While looking at a heap dump I noticed that a lot of instances of SocketTubes where kept alive in memory. I was able to determine that the reason was that the test had retained all CompletableFuture returned by HttpClient, and there was a back reference from CF -> HttpResponseImpl -> HttpConnection -> SocketTube.

The reference to HttpConnection from HttpResponseImpl is only needed when establishing a WebSocket, so maybe we should set it to null in the normal case to avoid keeping this objects alive when the returned HttpResponse is held and stored by user code?