foo.java

AsyncResCF<String> ar = new AsyncResCF<>();

vertx.executeBlocking(fut -> {
    // Your blocking code in here
}, ar);

ar.whenComplete((s, t) -> {
    // CF callbacks will be always be called on the correct Vert.x thread, no need to worry about
    // fork-join pools or anything  like that
});


// AsyncResCF is a simple utility class for adapting Handler<AsyncResult> and CompletableFuture:

public class AsyncResCF<T> extends CompletableFuture<T> implements Handler<AsyncResult<T>> {

    @Override
    public void handle(AsyncResult<T> ar) {
        if (ar.succeeded()) {
            complete(ar.result());
        } else {
            completeExceptionally(ar.cause());
        }
    }
}