How to maintain a single Realm instance throughout the app lifecycle and also close it?

Question

How can I maintain a single Realm Instance throughout the complete lifecycle of the App and also close it.

I can achieve maintaining the instance using a singleton class, but then how do I close it when the app is closed?

Also, is it safe to not close Realm Instance once opened?

Answer 1

I tend to use a singleton RealmManager for the UI thread, and for background threads I open/close the Realm using a try-with-sources block.

So for UI thread:

public class RealmManager {
    private static final String TAG = "RealmManager";

    static Realm realm;

    static RealmConfiguration realmConfiguration;

    public static void initializeRealmConfig(Context appContext) {
        if(realmConfiguration == null) {
            Log.d(TAG, "Initializing Realm configuration.");
            setRealmConfiguration(new RealmConfiguration.Builder(appContext).initialData(new RealmInitialData())
                    .deleteRealmIfMigrationNeeded()
                    .inMemory()
                    .build());
        }
    }

    public static void setRealmConfiguration(RealmConfiguration realmConfiguration) {
        RealmManager.realmConfiguration = realmConfiguration;
        Realm.setDefaultConfiguration(realmConfiguration);
    }

    private static int activityCount = 0;

    public static Realm getRealm() {
        return realm;
    }

    public static void incrementCount() {
        if(activityCount == 0) {
            if(realm != null) {
                if(!realm.isClosed()) {
                    Log.w(TAG, "Unexpected open Realm found.");
                    realm.close();
                }
            }
            Log.d(TAG, "Incrementing Activity Count [0]: opening Realm.");
            realm = Realm.getDefaultInstance();
        }
        activityCount++;
        Log.d(TAG, "Increment: Count [" + activityCount + "]");
    }

    public static void decrementCount() {
        activityCount--;
        Log.d(TAG, "Decrement: Count [" + activityCount + "]");
        if(activityCount <= 0) {
            Log.d(TAG, "Decrementing Activity Count: closing Realm.");
            activityCount = 0;
            realm.close();
            if(Realm.compactRealm(realmConfiguration)) {
                Log.d(TAG, "Realm compacted successfully.");
            }
            realm = null;
        }
    }
}

And for background thread:

try(Realm realm = Realm.getDefaultInstance()) {
   // ...
}