How to list all files from resources folder with scala

Question

Assume the following structure in your recources folder:

resources
├─spec_A
| ├─AA
| | ├─file-aev
| | ├─file-oxa
| | ├─…
| | └─file-stl
| ├─BB
| | ├─file-hio
| | ├─file-nht
| | ├─…
| | └─file-22an
| └─…
├─spec_B
| ├─AA
| | ├─file-aev
| | ├─file-oxa
| | ├─…
| | └─file-stl
| ├─BB
| | ├─file-hio
| | ├─file-nht
| | ├─…
| | └─file-22an
| └─…
└─…

The task is to read all files for a given specification spec_X one subfolder by one. For obvious reasons we do not want to have the exact names as string literals to open with Source.fromResource("spec_A/AA/…") for hundreds of files in the code.

Additionally, this solution should of course run inside the development environment, i.e. without being packaged into a jar.

Answer 1

The only option to list files inside a resource folder I found is with nio’s Filesystem concept, as this can load a jar-file as a file system. But this comes with two major downsides:

1. java.nio uses java Stream API, which I cannot collect from inside scala code: Collectors.toList() cannot be made to compile as it cannot determine the right type.
2. The filesystem needs different base paths for OS-filesystems and jar-file-based filesystems. So I need to manually differentiate between the two situations testing and jar-based running.

First lazy load the jar-filesystem if needed

  private static FileSystem jarFileSystem;

  static synchronized private FileSystem getJarFileAsFilesystem(String drg_file_root) throws URISyntaxException, IOException {
    if (jarFileSystem == null) {
      jarFileSystem = FileSystems.newFileSystem(ConfigFiles.class.getResource(drg_file_root).toURI(), Collections.emptyMap());
    }
    return jarFileSystem;
  }

next do the limbo to figure out whether we are inside the jar or not by checking the protocol of the URL and return a Path. (Protocol inside the jar file will be jar:

  static Path getPathForResource(String resourceFolder, String filename) throws IOException, URISyntaxException {
    URL url = ConfigFiles.class.getResource(resourceFolder + "/" + filename);
    return "file".equals(url.getProtocol())
           ? Paths.get(url.toURI())
           : getJarFileAsFilesystem(resourceFolder).getPath(resourceFolder, filename);
  }

And finally list and collect into a java list

  static List<Path> listPathsFromResource(String resourceFolder, String subFolder) throws IOException, URISyntaxException {
    return Files.list(getPathForResource(resourceFolder, subFolder))
      .filter(Files::isRegularFile)
      .sorted()
      .collect(toList());
  }

Only then we can go back do Scala and fetch is

class SpecReader {
  def readSpecMessage(spec: String): String = {
    List("CN", "DO", "KF")
      .flatMap(ConfigFiles.listPathsFromResource(s"/spec_$spec", _).asScala.toSeq)
      .flatMap(path ⇒ Source.fromInputStream(Files.newInputStream(path), "UTF-8").getLines())
      .reduce(_ + " " + _)
  }
}

object Main {
  def main(args: Array[String]): Unit = {
    System.out.println(new SpecReader().readSpecMessage(args.head))
  }
}

I put a running mini project to proof it here: https://github.com/kurellajunior/list-files-from-resource-directory

But of course this is far from optimal. I wanto to elmiminate the two downsides mentioned above so, that

1. scala files only
2. no extra testing code in my production library