src/main/java/io/reactivex/plugins/RxJavaPlugins.java
@SuppressWarnings("rawtypes")
    public static void setOnFlowableSubscribe(@Nullable BiFunction<? super Flowable, ? super Subscriber, ? extends Subscriber> onFlowableSubscribe) {
        if (lockdown) {
            throw new IllegalStateException("Plugins can't be changed anymore");
        }
        RxJavaPlugins.onFlowableSubscribe = onFlowableSubscribe;
    }
@SuppressWarnings("rawtypes")
    public static void setOnFlowableSubscribe(@Nullable BiFunction<? super Flowable, ? super Subscriber, ? extends Subscriber> onFlowableSubscribe) {
        if (lockdown) {
            throw new IllegalStateException("Plugins can't be changed anymore");
        }
        RxJavaPlugins.onFlowableSubscribe = onFlowableSubscribe;
    }
Sets the specific hook function.
@param onFlowableSubscribe the hook function to set, null allowed