AsyncStreamHandler

ApiClient.java

public static ListenableFuture<Response> readFile(final String documentUuid,
                                                  final OutputStream os) {
    final String requestUrl = ...;

    try {
        return new AsyncHttpClient().prepareGet(requestUrl)
                                    .addQueryParameter("documentUuid", documentUuid)
                                    .execute(new AsyncStreamHandler(os, documentUuid));
    } catch (final IOException e) {
        throw new UnexpectedException(e);
    }
}

Application.java

public static void download(final String uuid) throws Throwable {
    final Document document = ...;
    final PipedInputStream is = new PipedInputStream(8388608);
    ResourceApiClient.readFile(uuid, new PipedOutputStream(is));
    final String dispositionType = "attachment";
    final String contentDisposition = "%s; filename=\"%s\"";
    response.contentType = document.contentType;
    response.setHeader("Content-Disposition",
                       String.format(contentDisposition, dispositionType, document.name));
    response.setHeader("Content-Length", document.length + "");
    response.direct = is;
}

AsyncStreamHandler.java

public class AsyncStreamHandler implements AsyncHandler<Response> {

    private final Response.ResponseBuilder builder = new Response.ResponseBuilder();
    private final OutputStream os;
    private final Object id;

    public AsyncStreamHandler(final OutputStream os, final Object id) {
        this.os = os;
        this.id = id;
    }

    @Override
    public STATE onBodyPartReceived(final HttpResponseBodyPart content) throws Exception {
        content.writeTo(this.os);
        return STATE.CONTINUE;
    }

    @Override
    public STATE onStatusReceived(final HttpResponseStatus status) throws Exception {
        Logger.trace("Status %d received for stream %s", status.getStatusCode(), this.id);
        this.builder.accumulate(status);
        return STATE.CONTINUE;
    }

    @Override
    public STATE onHeadersReceived(final HttpResponseHeaders headers) throws Exception {
        this.builder.accumulate(headers);
        return STATE.CONTINUE;
    }

    @Override
    public Response onCompleted() throws Exception {
        Logger.trace("Stream body of %s completed. Closing OutputStream.", this.id);
        this.os.close();
        return this.builder.build();
    }

    @Override
    public void onThrowable(final Throwable t) {
        Logger.error(t, "Error while downloading stream %s", this.id);
    }

}