How to set entire application in portrait mode only?

Yes you can do this both programmatically and for all your activities making an AbstractActivity that all your activities extends.

public abstract class AbstractActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setRequestedOrientation (ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
    }
}

This abstract activity can also be used for a global menu.

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);


    //setting screen orientation locked so it will be acting as potrait
    setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_LOCKED);
}

Similar to Graham Borland answer...but it seems you dont have to create Application class if you dont want...just create a Base Activity in your project

public class BaseActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_base);
    setRequestedOrientation (ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);

}

And extend this class instead of AppCompatActivity where you want to use Potrait Mode

public class your_activity extends BaseActivity {}

For Xamarin Users:

If you extends all your activities to a BaseActivity Just add:

this.RequestedOrientation = ScreenOrientation.Portrait;

This will resolve the problem. If you want any particular activity to be in landscape override this in OnActivityCreated. As:

this.Activity.RequestedOrientation = ScreenOrientation.Landscape;

Well, I tried every answer but it didn't work in older versions of android. So, the final solution is to add this code to every activity just above setContentView:

    setRequestedOrientation (ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);

In kotlin -->

Use this in your Extends Application class fun onCreate()...

registerActivityLifecycleCallbacks(object : ActivityLifecycleCallbacks {
        override fun onActivityCreated(p0: Activity, p1: Bundle?) {
            p0.requestedOrientation = ActivityInfo.SCREEN_ORIENTATION_SENSOR_LANDSCAPE;
        }

        override fun onActivityStarted(p0: Activity) {
        }

        override fun onActivityResumed(p0: Activity) {
        }

        override fun onActivityPaused(p0: Activity) {
        }

        override fun onActivityStopped(p0: Activity) {
        }

        override fun onActivitySaveInstanceState(p0: Activity, p1: Bundle) {
        }

        override fun onActivityDestroyed(p0: Activity) {
        }
    }

    )}