concatMap throw an exception if applied after a custom operator (Rx 2.x)

Hello,

We’ve got some trouble when using concatMap when a custom operator is used in the upstream.

I think a code example worth more than thousands words. So let’s define a very simple custom operator that emits downstream the items.

import io.reactivex.Observable;
import io.reactivex.ObservableOperator;
import io.reactivex.Observer;
import io.reactivex.observers.DisposableObserver;

public class MyOperator implements ObservableOperator<Integer, Integer> {

  public static void main(String[] args) {
    Observable.range(0, 10)
        .lift(new MyOperator())
        .concatMap(Observable::just)
        .subscribe(System.out::println);
  }

  @Override
  public Observer<? super Integer> apply(Observer<? super Integer> observer) throws Exception {
    return new DisposableObserver<Integer>() {

      @Override
      public void onNext(Integer s) {
        observer.onNext(s);
      }

      @Override
      public void onError(Throwable e) {
        observer.onError(e);
      }

      @Override
      public void onComplete() {
        observer.onComplete();
      }
    };
  }
}

If you run the main function you will get a stack trace like that :

Exception in thread "main" java.lang.NullPointerException
	at io.reactivex.internal.operators.observable.ObservableConcatMap$SourceObserver.onNext(ObservableConcatMap.java:128)
	at MyOperator$1.onNext(MyOperator.java:21)
	at MyOperator$1.onNext(MyOperator.java:17)
	at io.reactivex.internal.operators.observable.ObservableRange$RangeDisposable.run(ObservableRange.java:64)
	at io.reactivex.internal.operators.observable.ObservableRange.subscribeActual(ObservableRange.java:35)
	at io.reactivex.Observable.subscribe(Observable.java:10838)
	at io.reactivex.internal.operators.observable.ObservableLift.subscribeActual(ObservableLift.java:57)
	at io.reactivex.Observable.subscribe(Observable.java:10838)
	at io.reactivex.internal.operators.observable.ObservableConcatMap.subscribeActual(ObservableConcatMap.java:52)
	at io.reactivex.Observable.subscribe(Observable.java:10838)
	at io.reactivex.Observable.subscribe(Observable.java:10824)
	at io.reactivex.Observable.subscribe(Observable.java:10727)
	at MyOperator.main(MyOperator.java:12)

Note : RxJava version is 2.1.1

[jcornaz]

Actually my operator inherit from DisposableObserver, and I thought that was the purpose of this class, because I don’t have to (and cannot) implement “onSubscribe” anymore, and also because the example given in the “Learning RxJava book” by Thomas Nield was written the same way.

But you are right, overriding DisposableObserver.onStart() by calling observer.onSubscribe() solves the problem.

May I suggest to make the method DisposableObserver.onStart() abstract to enforce its implementation ? (as it seems to be necessary)

Thank you for your help.

[jcornaz]

Ah ok. Thank you for that enlightenment. All is clear now.