在异常情况下关闭由 flatMap 打开的 Java 8 流？

Question

The Javadoc from Stream.flatMap (Function <T, Stream<U>> mapper) states:

Each mapped stream is closed after its contents have been placed into this stream.

Nice. But what about exception handling?

Consider the following:

public List<Path> search (List<Path> paths) {
  return paths.stream ()
    .flatMap (dir -> {
      try {
        return Files.list (dir);
      } catch (IOException e) {
        throw new UncheckedIOException (e);
      }
    })
    .filter (this::mightThrowException)
    .collect (toList ());
}

If Files.list (Path) throws an exception on the first call, its stream is never opened in the first place, so doesn't need closing. If it throws an exception on a later call, the streams generated from previous calls have all been fully processed and closed by .flapMap () per its specification.

But what happen if the filter throws an exception? We're in the middle of processing a file listing stream, so I guess I need to close something somewhere, but I don't get what exactly. The initial paths.stream () maybe? which would then call the close method on derived streams?

NB: for anyone not familiar with the Files.list (Path) static method, it returns a Stream<Path> that (unlike most streams) needs to be closed.

Answer 1

The source shows that the Stream returned to flatMap is immediately wrapped in a try with resources, so it will be closed even if a downstream operation throws an exception.

It might be a good idea to update the flatMap javadoc to explicitly state that this will happen.