Realm closes itself

Question

I use one static global Realm instance(never closed) on Application object only for use in UI Thread,

@UiThread
public static Realm getRealm() {
    if (Thread.currentThread() == Looper.getMainLooper().getThread()) {
        return realmInstance;
    } else {
        Timber.e("Illegal access to getRealmObservable");
        throw new IllegalStateException("Only UI Thread can access this realm");
    }
}

and another single use realm for WorkerThread as following:

@WorkerThread
public static void executeOnSingleUseRealm(final Realm.Transaction transaction) {
    if (Thread.currentThread() == Looper.getMainLooper().getThread()) {
        Timber.e("Wrong thread for Realm");
        throw new IllegalStateException("Wrong thread for Single use Realm");
    }

    Realm realm = null;
    try {
        realm = Realm.getDefaultInstance();
        realm.executeTransaction(transaction);
    } catch (Exception e) {
        Timber.e(e, "Exception in Single Use Realm transaction");
        throw e;
    } finally {
        if (realm != null) {
            realm.close();
        }
    }
}

However I still see crashes on use of single global Realm instance: This Realm instance has already been closed, making it unusable.

I don't know how it is even possible.

Here is how I initialise Realm instance:

Application onCreate

void onCreate(){
    ....
        Observable
            .fromCallable(() -> {
                Realm.init(SVApplication.this);
                RealmConfiguration realmConfiguration = new RealmConfiguration.Builder()
                        .deleteRealmIfMigrationNeeded()
                        .build();
                Realm.compactRealm(realmConfiguration);
                Realm.setDefaultConfiguration(realmConfiguration);
                return true;
            })
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe((v) -> onRealmLoaded());
    }
}

void onRealmLoaded(){
    realmInstance = Realm.getDefaultInstance();
    ....
}

Here is one of the crashes from one of the activities:

void onStart(){
    ....    subscribeUntilDetach(realmInstance.where(Notification.class).findAllAsync().asObservable()
                        .onBackpressureLatest()
                        .switchIfEmpty(emptyNotification())
                        .map(notifications -> notifications.where().isNull("readTime").or().isEmpty("readTime").count())
                        .onBackpressureLatest()
                        .observeOn(AndroidSchedulers.mainThread())
                        .subscribe(count -> {
                            if (count == 0L) {
                                mNotificationBadge.setVisibility(View.GONE);
                            } else {
                                mNotificationBadge.setText(String.format(Locale.getDefault(), "%d", count));
                                mNotificationBadge.setVisibility(View.VISIBLE);
                            }
                        }, throwable -> Timber.e(throwable, "Error setting notification count")));
}

@Override
protected void onPause() {
    super.onPause();
    if (isFinishing()) {
        mCompositeSubscription.clear();
    }
}

protected void subscribeUntilDetach(@NonNull Subscription subscription) {
    mCompositeSubscription.add(subscription);
}

@Override
protected void onDestroy() {
    super.onDestroy();
    if (mCompositeSubscription.hasSubscriptions()) {
        mCompositeSubscription.unsubscribe();
    }
}

on some activities this line also crashes with same error:

mCompositeSubscription.unsubscribe();

Here is a stacktrace from Crashlytics, may not be fully accurate. 

