feat: Add universe domain support for Java

[michaelpri10]

This adds a setUniverseDomain() method for the Publisher and Subscriber class so that users can easily set the universe domain, instead of having to create a StubSettings object. Additionally, this sets the default endpoint in the Publisher/Subscriber builders to null, which will still resolve to the default endpoint.

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Fixes: N/A ☕️

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[lqiu96]

Thanks! I think the change LGTM from my end. Were you able test this locally and connect properly?

[michaelpri10]

Thanks! I think the change LGTM from my end. Were you able test this locally and connect properly?

Yes, I was able to successfully publish and subscribe when setting the universeDomain in modified versions of PublisherExample and SubscribeAsyncExample.

[hongalex]

Is this still being used somewhere? I think it's autogenerated so it can't be removed IIRC

[michaelpri10]

This line was added when endpoint overriding was enabled for the Publisher/Subscriber here. If a user sets the endpoint when building the Publisher, we propagate that to the PublisherStubSettings builder, which is autogenerated. Otherwise, PublisherStubSettings uses the getDefaultEndpoint() method.

[lqiu96]

To add a bit more context from the GAPICs perspective. Gax-Java now will resolve the endpoint automatically so there is no need to set a default endpoint. One of the parameters that we take into consideration is the user set endpoint which is configured from the Builder's setEndpoint(). All these factors (i.e. mtls, user set endpoint, user set universe domain, and more) will be considered when creating the final endpoint to be used.

The reason why we're suggesting to not pass the getDefaultEndpoint to Gax is that Gax-Java won't be able to determine if the endpoint passed in is a user configuration or not. The new endpoint logic has user configuration override everything so it will always default to pubsub.googleapis.com if it is passed in.