Iteratees make it possible to consume streams of data in a non-blocking way. Conversely, enumerators produce data streams in a non-blocking way. They are useful when you need to send large results, or if your result is built from an intermittent data source. An Enumerator[A] object defines a stream of values of type A.

The simplest way to create an enumerator is to use one of the methods of the Enumerator object. For instance, you can easily convert a java.io.InputStream class or a java.io.File class to an Enumerator[Array[Byte]] object as follows:

import play.api.libs.iteratee.Enumerator
Enumerator.fromStream(inputStream)
Enumerator.fromFile(file)

To send a stream of data as a response body, it is better to explicitly set the Content-Length response header so that the connection can be kept alive to serve further requests. Alternatively, you can use the chunked transfer encoding as follows:

Ok.chunked(Enumerator.fromFile(new File("foo.txt")))

To use the chunked transfer encoding in Java, you have to pass a Chunks value to your result:

StringChunks chunks = (out) -> {
  out.write("foo");
  out.write("bar");
  out.close();
}
return ok(chunks);

Note, however, that in the case of files, it is even better to use the sendFile method:

Ok.sendFile(new File("foo.txt"))

The sendFile method reads the file size using the filesystem and sets the Content-Length header accordingly so that the web browser can display a nice progress bar while downloading the response content. The Content-Type header is also set according to the file extension. Finally, it also sets the Content-Disposition header to the attachment so that browsers download the file instead of displaying it.

In Java, simply pass File as a parameter of your result:

return ok(new File("foo.txt"));