以可关闭资源作为累加器的Java收集器

我认为您满足要求的唯一方法是向 Stream.onClose 方法假设您有以下类:

class CloseHandler implements Runnable {
    List<Runnable> children = new ArrayList<>();

    void add(Runnable ch) { children.add(ch); }

    @Override
    public void run() { children.forEach(Runnable::run); }
}

现在,您需要按如下方式使用流:

CloseHandler closeAll = new CloseHandler();
try (Stream<Something> stream = list.stream().onClose(closeAll)) {
    // Now collect
    stream.collect(Collectors.groupingBy(
        this::extractFileName, 
        toFile(closeAll)));
}

这使用 try-with-resources 构造,以便在使用或发生错误时自动关闭流。请注意,我们正在通过 closeAll 靠近 流动一旦关闭 方法

下面是下游收集器的草图,它将收集/写入/发送元素到 Closeable 资源(请注意,我们还传递 全部关闭 关闭处理程序):

static Collector<Something, ?, Void> toFile(CloseHandler closeAll) {

    class Acc {

        SomeResource resource; // this is your closeable resource

        Acc() {
            try {
                resource = new SomeResource(...); // create closeable resource
                closeAll.add(this::close);        // this::close is a Runnable
            } catch (IOException e) {
                throw new UncheckedIOException(e);
            }
        }

        void add(Something elem) {
            try {
                // TODO write/send to closeable resource here
            } catch (IOException e) {
                throw new UncheckedIOException(e);
            }
        }

        Acc merge(Acc another) {
            // TODO left as an exercise
        }

        // This is the close handler for this particular closeable resource
        private void close() {
            try {
                // Here we close our closeable resource
                if (resource != null) resource.close();
            } catch (IOException ignored) {
            }
        }
    }
    return Collector.of(Acc::new, Acc::add, Acc::merge, a -> null);
}

因此,它使用一个本地类(名为 Acc )包装可关闭的资源,并将方法声明为 add 流的一个元素到可关闭的资源,也到 merge 二 Acc公司 实例,以防流是平行的(留作练习,以防值得付出努力)。

Collector.of 用于根据 Acc公司 类的方法,具有返回 null ,因为我们不想在 Collectors.groupingBy 。

最后,还有 close 方法,该方法在已创建包装的可关闭资源的情况下关闭该资源。

当流通过 尝试使用资源 构造,即 CloseHandler.run 方法将自动执行,这将依次执行之前添加的所有子关闭处理程序 Acc公司 实例已创建。

好的,我已经看过了 Collectors 实施,您需要 CollectorImpl 创建自定义收集器,但它不是公共的。因此,我使用其副本实现了新的方法(您可能会感兴趣的最后2种方法):

public class CollectorUtils<T, A, R> implements Collector<T, A, R> {

    static final Set<Collector.Characteristics> CH_ID = Collections
            .unmodifiableSet(EnumSet.of(Collector.Characteristics.IDENTITY_FINISH));

    private final Supplier<A> supplier;
    private final BiConsumer<A, T> accumulator;
    private final BinaryOperator<A> combiner;
    private final Function<A, R> finisher;
    private final Set<Characteristics> characteristics;

    CollectorUtils(Supplier<A> supplier, BiConsumer<A, T> accumulator, BinaryOperator<A> combiner,
            Function<A, R> finisher, Set<Characteristics> characteristics) {
        this.supplier = supplier;
        this.accumulator = accumulator;
        this.combiner = combiner;
        this.finisher = finisher;
        this.characteristics = characteristics;
    }

    CollectorUtils(Supplier<A> supplier, BiConsumer<A, T> accumulator, BinaryOperator<A> combiner,
            Set<Characteristics> characteristics) {
        this(supplier, accumulator, combiner, castingIdentity(), characteristics);
    }

    @Override
    public BiConsumer<A, T> accumulator() {
        return accumulator;
    }

    @Override
    public Supplier<A> supplier() {
        return supplier;
    }

    @Override
    public BinaryOperator<A> combiner() {
        return combiner;
    }

    @Override
    public Function<A, R> finisher() {
        return finisher;
    }

    @Override
    public Set<Characteristics> characteristics() {
        return characteristics;
    }

    @SuppressWarnings("unchecked")
    private static <I, R> Function<I, R> castingIdentity() {
        return i -> (R) i;
    }

    public static <C extends Collection<File>> Collector<String, ?, C> toFile() {
        return new CollectorUtils<>((Supplier<List<File>>) ArrayList::new, (c, t) -> {
            c.add(toFile(t));
        }, (r1, r2) -> {
            r1.addAll(r2);
            return r1;
        }, CH_ID);
    }

    private static File toFile(String fileName) {
        try (Closeable type = () -> System.out.println("Complete! closing file " + fileName);) {
            // stuff
            System.out.println("Converting " + fileName);

            return new File(fileName);
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        throw new RuntimeException("Failed to create file");

    }

}

然后我调用流,如下所示:

public static void main(String[] args) {
        Stream.of("x.txt", "y.txt","z.txt").collect(CollectorUtils.toFile());
    }

输出:

Convertingx.txt
closing filex.txt
Convertingy.txt
closing filey.txt
Convertingz.txt
closing filez.txt