Five Mistakes to Avoid in Reactive Java

How not to look like a noob.

Image by OpenClipart-Vectors from Pixabay

Reactive Java has been around for almost a decade now but I still see people that are struggling with it and, even worse, abusing it. So I thought I’d jot down the kinds of things I never want to see in a pull request.

1. Blocking

This should be obvious. It’s supposed to be non-blocking, right? But I still see it on occasion because people don’t know how to link a method back into an original flow. As an example, let’s say you’re upgrading an existing application from regular Spring Boot to WebFlux. Your rest controller GET method used to return a String and now it returns a Mono<String>. But your service method that does the actual work still returns a String. So in your GET method, you use Mono.just() to return the correct type. Now it looks like this:

@GetMapping("/string")
Mono<String> returnsAMono() {
    return Mono.just(StringService.returnsAString());
}

At the same time, another programmer is upgrading the service to use the new WebClient instead of RestTemplate, but they have to return a String. So they block:

public interface StringService {
    static WebClient wc = WebClient.builder()
            .baseUrl("https://foaas.com/")
            .build();
    static String returnsAString()…