doOnSubscribe.java

        Observable.just(1)
                .doOnSubscribe(() -> System.out.println("before 1st doOnSubscribe: " + Thread.currentThread().getName()))
                .subscribeOn(Schedulers.newThread())
                .doOnSubscribe(() -> System.out.println("before 2nd doOnSubscribe: " + Thread.currentThread().getName()))
                .subscribeOn(Schedulers.io())
                .doOnSubscribe(() -> System.out.println("before 3rd doOnSubscribe: " + Thread.currentThread().getName()))
                .subscribeOn(Schedulers.computation())
                .doOnSubscribe(() -> System.out.println("before subscribe: " + Thread.currentThread().getName()))
                .subscribe(new Subscriber<Integer>() {
                    @Override
                    public void onCompleted() {

                    }

                    @Override
                    public void onError(Throwable e) {

                    }

                    @Override
                    public void onNext(Integer integer) {

                    }
                });
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        
Output:
before subscribe: main
before 3rd doOnSubscribe: RxComputationThreadPool-1
before 2nd doOnSubscribe: RxCachedThreadScheduler-1
before 1st doOnSubscribe: RxNewThreadScheduler-1

doOnSubscribe有什么用呢？doOnSubscribe可以通过subscribeOn来切换执行的线程。onStart做不了。

        // 当前线程不是UI线程
        Observable.create(...) // 这是一个IO操作
                .subscribeOn(Schedulers.io())
                .doOnSubscribe(() -> view.XXX) // 这是一个UI操作。
                .subscribeOn(AndroidSchedulers.mainThread())
                ...