Hey - I’ve got an immutable JPA Entity. It’s got a...

# server

[Rob Elliot]

Hey - I’ve got an immutable JPA Entity. It’s got a big String field on it which happens to contain JSON, and I’ve got a method that uses jackson to turn it into a Map.

I’d like to memoize the result of that, as it’s never going to change, but lazily so I don’t incur the cost until I need it.

But this doesn’t work:

@Entity data class MyImmutableEntity(val jsonString: String) {

  @javax.persistence.Transient private val jsonMap: Map<String, Any> by lazy {
    jacksonObjectMapper().readValue(jsonString)
  }

  fun getJsonMap(): Map<String, Any> = jsonMap
}

The compiler complains about @Transient - This annotation is not applicable to target 'member property with delegate'.

Anyone know of a workaround for this?

[Robert Jaros]

Instead of lazy property you could create a transient, nullable var property and put the logic into getJsonMap() function, something like that:

return jsonMap ?: run {
   jsonMap = jacksonObjectMapper().readValue(jsonString)
   jsonMap
}

[Rob Elliot]

Yes, I guess basically rolling my own lazy would work, but it’s got all the problems lazy solves around thread safety… I was hoping there’d be a nice way to keep using lazy.

[Robert Jaros]

Have you tried @delegate:Transient ?

[Robert Jaros]

https://stackoverflow.com/questions/50461756/jpa-by-kotlin-by-lazy-and-transient-not-work-with-hibernate

[Rob Elliot]

Looks good, thanks!

[Rob Elliot]

Sadly it doesn’t seem to work - something in the dynamic proxying I’d guess, it throws a NullPointerException.

[carlos]

I would rather use a JPAConverter instead of having the objectMapper inside my entity.

[carlos]

IMHO there shouldn’t be anything external to the entity inside it

[carlos]

the JPAConverter would take care of parsing and serializing the json

[Rob Elliot]

Wow, blast from the past - the post was 10 months ago & I left the project I was doing this on 6 months ago.

FWIW the objectMapper wasn't in the entity, it was statically referenced.

[carlos]

I just logged in into the channel after a long time and this was one of the first messages I saw… I did not realize it was that old 🤣

[carlos]

and the problem looks very similar to what I am doing in another project so I decided to reply 🙂