What exactly does onDestroy() destroy?

Question

I've been bothered by this "characteristics": When I use Back button to leave my app, I can tell onDestroy() is called, but the next time I run my app, all the static members of the Activity class still retain their values. See the code below:

public class HelloAndroid extends Activity {

private static int mValue;   // a static member here

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    TextView tv = new TextView(this);
    tv.setText((mValue != 0) ? 
        ("Left-over value = " + mValue) : "This is a new instance");
    setContentView(tv);
}

public void onDestroy() {
    super.onDestroy();
    mValue++;
}

}

The above code displays the left-over value in mValue, and it increments when the session ends so that I know for sure the onDestroy() is called.

I found a useful answer on this forum, and I understand in the above code mValue is a class member, instead of an instance member. But isn't it true that, in this particular case, I have only one single HelloAndroid activity, and so When he dies, everything is cleaned up, and the next time I come back, everything starts over again? (Or, is there some other misterious thing in the system still holds on to it after onDestroy() so that it just won't die???)

(The above is just a variable, what if it's a bunch of objec references? Each piece is a separately re-collectable memory. Is there a chance that GC collects some of them but not all-or-none? This really bugs me.)

Halvah

Answer 1

The OS decides when things "go away." The onDestroy is there to let your app have a final chance to clean things up before the activity does get destroyed but it does not mean that the activity will, in fact, be GCed. Here is a good article that I recommend people to read that relates to creating an exit button. While it's not exactly what you asked about, the concepts will help you understand what's going on.

Kirkkirkcaldy

Thanks, I wonder who decided to down vote this answer then. I wish they would have left a comment. – Kirkkirkcaldy

Answer 2

You don't just have the Activity though. You also have the application, and its process running in a Dalvik VM. Android will usually leave the application running in the background until it needs to reclaim the memory it is using for some other application. Your static member should remain in memory as long as the process is running. If you try running some memory-intensive application or forcefully closing the running application with some task manager, you may see the static value reset.

Masto

I see, thank you. Then can I assume that if one of the static members is not cleaned up, then NONE of them are cleaned up? – Halvah

Well, I guess the answer is yes. As long as I have them held on in hand, garbage collection should be a all-or-none thing. – Halvah