A little background to the need of such thing:
We want to connect to several different websocket-servers (lets say ~ 150).
The application runs in a container where no managed executor-services can be retreived via jndi-lookups. Therefore the ClientManager (or rather BaseContainer) will maintain its own thread-pools.
If we use a ClientManager per connection the thread usage is way to high (10 schedule-exec threads + 1-2 worker-threads = ~1800 threads)
If we share the same ClientManager accross all connections the reconnects are a bottleneck (only 10 threads for reconnects accross 150 connections). Furthermore we ran into the issue #640 from time to time.

One approach (because it seems that I'm the only one needing this) is a better extendability of the class org.glassfish.tyrus.client.ClientManager.
Currently the only visible constructor is the default one which internally calls the private constructor ClientManager(String containerProviderClassName, WebSocketContainer webSocketContainer). Therefore any extending class is required to use grizzly-client as container-provider.
I want to use the jdk-client so it would be helpful to have the constructor ClientManager(String containerProviderClassName, WebSocketContainer webSocketContainer) changed to visiblity protected
After that I would be able to override the methods BaseContainer.getExecutorService() and BaseContainer.getScheduledExecutorService() to return executor-services that are especially configured/customized (in terms of sizing etc.) for our needs.

Of course any other way to pass preconfigured executor-services would be sufficient, too (e.g. passing them via the ClientProperties).