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?

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
}

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.

Have you tried

@delegate:Transient

?

Looks good, thanks!

I would rather use a

JPAConverter

instead of having the

objectMapper

inside my entity.

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.

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 🤣