Spring WebFlux Webclient receiving an application/

I'm prototyping a small Spring WebFlux application in Kotlin. This application needs to GET a tar archive from a remote REST endpoint and store it locally on disk. Sounds simple.

I first created an integration test that starts the spring server and one other WebFlux server with a mock REST endpoint that serves the tar archive.

The test should go like:

1) app: GET mock-server/archive

2) mock-server: response with status 200 and tar archive in body as type attachment

3) app: block until all bytes received, then untar and use files

The problem I'm having is that when I try and collect the bytes into a ByteArray on the app, it blocks forever.

My mock-server/archive routes to the following function:

fun serveArchive(request: ServerRequest): Mono<ServerResponse> {
    val tarFile = FileSystemResource(ARCHIVE_PATH)
    assert(tarFile.exists() && tarFile.isFile && tarFile.contentLength() != 0L)
    return ServerResponse
            .ok()
            .contentType(MediaType.APPLICATION_OCTET_STREAM)
            .contentLength(tarFile.contentLength())
            .header("Content-Disposition", "attachment; filename=\"$ARCHIVE_FNAME\"")
            .body(fromResource(tarFile))
}

Then my app calls that with the following:

private fun retrieveArchive {
    client.get().uri(ARCHIVE_URL).accept(MediaType.APPLICATION_OCTET_STREAM)
            .exchange()
            .flatMap { response ->
                storeArchive(response.bodyToMono())
            }.subscribe()
}

private fun storeArchive(archive: Mono<ByteArrayResource>): Mono<Void> {
    val archiveContentBytes = archive.block() // <- this blocks forever
    val archiveContents = TarArchiveInputStream(archiveContentBytes.inputStream)
    // read archive
}

I've see How to best get a byte array from a ClientResponse from Spring WebClient? and that's why I'm trying to use the ByteArrayResource.

When I step through everything, I see that serveArchive seems to be working (the assert statement says the file I'm passing exists and there are some bytes in it). In retrieveArchive I get a 200 and can see all the appropriate information in the .headers (content-type, content-length all look good). When I get down to storeArchive and try to retrieve the bytes from the Mono using block, it simply blocks forever.

I'm at a complete loss of how to debug something like this.

You just have to return the converted body from the flatMap so it transforms from Mono<T> to T:

client.get().uri(ARCHIVE_URL).accept(MediaType.APPLICATION_OCTET_STREAM)
            .exchange()
            .flatMap { response ->
                response.bodyToMono(ByteArrayResource::class.java)
            }
            .map { archiveContentBytes ->
                archiveContentBytes.inputStream
            }
            .doOnSuccess { inputStream ->
                //here is you code to do anything with the inputStream
                val archiveContents = TarArchiveInputStream(inputStream)
            }
            .subscribe()