Spring 5 Web Reactive - Clarification about DTO response body conversion

Question

According to the doc :

The response body can be one of the following:

• Account — serialize without blocking the given Account; implies a synchronous, non-blocking controller method.

How come a synchronous controller method can be non-blocking? It sounds like a contradiction.

Answer 1

Here is an example of a synchronous, non-blocking controller method:

@GetMapping("/sample/account")
public Account sample() {
  return new Account("codependent");
}

It's synchronous because it does not return a type that signals a deferred result (like Mono or Flux ). It's non-blocking because no blocking operation is performed (I/O, waiting on a shared resource, etc).

You could wrap it with a Mono like Mono.just(new Account("codependent")) , but you'd just pay the cost of that async type for no reason.