How not to use CompletableFuture as a shared state with EventListener in Spring

How not to use CompletableFuture as a shared state with EventListener in Spring

Problem Description:

I need to asynchronously react to the @EventListener, therefore I’ve created something like this.

@Service
public class AsyncHandler {

    private CompletableFuture<My> future;

    @Async
    public CompletableFuture<My> getMy() {
        future = new CompletableFuture<>();
        return future;
    }

    @EventListener
    public void processEvent(MyEvent event) {
        future.complete(event.my());
    }
}

The problem here is that AsyncHandler is now stateful. Which is wrong.
And I don’t want to use database, so is there any other way to make the bean stateless while using @EventListener?

Solution – 1

You are right, your singleton has state, which is "not good".

One (possible) solution:

1. make the/refactor "statfeul part" to a "prototype" (request, session) scope bean.
2. make your "singleton" abstract!
3. inject the "stateful part" via "method injection" (we cannot "auto wire" lower(shorter-living) scope beans to higher ones…)

As code (example):

1. State holder:

   public class MyStateHolder {
      // State:
      private CompletableFuture<My> future;
   
      @Async // ?
      public CompletableFuture<My> getMy() {
        future = new CompletableFuture<>();
        return future;
      }
   }
   

2. Abstract, (no @Service ..yet, no state!):

   public abstract class AsyncHandler {
     @EventListener
     public void processEvent(MyEvent event) { 
        // !!
        delegate().getMy().complete(event.my());
     }
   
     // and now only (abstract!):
     public abstract MyStateHolder delegate();
   }
   

3. Wiring :

   @Configuration
   class MyConfig {
   
     @Bean
     @Scope("prototype") // !
     public MyStateHolder stateful() {
       return new MyStateHolder();
     }
   
     @Bean // singleton/service:
     public AsyncHandler asyncHandler() {
       return new AsyncHandler() { // !
         @Override // !
         public MyStateHolder delegate() {
           return stateful();// !;)
         }
       };
     }
   }
   

refs: (most of) https://docs.spring.io/spring-framework/docs/current/reference/html/core.html

Especially:
https://docs.spring.io/spring-framework/docs/current/reference/html/core.html#beans-factory-scopes-sing-prot-interaction