How to hold an Object as long as a service live?

Question

I have an Object that i use in all my activities which holds a lot of complex data in it, it would be quite an hassle to use Android framework of saving the object state and passing it around from activity to activity, so i thought it would be possible to make a Singleton that manages this object and makes it live as long as the application lives.

Tried to use regular Java Singleton scheme, with normal class and normal static instance, but the instance becomes null after a while (which is very confusing, why would an Object that is still referenced be turned to null and garbage collected?). so i decided to flow with Android designers and created a Service to manage this Object, the Service looks something like that :

public class DataService extends Service {
    private Data data;
    private static DataService instance;
    @Override
    public void onCreate() {
        instance = this;
        data= new Data(...);
        instance.data.addProgressListener(listener);
        (new Thread() {
            @Override
            public void run() {
                data.doInitProgress();
                listener = null;
            };
        }).start();

    }


    public static void listenToInitDataProcess(final ProgressBar progressBar,final Runnable onDone) {

        listener = new ProgressListener() {
            private int progress;
            private int max;
            @Override
            public void onUpdateProgress(final long i) {
                progressBar.setProgress(progress+=i);
            }

            @Override
            public void onProgressEndComputed(final long n) {
                progressBar.setMax(max=(int) n);
            }

            @Override
            public void onDone() {
                progressBar.setProgress(max);
                onDone.run();
            }
        };
            if (instance!=null) instance.data.addProgressListener(listener);
    }

    public static Data getData() {
        return instance.data;
    }
    @Override
    public IBinder onBind(Intent arg0) {
        return null;
    }
}

now the problem with that is that after a while that the app is on i get NPE caused by instance is null... notice that i was listenning to the data object creation and i was trying to get it only after it was once inited, so no way that instance was suppose to be null...

how to do this right then?

Using 'private static' variables can cause memory leaks, as they are mutable shared variables. I urge you to avoid this practice. — IgorGanapolsky, Mar 03 '15 at 16:21

score 2 · Accepted Answer · edited May 23 '17 at 12:32

If you want an Object that lives as long as your application lives (i.e. as long as its process is not killed by OS) you can extend android.app.Application, put your 'global' data there and use that subclass as your app context (needs to be declared in manifest)

However many argue that singletons provide essentially the same result as custom context e.g.
Singletons vs. Application Context in Android?

score -1 · Answer 2 · edited May 23 '17 at 11:58

but the instance becomes null after a while

First, understand Effective Java's singleton recommendations, and how something wouldn't become null:

// Singleton with public final field
public class Elvis {
    public static final Elvis INSTANCE = new Elvis();
    private Elvis() { ... }

    public void leaveTheBuilding() { ... }
}

But a Service is different, since there is no public constructor there. So, first check if the Service is running (as referenced in this post: Check if Service is running from a Broadcast Receiver):

private boolean isMyServiceRunning(Context context) {
    ActivityManager manager = (ActivityManager)      context.getSystemService(Context.ACTIVITY_SERVICE);
    for (RunningServiceInfo service : manager.getRunningServices(Integer.MAX_VALUE)) {
        if (DataService.class.getName().equals(service.service.getClassName())) {
            return true;
        }
    }

    return false;
}

Then define some methods in your Service to return your needed state variables.

How to hold an Object as long as a service live?

2 Answers2