Re: Reading into a buffer and writing from it at the same time

From:

Mike Amling <mamling@rmcis.com>

Newsgroups:

comp.lang.java.programmer

Date:

Tue, 20 Oct 2009 08:00:41 -0500

Message-ID:

<7k5qkfF381i4rU1@mid.individual.net>

A. Farber wrote:

final class CyclicBuffer
{
    private static int NBUFFERS = 4;

    private byte[][] _buffers;
    private int[] _bytesRead;
    private int _readIndex;
...
    public int read(InputStream in) throws IOException
    {
        _bytesRead[_readIndex] = in.read(_buffers[_readIndex]);

        if (_bytesRead[_readIndex] <= 0) {
            return -1;
        }

        synchronized(this) {
            _readIndex = (_readIndex + 1) % NBUFFERS;

I haven't read all your code, but this part does not seem right. If
this read(InputStream) is only called from one Thread, then you don't
need synchronized(). If it's called from more than one Thread, then you
need to refrain from using _readIndex outside the synchronized(). For
example, what prevents some other Thread from bumping _readIndex after
your use of it in the first _bytesRead[_readIndex] and before your use
of it in _buffers[_readIndex] or the second _bytesRead[_readIndex]?

--Mike Amling