AtomicInteger counter = new AtomicInteger();

PublishSubject<Integer> testSubject = PublishSubject.create();

testSubject
.observeOn(Schedulers.io())
.doOnNext(val -> {
  int count = counter.incrementAndGet();

  System.out.println(String.format(
      "doOnNext - #%s %s %d",
      Thread.currentThread().getId(),
      val,
      count
  ));
  Thread.sleep(20);

  if(count > 1)
    System.out.println("Whoa!!!!"); // <- never happens

  counter.decrementAndGet();
})
.subscribe();