Using scala.concurrent.Future on Android

From Scala 2.10, a concurrent execution function scala.concurrent.ops.spawn is deprecated, replaced with scala.concurrent.Future, and will be removed from Scala 2.11.

I rewrote my app with scala.concurrent.Future as an official Scala documentation suggests:

import ExecutionContext.Implicits.global
Future {
// ...
}

However, after I uploaded the app to Google Play, I received a ton of crash report:

java.lang.ExceptionInInitializerError
at scala.concurrent.impl.ExecutionContextImpl.createExecutorService(ExecutionContextImpl.scala:77)
at scala.concurrent.impl.ExecutionContextImpl.<init>(ExecutionContextImpl.scala:28)
at scala.concurrent.ExecutionContext$Implicits$.global$lzycompute(ExecutionContext.scala:63)
at scala.concurrent.ExecutionContext$Implicits$.global(ExecutionContext.scala:63)
at com.soundcorset.client.android.SoundcorsetService$Metronome.start(SoundcorsetService.scala:38)
...
Caused by: java.lang.Error: java.lang.NoSuchFieldException: parkBlocker
at scala.concurrent.forkjoin.ForkJoinPool.<clinit>(ForkJoinPool.java:2852)
... 20 more
Caused by: java.lang.NoSuchFieldException: parkBlocker
at java.lang.ClassCache.findFieldByName(ClassCache.java:510)
at java.lang.Class.getDeclaredField(Class.java:683)
at scala.concurrent.forkjoin.ForkJoinPool.<clinit>(ForkJoinPool.java:2847)
... 20 more

Scala library has its own copy of ForkJoinPool implementation, which includes dynamic invocations, that is parkBlocker in this case. Unfortunately, some Android devices does not have this method, so we've got this awful crash report.

The solution is very simple: Do not use ExecutionContext.Implicits.global. Declare a custom implicit ExecutionContext instead. For example:

implicit val exec = ExecutionContext.fromExecutor(
  new ThreadPoolExecutor(100, 100, 1000, TimeUnit.SECONDS,
    new LinkedBlockingQueue[Runnable]))

Future {
// ...
}

This works perfect in production.

EDIT: If the minimum API level of your app is above 11, consider using AsyncTask.THREAD_POOL_EXECUTOR instead of making your own thread executor:

implicit val exec = ExecutionContext.fromExecutor(
  AsyncTask.THREAD_POOL_EXECUTOR)

Thanks Niklas Klein for this suggestion.