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I haven't finish a full review but I have two big picture comments:

1. The amount of @NonNull annotations is suspicious, since this is the default. You should very rarely need to use it. There are also many places where you apply @Nullable to a type variable, which is usually rare, but I think might be fine in some of these cases.
2. It feels like the many subclasses really might be simple as just functions / a switch statement.

Thank you so much for the review! I have refactored the ProtoBeamConverter class.

I've cherry-picked the commit at #35217. Thanks!

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@kennknowles could you please proceed with the review when you have time? Thank you.

I haven't gotten to make another pass yet. I did just look at the tests and there is a prism failure which is pretty rare. I restarted the test job anyhow but you might take a look at the gradle scan at https://develocity.apache.org/s/o35wszaauzfvy/tests/task/:runners:prism:java:test/details/org.apache.beam.runners.prism.PrismRunnerTest/windowing?top-execution=1 just to make sure it really was a flake

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I haven't gotten to make another pass yet. I did just look at the tests and there is a prism failure which is pretty rare. I restarted the test job anyhow but you might take a look at the gradle scan at https://develocity.apache.org/s/o35wszaauzfvy/tests/task/:runners:prism:java:test/details/org.apache.beam.runners.prism.PrismRunnerTest/windowing?top-execution=1 just to make sure it really was a flake

The test passed locally, so I triggered a rerun and it's now green.

Reminder, please take a look at this pr: @m-trieu @damccorm

R: @kennknowles

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@ahmedabu98

I think that the many cases of @NonNull and @Nullable applied to type variables indicates a problem. Each one really needs a comment justifying it, because it is unusual and actually a somewhat inflexible situation. Probably you can remove most of them. And some places where you have a type variable and you need to constrain what types it can be instantiated with, you would use extends @NonNull Object (the default if you do not annotate is extends @Nullable Object which allows the type variable to be instantiated with any type)

I've removed @NonNull and @Nullable from generic types in the ProtoBeamConverter class.

I realized I had significantly misunderstood their proper usage. Nullability annotations on generics are only for specific use cases with strong reasoning (apparently a container-like class?).

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@kennknowles could you review this pr again? Thanks