Do terminal operations on streams close the source? [duplicate]

Answer 1

Terminal operators do NOT close the stream automatically. Consider this code:

Stream<Path> list = Files.list(directory).onClose(() -> System.out.println("Closed"));
list.forEach(System.out::println);

This does NOT print "Closed".

However, the following does print "Closed":

try (Stream<Path> list = Files.list(directory).onClose(() -> System.out.println("Closed"))) {
    list.forEach(System.out::println);
}

So the best way to do it is to use the try-with-resources mechanism.

Answer 2

So a quick-check reveals that forEach does not close the DirectoryStream:

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.function.Consumer;
import java.util.stream.Stream;

/**
 * Created for http://stackoverflow.com/q/27381329/1266906
 */
public class FileList {
    public static void main(String[] args) {
        Path directory = Paths.get("C:\\");
        try {
            Stream<Path> list = Files.list(directory).onClose(() -> System.out.println("Close called"));
            list.forEach(System.out::println);
            // Next Line throws "java.lang.IllegalStateException: stream has already been operated upon or closed" even though "Close called" was not printed
            list.forEach(System.out::println);
        } catch (IOException | IllegalStateException e) {
            e.printStackTrace();  // TODO: implement catch
        }

        // The mentioned try-with-resources construct
        try (Stream<Path> list = Files.list(directory)) {
            list.forEach(System.out::println);
        } catch (IOException | IllegalStateException e) {
            e.printStackTrace();  // TODO: implement catch
        }

        // Own helper-method
        try {
            forEachThenClose(Files.list(directory), System.out::println);
        } catch (IOException | IllegalStateException e) {
            e.printStackTrace();  // TODO: implement catch
        }
    }

    public static <T> void forEachThenClose(Stream<T> list, Consumer<T> action) {
        try {
            list.forEach(action);
        } finally {
            list.close();
        }
    }
}

I see the two presented mitigations:

1. use try-with-resources as stated in the Files.list JavaDoc
2. write your own helper-method which utilizes a finally-block

Which is more maintainable depends probably on how-many helper-methods you would need.