Do terminal operations close the stream?

Question

dirPath contains 200k files. I want to read them one by one and do some processing. The following snippet causes java.nio.file.FileSystemException: dirPat

Answer 1

A nested forEach is the wrong tool, in most cases.

The code

Files.list(dirPath).forEach(filePath -> Files.lines(filePath).forEach(line -> { ... });

can and should be replaced by

Files.list(dirPath).flatMap(filePath -> Files.lines(filePath)).forEach(line -> { ... });

or well, since it’s not that easy in this case:

Files.list(dirPath).flatMap(filePath -> {
    try { return Files.lines(filePath);}
    catch(IOException ex) { throw new UncheckedIOException(ex); }
}).forEach(line -> {  });

as a side-effect, you get the following for free:

Stream.flatMap(…):

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

So that’s the preferred solution. Or well, to make it entirely correct:

try(Stream dirStream = Files.list(dirPath)) {
    dirStream.flatMap(filePath -> {
        try { return Files.lines(filePath);}
        catch(IOException ex) { throw new UncheckedIOException(ex); }
    }).forEach(line -> { });
}