How to write a cache loader for Caffeine LoadingCache in "Scala" for "refreshAfterWrite" to work

Question

Scala application use case:

We have a Scala based that module reads the data from global cache (Redis) and save the same into local cache(Caffeine LoadingCache). As we want this data to be refreshed asynchronously, we are using LoadingCache with refreshAfterWrite duration set to refresh window of 2.second.

Question: Not question but need help with the following code that is giving warning and also compile time errors

Warning: For build method, it gives warning as Implements member load in CacheLoader (com.github.benmanes.caffeine.cache)

Compile time error 1: type arguments [Int,redisToCaffeine.DataObject] conform to the bounds of none of the overloaded alternatives of value build: [K1 <: Object, V1 <: Object](x$1: com.github.benmanes.caffeine.cache.CacheLoader[_ >: K1, V1])com.github.benmanes.caffeine.cache.LoadingCache[K1,V1] <and> [K1 <: Object, V1 <: Object]()com.github.benmanes.caffeine.cache.Cache[K1,V1] .build[Int, DataObject](key => loader(key))

Compile time error 2: wrong number of type parameters for overloaded method value build with alternatives: [K1 <: Object, V1 <: Object](x$1: com.github.benmanes.caffeine.cache.CacheLoader[_ >: K1, V1])com.github.benmanes.caffeine.cache.LoadingCache[K1,V1] <and> [K1 <: Object, V1 <: Object]()com.github.benmanes.caffeine.cache.Cache[K1,V1] .build[Int, DataObject](key => loader(key))

Code:

package redisToCaffeine

import scala.concurrent.duration._

import com.github.benmanes.caffeine.cache.{ CacheLoader, Caffeine, LoadingCache }
import com.twitter.finagle.stats.InMemoryStatsReceiver
import javax.annotation.Nullable
import redisToCaffeine.CacheImplicits.StatsReportingCaffeineCache

class LocalDealService {

  class DataObject(data: String) {
    override def toString: String = {
      "[ 'data': '" + this.data + "' ]"
    }
  }

  val defaultCacheExpireDuration: FiniteDuration = 2.second
  val stats: InMemoryStatsReceiver = new InMemoryStatsReceiver

  // loader helper
  @Nullable
  @throws[Exception]
  protected def loader(key: Int): DataObject = { // this will replace to read the data from Redis Cache
    new DataObject(s"LOADER_HELPER_$key")
  }

  def initCache(maximumSize: Int = 5): LoadingCache[Int, DataObject] = {
    Caffeine
      .newBuilder()
      .maximumSize(maximumSize)
      .refreshAfterWrite(defaultCacheExpireDuration.length, defaultCacheExpireDuration.unit)
      .recordStats()
      .build[Int, DataObject](key => loader(key))
      .enableCacheStatsReporting("deal-service", stats)
  }
}

I'm new to Scala and Caffeine both so not sure what I'm be doing wrong; I tried different ways mentioned here and here to write loader but nothing worked (mainly they are in Java). Little research around Scala bounds doesn't helped here any way. Kindly help.

Answer 1

Depends on which Scala version is being used here.

Although Scala (2.12 and later) Functions support conversions to Java SAM, these are done only when explicitly required. So if you are using Scala 2.12 or later, you can explicitly ask the compiler to convert the Scala function to SAM,

Also, don't use Int as key for the cache. Although it will work because of implicit conversions to Integer, that is not a good practice.

def initCache(maximumSize: Int = 5): LoadingCache[Integer, DataObject] = {
  Caffeine
    .newBuilder()
    .maximumSize(maximumSize)
    .refreshAfterWrite(defaultCacheExpireDuration.length, defaultCacheExpireDuration.unit)
    .recordStats()
    .build[Integer, DataObject]((key => loader(key)): CacheLoader[Integer, DataObject])
    .enableCacheStatsReporting("deal-service", stats)
  }

And if you are dealing with older Scala versions, then just forget that SAM exists and do it old style.

def initCache(maximumSize: Int = 5): LoadingCache[Integer, DataObject] = {
  Caffeine
    .newBuilder()
    .maximumSize(maximumSize)
    .refreshAfterWrite(defaultCacheExpireDuration.length, defaultCacheExpireDuration.unit)
    .recordStats()
    .build[Int, DataObject](new CacheLoader[Integer, DataObject] {
      override def load(key: Integer): DataObject = loader(key)
    })
    .enableCacheStatsReporting("deal-service", stats)
  }