Can I return an API response without waiting on other external API calls in Spring Boot?

Is this the correct way to use @Async in Spring Boot?

@Service
class someServiceImpl {
...
  public someResponseDTO getUsers(int userId) {
   // Do some logic
   ...
   // Call external API with another service method from another service impl
   anotherService.emailUserInTheBackground(userId);
   return someResponseDTO;
  }
...
}

@Service
public class AnotherService {
  @Async
  public void emailUserInTheBackground(int userId) {
    // This might take a while...
    ...
  }
}

Since emailUserInTheBackground() has @Async annotation and void return type, does it block the line return someResponseDTO at all?

All I wanted is to return the response to the caller without waiting because emailUserInTheBackground() takes too long to complete and isn't directly tied to the response object.

Yes that is the correct way to run a task in the background, you can mimick the thread blocking behavior by introducing a delay.

@SpringBootApplication
@EnableAsync
public class MyApplication {

    public static void main(String[] arg) {
       SpringApplication.run(MyApplication.class);
    }
}

then you need to mark the emailUserInTheBackground method with @Async annotation.

@Service
class AnotherService {

    @Async
    public void emailUserInTheBackground(int userId) {
       try {
          TimeUnit.SECONDS.sleep(10);
          System.out.println("Print from async: "+ Thread.currentThread().getName());
       } catch (InterruptedException e) {
          e.printStackTrace();
       }
    }
}

Now add one more logger after a method call, you'll see getUsers(...) call completing first in a different thread even though the emailService thread is blocked for 10 seconds.

anotherService.emailUserInTheBackground(userId);
System.out.println("Print from service: "+ Thread.currentThread().getName());

you can also use CompletableFuture to run a task in the background.

 public someResponseDTO getUsers(int userId) {
   // some other task
   ...
   // Call external API with another service method from another service impl
   CompletableFuture.runAsync(() -> anotherService.emailUserInTheBackground(userId)) 
   return someResponseDTO;
  }