How to properly close a variable amount of streams?

Question

I'm creating multiple streams which I have to access in parallel (or possibly-parallel). I know how to make a try-with-resources when the amount of resources is fixed at compile-time, but what if the amount of resources is determined by a parameter?

I have something like this:

private static void foo(String path, String... files) throws IOException {
    @SuppressWarnings("unchecked")
    Stream<String>[] streams = new Stream[files.length];

    try {
        for (int i = 0; i < files.length; i++) {
            final String file = files[i];
            streams[i] = Files.lines(Paths.get(path, file))
                .onClose(() -> System.out.println("Closed " + file));
        }

        // do something with streams
        Stream.of(streams)
            .parallel()
            .flatMap(x -> x)
            .distinct()
            .sorted()
            .limit(10)
            .forEach(System.out::println);
    }
    finally {
        for (Stream<String> s : streams) {
            if (s != null) {
                s.close();
            }
        }
    }
}

Answer 1

You could write a composite AutoCloseable for managing a dynamic amount of AutoCloseable:

import java.util.ArrayList;
import java.util.List;

public class CompositeAutoclosable<T extends AutoCloseable> implements AutoCloseable {
    private final List<T> components= new ArrayList<>();

    public void addComponent(T component) { components.add(component); }

    public List<T> getComponents() { return components; }

    @Override
    public void close() throws Exception {
        Exception e = null;
        for (T component : components) {
            try { component.close(); }
            catch (Exception closeException) {
                if (e == null) { e = closeException; }
                else { e.addSuppressed(closeException); }
            }
        }
        if (e != null) { throw e; }
    }
}

and you could use it in your method:

private static void foo(String path, String... files) throws Exception {
    try (CompositeAutoclosable<Stream<String>> streams 
            = new CompositeAutoclosable<Stream<String>>()) {
        for (int i = 0; i < files.length; i++) {
            final String file = files[i];
            streams.addComponent(Files.lines(Paths.get(path, file))
                .onClose(() -> System.out.println("Closed " + file)));
        }
        streams.getComponents().stream()
            .parallel()
            .flatMap(x -> x)
            .distinct()
            .sorted()
            .limit(10)
            .forEach(System.out::println);
    }
}