Does subscribeOn call order matter?

Hi, I am using RxJava 2.1.1. I thought I knew how subscribeOn works, but sometimes I still have some doubts about it.

As far as I know, for subscribeOn it doesn’t metter the position where you use it (in the chain).

Assuming that I am executing this code from my MAIN THREAD.

Observable.just("")
                .doOnSubscribe(/* NEW THREAD */)
                .subscribeOn(Schedulers.newThread())
                .subscribe(/* NEW THREAD */);

As expected the doOnSubscribe() is called on the NEW THREAD, because of subscribeOn(Schedulers.newThread())

But, if I call subscribeOn before doOnSubscribe the result will be different:

Observable.just("")
                .subscribeOn(Schedulers.newThread())
                .doOnSubscribe(/* MAIN THREAD */)
                .subscribe(/* NEW THREAD */);

Now the doOnSubscirbe() seems to be executed in the MAIN THREAD, regardless of subscribeOn(Schedulers.newThread()).

Why is this happening?

[akarnokd]

subscribeOn moves the subscription side-effects of the upstream to another thread but doesn’t affect the downstream’s subscription side-effects. Therefore, it matters were you put it if you have subscription side-effecting operators before it.

Let’s walk through what happens in the first case. The chain consist of Just -> DoOnSubscribe -> SubscribeOn -> LambdaObserver operators but the subscription process walks in reverse:

Step	Thread	Action
1	main	Creation of LambdaObserver
2	main	SubscribeOn.subscribe(LambdaObserver)
3	main	SubscribeOn calls onSubscribe on LambdaObserver
4	main	SubscribeOn schedules a subscribe action on newThread
5	newthread	DoOnSubscribe.subscribe(SubscribeOnObserver)
6	newthread	Just.subscribe(DoOnSubscribeObserver)
7	newthread	Just calls onSubscribe on DoOnSubscribeObserver
8	newthread	DoOnSubscribe calls the lambda and then onSubscribe on SubscribeOnObserver
9	newthread	Since SubscribeOnObserver already called onSubscribe, the call returns to Just
10	newthread	Just emits onNext

Now if doOnSubscribe is after subscribeOn:

Step	Thread	Action
1	main	Creation of LambdaObserver
2	main	DoOnSubscribe.subscribe(SubscribeOnObserver)
3	main	SubscribeOn.subscribe(DoOnSubscribeObserver)
4	main	SubscribeOn calls onSubscribe on DoOnSubscribeObserver
5	main	DoOnSubscribe calls the lambda and then onSubscribe on LambdaObserver
6	main	SubscribeOn schedules a subscribe action on newThread
7	newthread	Just.subscribe(SubscribeOnObserver)
8	newthread	Just calls onSubscribe on SubscribeOnObserver
9	newthread	Since SubscribeOnObserver already called onSubscribe, the call returns to Just
10	newthread	Just emits onNext

[leinli03]

https://stackoverflow.com/questions/50075574/doonsubscribe-gets-called-on-main-thread the answer here seems not quite match the explaintion