ContentServiceAsync.retrieve does not return the contents that you retrieved, and usage of @MustBeClosed is wrong #489
Description
ContentServiceAsync.retrieve() just returns a CompletableFuture<Void>. So there's no way to actually access what was retrieved by the API call.
The only way to do it is with ContentServiceAsync.withRawResponse().retrieve(). This returns a CompletableFuture<HttpResponse> from which you can access the body as an InputStream. This works, but has its own problem. This method is annotated with @MustBeClosed, which does not apply to CompletableFuture. The contract of @MustBeClosed is that the actual return value has to be closed, but you can't close a CompletableFuture. What you want here is to ensure that the HttpResponse is closed, but that's not how the check works.
This CompletableFuture problem is actually not limited to ContentServiceAsync. I see the same problem in FileServiceAsync, and it may be in others as well.