Wrong ClassTag in DefaultAsyncCacheApi

Play Version 2.6.x

API Java to Scala

JDK Oracle java version "1.8.0_151"

DefaultAsyncCacheApi internally invokes Scala implementation of the API, which requires
passing there the ClassTag of type T. This works fine with DefaultSyncCacheApi as the
type T usually is not nested in some other type such as X<T>. However, in DefaultAsyncCacheAPI,
the type T is nested inside the CompletionStage<T>. That causes the type T being erased at runtime, which results in Scala.<T>classTag() returning Any.

When the Scala implementation assumes a valid ClassTag and uses it, e.g., for serialization or deserialization of the value, it fails as the ClassTag is not correct.

Note: Scala implementation works because there is a correct ClassTag.

Example

play-redis implements the CacheApi against the Redis server and uses the ClassTag for deserialization of the value.

play.cache.AsyncCacheApi cache; // impl is DefaultAsyncCacheApi
cache.getOrElseUpdate(key, () -> CompletableFuture.completedFuture("Stored value"), 5)

The type T is a String here, but DefaultAsyncCacheApi passes inside the Scala implementation Any. See here.

Start at framework/src/main/java/play/cache/DefaultAsyncCacheApi.java, especially the ClassTag invocation around line 47, and compare it with DefaultSyncCacheApi. Trace the Java-to-Scala call for getOrElseUpdate using the provided CompletableFuture example; done means the Scala implementation receives the actual value type rather than Any for serialization or deserialization.