Github user zsxwing commented on a diff in the pull request: https://github.com/apache/spark/pull/13335#discussion_r65411387 --- Diff: sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/Offset.scala --- @@ -17,11 +17,14 @@ package org.apache.spark.sql.execution.streaming +import com.fasterxml.jackson.annotation.JsonTypeInfo + /** * An offset is a monotonically increasing metric used to track progress in the computation of a * stream. An [[Offset]] must be comparable, and the result of `compareTo` must be consistent * with `equals` and `hashcode`. */ +@JsonTypeInfo(use = JsonTypeInfo.Id.CLASS, include = JsonTypeInfo.As.PROPERTY, property = "@class") --- End diff -- > What does this mean? What JSON does this produce? This is for polymorphic serialization. It will write the class name to the field `@class` (set by property = "@class"). When deserializing an object, it needs the class name to know the concrete class since the reflection APIs will only provide the interface name.
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