How can I read a file to an InputStream then write it into an OutputStream in Scala?

Question

I'm trying to use basic Java code in Scala to read from a file and write to an OutputStream, but when I use the usual while( != -1 ) in Scala gives me a warning "comparing types of Unit and Int with != will always yield true".

The code is as follows:

    val file = this.cache.get(imageFileEntry).getValue().asInstanceOf[File]
    response.setContentType( "image/%s".format( imageDescription.getFormat() ) )

    val input = new BufferedInputStream( new FileInputStream( file ) )
    val output = response.getOutputStream()

    var read : Int = -1

    while ( ( read = input.read ) != -1 ) {
        output.write( read )
    }

    input.close()
    output.flush()

How am I supposed to write from an input stream to an output stream in Scala?

I'm mostly interested in a Scala-like solution.

Answer 1

You could do this:

Iterator 
.continually (input.read)
.takeWhile (-1 !=)
.foreach (output.write)

Answer 2

If this is slow:

Iterator 
.continually (input.read)
.takeWhile (-1 !=)
.foreach (output.write)

you can expand it:

val bytes = new Array[Byte](1024) //1024 bytes - Buffer size
Iterator
.continually (input.read(bytes))
.takeWhile (-1 !=)
.foreach (read=>output.write(bytes,0,read))
output.close()

Answer 3

Assignment statements always return Unit in Scala, so read = input.read returns Unit, which never equals -1. You can do it like this:

while ({read = input.read; read != -1}) {
  output.write(read)
}

Answer 4

def stream(inputStream: InputStream, outputStream: OutputStream) =
{
  val buffer = new Array[Byte](16384)

  def doStream(total: Int = 0): Int = {
    val n = inputStream.read(buffer)
    if (n == -1)
      total
    else {
      outputStream.write(buffer, 0, n)
      doStream(total + n)
    }
  }

  doStream()
}

Answer 5

We can copy an inputstream to an outputstream in a generic and type-safe manner using typeclasses. A typeclass is a concept. It's one approach to polymorphism. In particular, it's parametric polymorphism because the polymorphic behavior is encoded using parameters. In our case, our parameters will be generic types to Scala traits.

Let's make Reader[I] and Writer[O] traits, where I and O are input and output stream types, respectively.

trait Reader[I] {
  def read(input: I, buffer: Array[Byte]): Int
}

trait Writer[O] {
  def write(output: O, buffer: Array[Byte], startAt: Int, nBytesToWrite: Int): Unit
}

We can now make a generic copy method that can operate on things that subscribe to these interfaces.

object CopyStreams {

  type Bytes = Int

  def apply[I, O](input: I, output: O, chunkSize: Bytes = 1024)(implicit r: Reader[I], w: Writer[O]): Unit = {
    val buffer = Array.ofDim[Byte](chunkSize)
    var count = -1

    while ({count = r.read(input, buffer); count > 0})
      w.write(output, buffer, 0, count)
  }
}

Note the implicit r and w parameters here. Essentially, we're saying that CopyStreams[I,O].apply will work iff there are Reader[I] and a Writer[O] values in scope. This will make us able to call CopyStreams(input, output) seamlessly.

Importantly, however, note that this implementation is generic. It operates on types that are independent of actual stream implementations.

In my particular use case, I needed to copy S3 objects to local files. So I made the following implicit values.

object Reader {

  implicit val s3ObjectISReader = new Reader[S3ObjectInputStream] {
    @inline override def read(input: S3ObjectInputStream, buffer: Array[Byte]): Int =
      input.read(buffer)
  }
}


object Writer {

  implicit val fileOSWriter = new Writer[FileOutputStream] {
    @inline override def write(output: FileOutputStream,
                               buffer: Array[Byte],
                               startAt: Int,
                               nBytesToWrite: Int): Unit =
      output.write(buffer, startAt, nBytesToWrite)
  }
}

So now I can do the following:

val input:S3ObjectStream = ...
val output = new FileOutputStream(new File(...))
import Reader._
import Writer._
CopyStreams(input, output)
// close and such...

And if we ever need to copy different stream types, we only need to write a new Reader or Writer implicit value. We can use the CopyStreams code without changing it!