how to get main thread variables in RxJava3

Question

Say I have some variables stored in ThreadLocal and how can I fetch them in below 'mapper' method?

ThreadLocal<String> data = new ThreadLocal<>();
data.set("some data");

Flowable.fromIterable(xxx)
                .parallel(4)
                .runOn(io())
                .map(this::mapper)
                .sequential()
                .blockingStream().collect(Collectors.toList());