Unique id of an android activity

Question

I need to get an unique id of an android activity instance. I\'d like to take the string which the activitymanager writes in the log (for example: ActivityManager: Activity

Answer 1

As correctly pointed out in the other answer, activities can be recreated by the OS, so using Activity.hashCode() is not a reliable solution. Moreover, the whole app process can be recreated (after permission change, for example), which means any runtime/non-persisted ID cannot be trusted. A correct solution is manually generate a unique ID during onCreate(Bundle) (or read it from savedInstanceState, if exists) and save it during onSaveInstanceState(Bundle).

Another problem is generating truly unique ID. The simplest and most reliable solution is too keep a static counter, but since app process may die, it requires a little more care.

To bring boilerplate code to a minimum, I've put together a specialized class:

public class ActivityUUID {

    private static final String INSTANCE_DATA_ID = "BwsKCwYBDg8GAQ4LBAgAAA"; // NOTE Any globally unique ID
    private static final String UUID_NODE_ID = "a";
    private static final long INVALID_UUID = 0;

    private static long _lastActivityUUID = INVALID_UUID; // NOTE All activity callbacks are guaranteed to be made from UI thread, so no synchronization is required
    private static SharedPreferences _persistentData;

    private final long _uuid;

    public ActivityUUID(Context context, Bundle savedInstanceState) {
        _uuid = readUUID(context, savedInstanceState);
    }

    public long get() {
        return _uuid;
    }

    public void onSaveInstanceState(Bundle outState) {
        outState.putLong(INSTANCE_DATA_ID, _uuid);
    }

    private static long readUUID(Context context, Bundle savedInstanceState) {
        long result = (savedInstanceState != null) ? savedInstanceState.getLong(INSTANCE_DATA_ID, INVALID_UUID) : INVALID_UUID;
        return (result != INVALID_UUID) ? result : generateUUID(context);
    }

    private static long generateUUID(Context context) {
        if (_persistentData == null) {
            _persistentData = context.getSharedPreferences(INSTANCE_DATA_ID, Context.MODE_PRIVATE);
            _lastActivityUUID = _persistentData.getLong(UUID_NODE_ID, INVALID_UUID);
        }

        long result = ++_lastActivityUUID;
        if (result != INVALID_UUID) {
            _persistentData.edit().putLong(UUID_NODE_ID, result).apply();
        } else {
            throw new RuntimeException("You have somehow managed to create a total of 9223372036854775807 activities during lifetime of your app! =)");
        }
        return result;
    }

}

Usage is pretty simple:

private ActivityUUID _activityUUID;

@Override
public final void onCreate(Bundle savedInstanceState) {
    _activityUUID = new ActivityUUID(this, savedInstanceState);
}

@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    _activityUUID.onSaveInstanceState(outState);
}

// ...somewhere else in the code
long activityUUID = _activityUUID.get();