Register a Stream “completion” hook

Question

Using the Java 8 Stream API, I would like to register a \"completion hook\", along the lines of:

Stream stream = Stream.of(\"a\",          


        

Answer 1

The solution I've come up with looks like this:

class AutoClosingStream implements Stream {

    AutoClosingStream(Stream delegate, Consumer> onComplete) {}

    // Pipeline ops delegate the op to the real stream and wrap that again
    @Override
    public Stream limit(long maxSize) {
        return new AutoClosingStream(delegate.limit(maxSize), onComplete);
    }

    // Terminal ops intercept the result and call the onComplete logic
    @Override
    public void forEach(Consumer action) {
        terminalOp(() -> delegate.forEach(action));
    }

    private void terminalOp(Runnable runnable) {
        terminalOp(() -> { runnable.run(); return null; });
    }

    private  R terminalOp(Supplier supplier) {
        R result = null;

        try {
            result = supplier.get();
            onComplete.accept(Optional.empty());
        }
        catch (Throwable e) {
            onComplete.accept(Optional.of(e));
            Utils.sneakyThrow(e);
        }

        return result;
    }
}

This is only a simplified sketch to illustrate the idea. The real solution would also support the primitive IntStream, LongStream, and DoubleStream