Utilities of auto close

Zhong Yu zhong.j.yu at gmail.com
Fri Jun 21 07:41:48 PDT 2013

EG is discussing the issue of closing a Stream that ties to IO
resources, and I'd like to throw in my 2 cents.

Background: some new IO methods return Stream of things, for example

    class Files
        /** return a stream of entries in the dir */
        static Stream<Path> list(Path dir);

The problem is some IO resources are tied up and they need to be freed
as soon as the Stream is no longer used.

The current solution is

        interface CloseableStream<T> extends Stream<T>, AutoCloseable

        static CloseableStream<Path> list(Path dir);

EG is discussing an alternative that makes all Streams AutoCloseable

        interface Stream<T> extends AutoCloseable

Some disagree with that approach, since most Streams do not need to be closed.

My proposal:

It's better to move the business of close() away from Stream to some
surrounding constructs. For example:

    try(Scope scope = Scope.newInstance())
        Stream<Path> entries = Files.list(scope, dir);

   interface Scope extends AutoCloseable
        void onClose(Runnable action);

        void close();

    static Stream<Path> list(Scope scope, Path dir)
        scope.onClose( ()->{ free resources } );

Scope provides generic destructor-like functionality, orthogonal to
resource types.

Zhong Yu

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