Resource leak in Files.list(Path dir) when stream is not explicitly closed?

Question

I recently wrote a small app that periodically checked the content of a directory. After a while, the app crashed because of too many open file handles. After some debugging

Answer 1

If you close the Stream, Files.list() does close the underlying DirectoryStream it uses to stream the files, so there should be no resource leak as long as you close the Stream.

You can see where the DirectoryStream is closed in the source code for Files.list() here:

return StreamSupport.stream(Spliterators.spliteratorUnknownSize(it, Spliterator.DISTINCT), false)
                    .onClose(asUncheckedRunnable(ds));

The key thing to understand is that a Runnable is registered with the Stream using Stream::onClose that is called when the stream itself is closed. That Runnable is created by a factory method, asUncheckedRunnable that creates a Runnable that closes the resource passed into it, translating any IOException thrown during the close() to an UncheckedIOException

You can safely assure that the DirectoryStream is closed by ensuring the Stream is closed like this:

try (Stream files = Files.list(Paths.get(destination))){
    files.forEach(path -> {
         // Do stuff
    });
}