Reactive – Improve Mono syntax

I have this block of code that works fine:

return isBlacklistedToken(refreshToken, Boolean.TRUE).flatMap(isBlacklisted -> {
    if (isBlacklisted)
        return Mono.error(new UnauthorizedException(format("The user %s has already logged out.", username)));
    else
        return isBlacklistedToken(accessToken, Boolean.FALSE).flatMap(isABlacklisted -> {
            if (isABlacklisted)
                return Mono.error(new UnauthorizedException(format("The user %s has already logged out.", username)));
            else
                return blacklistTokens(username, refreshToken, accessToken);
        });
});

To summarize it:

• calls the isBlacklistedToken function (returns a Mono<Boolean> with the result of the refresh token)
• If the refresh token is blacklisted, throws an UnauthorizedException
• If the refresh token is not blacklisted, does the same process for the access token
• If both tokens are not blacklisted, finally blacklists them.

This syntax, while it works, seems a bit sloppy. Is there a way to improve that? I wrote this piece of code, and while it throws an exception, the last part (blacklisting the tokens) always executes – peraphs my knowledge of reactive programming is a bit off.

return isBlacklistedToken(refreshToken, Boolean.TRUE)
                .flatMap(isBlacklisted -> isBlacklisted ? Mono.error(new UnauthorizedException(format("The user %s has already logged out.", username))) : Mono.empty())
                .then(isBlacklistedToken(accessToken, Boolean.FALSE))
                .flatMap(isBlacklisted -> isBlacklisted ? Mono.error(new UnauthorizedException(format("The user %s has already logged out.", username))) : Mono.empty())
                .then(blacklistTokens(username, refreshToken, accessToken));

Edit: adding the isBlacklistedToken method

private Mono<Boolean> isBlacklistedToken(final String token, final Boolean type) {

        return blacklistService.isBlacklisted(token, type);
    }

and the respective blacklistService call (just a repository call, really simple)

public Mono<Boolean> isBlacklisted(final String token, final Boolean isRefresh) {
        return Mono.just(this.blacklistRepository.existsBlacklistByTokenAndIsRefresh(token, isRefresh));
    }

Answer

I would suggest the following:

return isBlacklistedToken(refreshToken, Boolean.TRUE)                 
    .filter(isBlacklisted -> !isBlacklisted)                 
    .flatMap(isBlacklisted -> isBlacklistedToken(accessToken, Boolean.FALSE))                 
    .filter(isBlacklisted -> !isBlacklisted)                 
    .flatMap(isBlacklisted -> blacklistTokens(username, refreshToken, accessToken))                 
    .switchIfEmpty(Mono.error(new UnauthorizedException(format("The user %s has already logged out.", username))));

Sorry if there is some compile error but I tried this in Kotlin and needed to translate it to Java, which is become less and less easy.