DataRepository.java

public class DataRepository extends RealmRepository {
    private static DataRepository instance;

    synchronized public static DataRepository getInstance() {
        if (instance == null) {
            instance = new BotRepository();
        }
        return instance;
    }

    public Data getData(long memberId) {
        return execute(realm -> {
            Bot bot = realm.where(Data.class)
                    .equalTo("id", memberId)
                    .findFirst();
            if (bot != null) {
                return realm.copyFromRealm(bot);
            } else {
                return null;
            }
        });
    }
}

RealmRepository.java

public class RealmRepository {
    private Lock lock;
    protected RealmRepository() {lock = new ReentrantLock();}

    protected <T> T execute(Executor<T> executor) {
        Realm realm = null;
        try {
            lock.lock();
            realm = Realm.getDefaultInstance();
            return executor.execute(realm);
        } finally {
            if (realm != null && !realm.isClosed()) {
                realm.close();
            }
            lock.unlock();
        }
    }
    public interface Executor<T> {
        T execute(Realm realm);
    }
}