How can I get content from InputStream based results in Play tests

I am on Play 2.6, using Java

My controller returns:

public Result xml() {
    return Results.ok(new ByteArrayInputStream("<someXml />".getBytes()));
}

I want to parse the result in tests:

Result result = new MyController().xml();    
play.test.Helpers.contentAsString(result)

This throws

failed: java.lang.UnsupportedOperationException: Tried to extract body from a non strict HTTP entity without a materializer, use the version of this method that accepts a materializer instead

How can I retrieve the content of results issued from input streams in tests?

As the exception message states, since your result is a streamed entity, use the version of contentAsString that takes a Materializer. Here's an example from HelpersTest.java in the Play repository that uses that method:

@Test
public void shouldExtractContentAsStringFromAResultUsingAMaterializer() throws Exception {
    ActorSystem actorSystem = ActorSystem.create("TestSystem");

    try {
        Materializer mat = ActorMaterializer.create(actorSystem);

        Result result = Results.ok("Test content");
        String contentAsString = Helpers.contentAsString(result, mat);
        assertThat(contentAsString, equalTo("Test content"));
    } finally {
        Future<Terminated> future = actorSystem.terminate();
        Await.result(future, Duration.create("5s"));
    }
}