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https://issues.apache.org/jira/browse/FLINK-35555?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17853132#comment-17853132
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Zhanghao Chen commented on FLINK-35555:
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FLINK-23420 is similar to our case here. We can use a binary mask for marking
null values and rely on TypeSerializerSnapshot to deal with state-compatibility.
> Serializing List with null values throws NPE
> --------------------------------------------
>
> Key: FLINK-35555
> URL: https://issues.apache.org/jira/browse/FLINK-35555
> Project: Flink
> Issue Type: Sub-task
> Components: API / Type Serialization System
> Affects Versions: 1.20.0
> Reporter: Zhanghao Chen
> Priority: Critical
> Fix For: 1.20.0
>
>
> FLINK-34123 introduced built-in serialization support for java.util.List,
> which relies on the existing {{ListSerializer}} impl. However,
> {{ListSerializer}} does not allow null values, as it is originally designed
> for serializing {{ListState}} only where null value is explicitly forbidden
> in the contract.
> Directly adding null marker to allow null values will break backwards state
> compatibility, so we'll need to introduce a new List serializer called
> {{NullableElementListSerializer}} and the corrsponding TypeInformation called
> {{NullableElementListTypeInfo}} that allows null values for serializing user
> objects, and leaves the existing {{ListSerializer}} and {{ListTypeInfo}} for
> Flink's internal state use.
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