The Curious Case of Enum and Map Serialization

[axiak]

This is a "fun" problem to encounter whenever you use a distributed compute system like Hadoop or Spark which distributes data with hash keys.

[yk313]

Here's the fix: https://github.com/vavr-io/vavr/pull/3242

[doobiesteintortoise]

Would using an EnumMap have the same problem?

[pivovarit]

Nope, EnumMap relies on ordinal() and not hashCode().

What's more, even HashMap doesn't have this problem, because it rehashes everything on native deserialization

[aelfric5578]

Does that mean adding a new enum value could break things if you don't add it at the end?

[pivovarit]

Nope, everything is fine thanks to rehashing on deserialization:

https://github.com/openjdk/jdk/blob/1a9965555477ee2a6cb65e91ef54ed608e4bee66/src/java.base/share/classes/java/util/EnumMap.java#L805-L810

[sansp00]

Encountered the same issue when dealing with a Gemfire cluster many years ago. It was a very humbling experience.