代码如下但是看不太懂，希望有人帮梳理一下逻辑。

private void fill() throws IOException {
        byte[] buffer = getBufIfOpen();
    if (markpos < 0)
        pos = 0;        /* no mark: throw away the buffer */
    else if (pos >= buffer.length)  /* no room left in buffer */
        if (markpos > 0) {  /* can throw away early part of the buffer */
        int sz = pos - markpos;
        System.arraycopy(buffer, markpos, buffer, 0, sz);
        pos = sz;
        markpos = 0;
        } else if (buffer.length >= marklimit) {
        markpos = -1;   /* buffer got too big, invalidate mark */
        pos = 0;    /* drop buffer contents */
        } else {        /* grow buffer */
        int nsz = pos * 2;
        if (nsz > marklimit)
            nsz = marklimit;
        byte nbuf[] = new byte[nsz];
        System.arraycopy(buffer, 0, nbuf, 0, pos);
                if (!bufUpdater.compareAndSet(this, buffer, nbuf)) {
                    // Can't replace buf if there was an async close.
                    // Note: This would need to be changed if fill()
                    // is ever made accessible to multiple threads.
                    // But for now, the only way CAS can fail is via close.
                    // assert buf == null;
                    throw new IOException("Stream closed");
                }
                buffer = nbuf;
        }
        count = pos;
    int n = getInIfOpen().read(buffer, pos, buffer.length - pos);
        if (n > 0)
            count = n + pos;
}

顺便问下BufferedInputStream有缓冲区跟FileInputStream无缓冲区的区别（请用源码来解释）

http://blog.csdn.net/songwei128/article/details/23355045
参考这个