Spring-boot async controller via CompletableFuture works synchronicity - spring

Question

I created 2 spring-boot services: slow-service and gateway
slow-service is service which reverses string but do it very slowly.
#Service
public class SlowReverseService implements ReverseService {
#Override
public String reverse(String message) {
StringBuilder builder = new StringBuilder(message);
try {
TimeUnit.SECONDS.sleep(30);
} catch (InterruptedException e) {
e.printStackTrace();
}
return builder.reverse().toString();
}
}
And Controller:
#RestController
public class SlowController {
private final ReverseService reverseService;
public SlowController(ReverseService reverseService) {
this.reverseService = reverseService;
}
#GetMapping(value = "/reverse/{message}")
public String reverseMessage(#PathVariable("message") String message){
return reverseService.reverse(message);
}
}
gateway uses slow-service and returns an async result.
#Service
public class AsyncReverseService implements ReverseService {
#Async
#Override
public CompletableFuture<String> reverse(String message) {
RestTemplate restTemplate = new RestTemplate();
return CompletableFuture.completedFuture(
restTemplate.getForObject("http://localhost:8081/reverse/" + message, String.class));
}
}
And Controller:
#RestController
public class GatewayController {
private final ReverseService reverseService;
public GatewayController(ReverseService reverseService) {
this.reverseService = reverseService;
}
#GetMapping(value = "/reverse/{message}")
public CompletableFuture<String> reverseMessage(#PathVariable("message") String message) {
String name = Thread.currentThread().getName();
System.out.println(new Date() + ":" + name);
return reverseService.reverse(message);
}
}
I set this param in the properties of gateway: server.tomcat.threads.max=2
I try to send 6 requests to the gateway and see this log:
Thu Aug 13 10:59:28 MSK 2020:http-nio-8080-exec-1
Thu Aug 13 10:59:29 MSK 2020:http-nio-8080-exec-2
Thu Aug 13 10:59:58 MSK 2020:http-nio-8080-exec-1
Thu Aug 13 10:59:59 MSK 2020:http-nio-8080-exec-2
Thu Aug 13 11:00:28 MSK 2020:http-nio-8080-exec-1
Thu Aug 13 11:00:29 MSK 2020:http-nio-8080-exec-2
My gateway controller receives 2 requests and sends it to slow-service. Slow-service works 30 sec and returns the result to the gateway. The gateway can process the next 2 requests. The Gateway can process 2 requests each 30 sec.
I Expected another behavior: Gateway receives 2 requests and delegates it to CompletableFuture. And frees threads. And Can receive the next 2 requests. Connections increases but my 2 threads are always available.