How to create a generic wrapper for just any method call?

Question

I want to create a helper method that can wrap/convert just any sync method call into an async Mono.

The following is close, but shows an error:

Required type: Mono <T>
Provided: Mono<? extends Callable<? extends T>>

This is my code:

public <T> Mono<T> wrapAsync(Callable<? extends T> supplier) {
    return Mono.fromCallable(() -> supplier)
            .subscribeOn(Schedulers.boundedElastic());
}

public void run() {
    Mono<Boolean> mono = wrapAsync(() -> syncMethod());
}

private Boolean mySyncMethod() {
    return true; //for testing only
}

If `Mono.fromCallable` takes a `Callable`, then `(() -> supplier)` is the wrong argument (it's more like a `Callable>`). — ernest_k, Oct 20 '20 at 10:16

JEY · Accepted Answer · 2020-10-20T11:24:11.633

First you call Mono.fromCallable with a Callable<Callable<? extend T>>. You need to change the call like this: Mono.fromCallable(supplier).

Then you will have a problem because Mono.fromCallable will be inferred as Callable<? extend ? extend T> so your Mono will be Mono<? extend T> instead of Mono<T>. To avoid this, two solutions:

Change the signature of wrapAsync:

public <T> Mono<T> wrapAsync(Callable<T> supplier) {
    return Mono.fromCallable(supplier)
            .subscribeOn(Schedulers.boundedElastic());
}

Or if you want to keep the signature you need to provide type:

public <T> Mono<T> wrapAsync(Callable<? extends T> supplier) {
    return Mono.<T>fromCallable(supplier)
            .subscribeOn(Schedulers.boundedElastic());
}

Okay, yes that makes sense. My problem was I though I'd have to provide the supplier itself also as a lambda call. — membersound, Oct 20 '20 at 11:35

How to create a generic wrapper for just any method call?

1 Answers1