Guice: how do you configure an @Provides and @Singleton in a module in this case?

I have a provider method in a module annotated with @Provides:

@Provides
public ChatServicePerformanceMonitor getChatServicePerfMon() {
  ...
}

and I have annotated my ChatServicePerformanceMonitor with @Singleton. In my code, where I use this instance, I can't have guice "passively" inject it, due to a framework I'm using that's constructing the enclosing class (it does not use Guice, so this is the only way I know of to get the reference):

chatServicePerfMon = injector.getInstance(ChatServicePerformanceMonitor.class);

It seems Guice does not respect the @Singleton annotation on my ChatServicePerformanceMonitor class. I get an instance for every call to injector.getInstance(ChatServicePerformanceMonitor.class).

Adding the @Singleton to the provider method seems to fix this:

@Provides @Singleton
public ChatServicePerformanceMonitor getChatServicePerfMon() {
  ...
}

Is that the expected behavior? It seems an @Singleton on the instance should be all I would need.

Solution

If you're creating the ChatServicePerformanceMonitor like this:

@Provides
public ChatServicePerformanceMonitor getChatServicePerfMon() {
  return new ChatServicePerformanceMonitor();
}

then your class level @Singleton annotation will have no effect because Guice isn't creating the object, you are. Guice can only enforce scope on objects it creates. There's nothing wrong with adding @Singleton to your getChatServicePerfMon() method.

If you have a no-argument constructor (or an @Inject constructor) on the ChatServicePerformanceMonitor class and you remove your @Provides method then continual calls to the injector will return the same singleton.