Throw exception by condition. Spring web flux

Question

I have the following reactive code. And I want to throw my custom exception, if return value is null

ReactiveSecurityContextHolder.getContext()
                .map(SecurityContext::getAuthentication)
                .map(authentication ->(UserAuthenticationToken) authentication)
                .map(UserAuthenticationToken::getUserPrincipalName)
                //Here I want to throw exception, if 
                 //UserAuthenticationToken::getUserPrincipalName will return null

something like :

 .map(UserAuthenticationToken::getUserPrincipalName)
               .switchIfEmpty(Mono.error(new MissingPrincipalException("Missing email field in the JWT token")));

or :

.map(UserAuthenticationToken::getUserPrincipalName)
               .filter(principal -> !Objects.isNull(principal))
               .switchIfEmpty(Mono.error(new MissingPrincipalException("Missing email field in the JWT token")))

does not work.

Answer 1

The reactive streams specification disallows null values in a sequence, therefore you can't use map(UserAuthenticationToken::getUserPrincipalName).

One way to handle such case is to use flatMap

ReactiveSecurityContextHolder.getContext()
     .map(ctx -> (UserAuthenticationToken) ctx.getAuthentication())
     .flatMap(token -> {
        if (token.getUserPrincipalName() == null) {
             return Mono.error(new MissingPrincipalException("Missing email field in the JWT token"));
        }
        
        return Mono.just(token.getUserPrincipalName());
     })

You can also use handle operator to remove any nulls. Reactor Reference Guide has a good example for Using handle for a "map and eliminate nulls" scenario