Android instance life cycle

Question

This is a very simple question, I am implementing a kind of Singleton pattern in my main activity , obviously it is not well implemented and not working as expected, but I will like to know more in detail why.

This is my code:

public class UIthread extends SherlockFragmentActivity implements 
                                    WsP12GetDocPendantToSign.OnCallbackFunction{

private static final String TAG = "UIthread";

//mode
private static UIthread instance;
....

public static UIthread getInstance() {
    return instance;
}

protected void onCreate(Bundle savedInstanceState) {

    if (instance == null)
        instance = this;
    super.onCreate(savedInstanceState);

    instance.initialize();

}

When I close my app and re-run, the instance still alive and the application has problems, so the instance is not being deleted when I close the app the first time, I will be very grateful if someone can explain why the instance still alive.

Thanks!