RxJava just（）计算线程

Question

Im confused. I have a method which do some longtime job (about 5 second) and returns a String as a result. I wrapped that function into an Observable smth like this:

private Observable<String> getJobObservable() {
    return Observable.just(doJob());
}

But even with .subscribeOn(Schedulers.computation()) and .observeOn(Schedulers.computation()) my doJob() method calculates on mainThread But, after i have changed Observable.just() to Observable.fromCallable() everything works fine. Why Observable.just() doesn't react on thread changes?

Answer 1

If you write

return Observable.just(doJob());

that is equivalent to

Object o = doJob();
return Observable.just(o);

Now if you don't have Observable.just(o) at all, where does doJob() execute? Just because doJob() or o are put into between the parenthesis of just , it doesn't mean somehow the whole execution of doJob gets deferred, unlike when you ensure it via fromCallable .