute android:name="your.package.name.App".

Any time you need the application context, just get it from App.context.

Application is always initialized first whether your process runs, whether it's an activity, a service, or something else. You will always have access to the application context.

回答3:

There is only one application context, so you should get the same one. You can have just one constructor that takes a Context, you don't really need two. Or if you wanted to make sure that you are getting the application context, and not, say, an activity one, you can have your constructor take Application as a parameter which is a Context.

回答4:

You can go for getApplicationContext() if you wanna get context of whole application. If you want to get context of current class you can use getBaseContext() instead.

回答5:

I have adapted yuku's answer with a non static direct context reference.

Create a class domain.company.pseudo.ApplicationName which extends android.app.Application.

package hypersoft.systems.android;

import android.app.Application;

public class Starbox extends Application {

  public static Starbox instance;

  @Override
  public void onCreate() {
    super.onCreate();
    instance = this;
  }

}

In this sample, my full application package name is hypersoft.systems.android.starbox.

Now, modify your AndroidManifest.xml <application> tag to have the attribute android:name="hypersoft.systems.android.Starbox", and be sure the Starbox.java class file is located in the project component directory: android rather than starbox.

With all this done, you can now import hypersoft.systems.android.Starbox, and in your code you can get the ApplicationContext by calling Starbox.instance.getApplicationContext()

Successfully compiling with build tools 26 and api 26 (Android 8.0) with min sdk version 14 (4.0).

回答6:

Application Context add Activity Context both are different.Downcasting is risky .Use this code to use context object .

public class App extends Application {
public static Context context;

    @Override public void onCreate() {
        super.onCreate();
        context = getApplicationContext();
    }
} 

In Your Activities and in fragments Class :

Conetext context=App.context;