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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