Fatal Exception: java.lang.RuntimeException: Unable to resume activity {com.myapp.mobile/com.teknoloji.myapp.ui.pages.HomeActivity}: rx.b.f: This Realm instance has already been closed, making it unusable.
   at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3353)
   at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3384)
   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1443)
   at android.os.Handler.dispatchMessage(Handler.java:102)
   at android.os.Looper.loop(Looper.java:168)
   at android.app.ActivityThread.main(ActivityThread.java:5885)
   at java.lang.reflect.Method.invoke(Method.java)
   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:797)
   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:687)
Caused by rx.b.f: This Realm instance has already been closed, making it unusable.
   at rx.internal.util.InternalObservableUtils$ErrorNotImplementedAction.call(Unknown Source)
   at rx.internal.util.InternalObservableUtils$ErrorNotImplementedAction.call(Unknown Source)
   at rx.internal.util.ActionSubscriber.onError(Unknown Source)
   at rx.observers.SafeSubscriber._onError(Unknown Source)
   at rx.observers.SafeSubscriber.onError(Unknown Source)
   at rx.exceptions.Exceptions.propagate(Unknown Source)
   at rx.observers.SafeSubscriber.onNext(Unknown Source)
   at rx.internal.producers.SingleProducer.request(Unknown Source)
   at rx.Subscriber.setProducer(Unknown Source)
   at rx.Subscriber.setProducer(Unknown Source)
   at rx.internal.operators.OperatorSingle$ParentSubscriber.onCompleted(Unknown Source)
   at rx.internal.operators.OperatorTake$1.onNext(Unknown Source)
   at rx.internal.operators.NotificationLite.next(Unknown Source)
   at rx.subjects.SubjectSubscriptionManager$SubjectObserver.accept(Unknown Source)
   at rx.subjects.SubjectSubscriptionManager$SubjectObserver.emitNext(Unknown Source)
   at rx.subjects.SubjectSubscriptionManager$SubjectObserver.emitFirst(Unknown Source)
   at rx.subjects.BehaviorSubject$1.call(Unknown Source)
   at rx.subjects.BehaviorSubject$1.call(Unknown Source)
   at rx.subjects.SubjectSubscriptionManager.call(Unknown Source)
   at rx.subjects.SubjectSubscriptionManager.call(Unknown Source)
   at rx.subjects.SubjectSubscriptionManager.call(Unknown Source)
   at rx.internal.operators.OnSubscribeLift.call(Unknown Source)
   at rx.internal.operators.OnSubscribeLift.call(Unknown Source)
   at rx.internal.operators.OnSubscribeLift.call(Unknown Source)
   at rx.internal.operators.OnSubscribeLift.call(Unknown Source)
   at rx.Observable.create(Unknown Source)
   at rx.Observable.unsafeCreate(Unknown Source)
   at rx.Observable.create(Unknown Source)
   at com.teknoloji.myapp.ui.pages.HomeActivity.onStart(Unknown Source)
   at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1288)
   at android.app.Activity.performStart(Activity.java:6279)
   at android.app.Activity.performRestart(Activity.java:6325)
   at android.app.Activity.performResume(Activity.java:6330)
   at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3336)
   at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3384)
   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1443)
   at android.os.Handler.dispatchMessage(Handler.java:102)
   at android.os.Looper.loop(Looper.java:168)
   at android.app.ActivityThread.main(ActivityThread.java:5885)
   at java.lang.reflect.Method.invoke(Method.java)
   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:797)
   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:687)
Caused by java.lang.IllegalStateException: This Realm instance has already been closed, making it unusable.
   at io.realm.BaseRealm.checkIfValid(Unknown Source)
   at io.realm.Realm.init(Unknown Source)
   at com.teknoloji.myapp.ui.pages.HomeActivity.lambda$onStart$3(Unknown Source)
   at com.teknoloji.myapp.ui.pages.HomeActivity$$Lambda$1.call(Unknown Source)
   at rx.internal.util.ActionSubscriber.onNext(Unknown Source)
   at rx.observers.SafeSubscriber.onNext(Unknown Source)
   at rx.internal.producers.SingleProducer.request(Unknown Source)
   at rx.Subscriber.setProducer(Unknown Source)
   at rx.Subscriber.setProducer(Unknown Source)
   at rx.internal.operators.OperatorSingle$ParentSubscriber.onCompleted(Unknown Source)
   at rx.internal.operators.OperatorTake$1.onNext(Unknown Source)
   at rx.internal.operators.NotificationLite.next(Unknown Source)
   at rx.subjects.SubjectSubscriptionManager$SubjectObserver.accept(Unknown Source)
   at rx.subjects.SubjectSubscriptionManager$SubjectObserver.emitNext(Unknown Source)
   at rx.subjects.SubjectSubscriptionManager$SubjectObserver.emitFirst(Unknown Source)
   at rx.subjects.BehaviorSubject$1.call(Unknown Source)
   at rx.subjects.BehaviorSubject$1.call(Unknown Source)
   at rx.subjects.SubjectSubscriptionManager.call(Unknown Source)
   at rx.subjects.SubjectSubscriptionManager.call(Unknown Source)
   at rx.subjects.SubjectSubscriptionManager.call(Unknown Source)
   at rx.internal.operators.OnSubscribeLift.call(Unknown Source)
   at rx.internal.operators.OnSubscribeLift.call(Unknown Source)
   at rx.internal.operators.OnSubscribeLift.call(Unknown Source)
   at rx.internal.operators.OnSubscribeLift.call(Unknown Source)
   at rx.Observable.create(Unknown Source)
   at rx.Observable.unsafeCreate(Unknown Source)
   at rx.Observable.create(Unknown Source)
   at com.teknoloji.myapp.ui.pages.HomeActivity.onStart(Unknown Source)
   at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1288)
   at android.app.Activity.performStart(Activity.java:6279)
   at android.app.Activity.performRestart(Activity.java:6325)
   at android.app.Activity.performResume(Activity.java:6330)
   at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3336)
   at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3384)
   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1443)
   at android.os.Handler.dispatchMessage(Handler.java:102)
   at android.os.Looper.loop(Looper.java:168)
   at android.app.ActivityThread.main(ActivityThread.java:5885)
   at java.lang.reflect.Method.invoke(Method.java)
   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:797)
   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:687)

Answer 1

The problem is:

A closed exception indicates that you're attempting to use a closed realm, utilizing Realm in a way that would not be promoted as the best practice. ... Each activity is supposed to have its own realm instance for best performance. ... create a realm instance at onCreate() and destroy the instance at onDestory(). https://github.com/realm/realm-java/issues/2594#issuecomment-211793848

Furthermore

...it is not a good idea that keep a static field of Realm in the class. It is recommended to control the Realm's life cycle within the Activity/Fragment/etc https://realm.io/docs/java/latest/#controlling-the-lifecycle-of-realm-instances

Because creating new instances in Realm, when there is at least one open, is super fast, don't be afraid to create multiple instances of Realm.

A senior software engineer from Realm points that out here:

As long as you have at least one instance open on a thread calling Realm.getInstance() it is just a HashMap lookup... Best practise is to keep the Realm instance open for as long as your thread lives.

The best practice is described in the Realm documentation:

// Setup Realm in your Application
public class MyApplication extends Application {
    @Override
    public void onCreate() {
        super.onCreate();
        Realm.init(this);
        RealmConfiguration realmConfiguration = new RealmConfiguration.Builder().build();
        Realm.setDefaultConfiguration(realmConfiguration);
    }
}

// onCreate()/onDestroy() overlap when switching between activities.
// Activity2.onCreate() will be called before Activity1.onDestroy()
// so the call to getDefaultInstance in Activity2 will be fast.
public class MyActivity extends Activity {
    private Realm realm;
    private RecyclerView recyclerView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        realm = Realm.getDefaultInstance();
        // ...
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        realm.close();
    }
}

For worker threads the documentation recommends to get a new instance of Realm at the beginning and close it at the end.

protected Void doInBackground(Void... params) {
    Realm realm = Realm.getDefaultInstance();
    try {
        // ... Use the Realm instance ...
    } finally {
        realm.close();
    }

    return null;
}