Realm test

Test1.java

// Test 1
for (int i = 0; i < 10; i++) {
    // spawn new thread which call queryNoTransaction()
}

void queryNoTransaction() {
  try (Realm realm = Realm.getDefaultInstance()) {
    long start = System.currentTimeMillis();
    RealmResults<UserProfile> users = realm.where(UserProfile.class).findAll();
    List<UserProfile> userCopy = realm.copyFromRealm(users);
    long end = System.currentTimeMillis();
    L.d("time: " + (end - start));
  }
}
// output 
// time: 4949
// time: 4986
// time: 5022
// time: 5040
// ...

Test2.java

// Test 2
for (int i = 0; i < 10; i++) {
    // spawn new thread which call queryTransaction()
}

void queryTransaction() {
  Realm.getDefaultInstance().executeTransaction(new Realm.Transaction() {
      @Override
      public void execute(Realm realm) {
        long start = System.currentTimeMillis();
        RealmResults<UserProfile> users = realm.where(UserProfile.class).findAll();
        List<UserProfile> userCopy = realm.copyFromRealm(users);
        long end = System.currentTimeMillis();
        L.d("time: " + (end - start));
      }
  });
}

// output 
// time: 46
// time: 85
// time: 61
// time: 49
// ...

Conclusion: if multiple queries are trying to query realm without transaction realm will be locked, until all queries are completed, when last query is completed - realm will be released. To prevent this use transaction.