[akka-streams-java] Exception in completionstage handling disappears without a trace

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murtuza chhil

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Oct 18, 2016, 1:42:35 AM10/18/16

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Hi,

Just some simple java code to learn some basics of materialized values.

There is an intentional error in the code snippet in the String.format that uses an invalid %l which actually throws an exception but nothing gets printed in the console.

What exactly is happening and how does one handle a scenarios like this to actually see/debug the exception?


public class Sample {

    public static void main(String[] args) throws IOException{

        ActorSystem system = ActorSystem.create("system");
        ActorMaterializer mat = ActorMaterializer.create(system);

        Source<ByteString, CompletionStage<IOResult>> source = FileIO.fromFile(new File("blah.txt"));

        Sink<ByteString, CompletionStage<IOResult>> sink = FileIO.toFile(new File("c:\\temp\\blah.txt.copy"));

        RunnableGraph<CompletionStage<IOResult>>  runnableGraph = source.to(sink);

        //RunnableGraph<CompletionStage<IOResult>> runnableGraph1 = source.toMat(sink, Keep.left());

        //RunnableGraph<CompletionStage<IOResult>> runnableGraph2 = source.toMat(sink, Keep.right());

        //RunnableGraph<Pair<CompletionStage<IOResult>, CompletionStage<IOResult>>> pairRunnableGraph = source.toMat(sink, Keep.both());

        final CompletionStage<IOResult> cs = runnableGraph.run(mat).handle((x, t) -> {
            if(t!=null){
                System.out.println("Error: " + t);
            }
            if (x!=null){
                String s = String.format("%s , %l bytes read",x.status(),x.count());
                System.out.println(s);
            }
            return null;
        });