How to convert Source[ByteString, Any] to InputStream

Question

akka-http represents a file uploaded using multipart/form-data encoding as Source[ByteString, Any]. I need to unmarshal it using Java library that expects an InputStream.

How Source[ByteString, Any] can be turned into an InputStream?

Answer 1

As of version 2.x you achieve this with the following code:

import akka.stream.scaladsl.StreamConverters
...
val inputStream: InputStream = entity.dataBytes
        .runWith(
           StreamConverters.asInputStream(FiniteDuration(3, TimeUnit.SECONDS))
        )

See: http://doc.akka.io/docs/akka-stream-and-http-experimental/2.0.1/scala/migration-guide-1.0-2.x-scala.html

Note: was broken in version 2.0.2 and fixed in 2.4.2

Answer 2

You could try using an OutputStreamSink that writes to a PipedOutputStream and feed that into a PipedInputStream that your other code uses as its input stream. It's a little rough of an idea but it could work. The code would look like this:

import akka.util.ByteString
import akka.stream.scaladsl.Source
import java.io.PipedInputStream
import java.io.PipedOutputStream
import akka.stream.io.OutputStreamSink
import java.io.BufferedReader
import java.io.InputStreamReader
import akka.actor.ActorSystem
import akka.stream.ActorFlowMaterializer

object PipedStream extends App{
  implicit val system = ActorSystem("flowtest")
  implicit val mater = ActorFlowMaterializer()

  val lines = for(i <- 1 to 100) yield ByteString(s"This is line $i\n")
  val source = Source(lines)

  val pipedIn = new PipedInputStream()
  val pipedOut = new PipedOutputStream(pipedIn)      
  val flow = source.to(OutputStreamSink(() => pipedOut))
  flow.run()

  val reader = new BufferedReader(new InputStreamReader(pipedIn))
  var line:String = reader.readLine
  while(line != null){
    println(s"Reader received line: $line")
    line = reader.readLine
  }           
}

Answer 3

You could extract an interator from ByteString and then get the InputStream. Something like this (pseudocode):

source.map { data: ByteString =>
  data.iterator.asInputStream
}

Update

A more elaborated sample starting with a Multipart.FormData

def isSourceFromFormData(formData: Multipart.FormData): Source[InputStream, Any] = 
 formData.parts.map { part => 
   part.entity.dataBytes
   .map(_.iterator.asInputStream)
}.flatten(FlattenStrategy.concat)