The right way to arrange the basic operators in RxJava - exception

Question

mSubscriptions.add(api.signIn(phoneNumber, otp)
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.doOnError(throwable -> Timber.e(throwable.getMessage()))
.onErrorResumeNext(throwable -> Observable.empty())
.subscribe(user -> {
// Handle user object logic here
}
}));
I generally use this pattern in all my apps for the schedulers and to handle exceptions, but sometimes i get an illegal state exceptions like this : Exception thrown on Scheduler.Worker thread. Add onError handling. Caused by rx.exceptions.OnErrorNotImplementedException, so i was wondering if this is right.
Thanks in advance.

Answer 1

As #yosriz mentioned - the right way is to implement onError callback in every subscriber. In your case, you're getting OnErrorNotImplementedException most likely due to your "Handle user object logic" throws.
Simple examples:
Exception in the stream:
Observable.just("value")
.flatMap(s -> Observable.error(new IllegalStateException()))
.onErrorResumeNext(t -> Observable.empty())
.doOnCompleted(() -> System.out.println("Completed"))
.subscribe(s -> {});
Completed
Exception in onNext callback:
Observable.just("value")
.onErrorResumeNext(t -> Observable.empty())
.doOnCompleted(() -> System.out.println("Completed"))
.subscribe(s -> {
throw new IllegalStateException();
});
rx.exceptions.OnErrorNotImplementedException
...
Caused by: java.lang.IllegalStateException
Exception in onNext callback, subscriber's onError implemented:
Observable.just("value")
.onErrorResumeNext(t -> Observable.empty())
.doOnCompleted(() -> System.out.println("Completed"))
.subscribe(s -> {
throw new IllegalStateException();
}, t -> {
System.out.println("Subscriber's onError triggered");
});
Subscriber's onError triggered

Answer 2

Arranging Schedulers has nothing to do with this type of error you get. The problem can happen if you don't have onError() handling on your subscriber (at the subscribe() method), and you get error somewhere in the stream without any handling.
doOnError() will not solve this, as it's just a side effect operator that will perform some operation with any onError().
In fact, In your example onErrorResumeNext() will handle the error as it will swallow any error, so probably you have other scenarios, where you do not handle all errors scenarios and thus got the on error not implemented exception.
As a general pattern, it's always better to prepared to any case and implement onError() at the subscriber to avoid cases where error was not handled down the stream.