Java.IO.BufferedInputStream

BufferedInputStream流的核心在于预先向缓冲区读入流，read()方法是在缓冲区上操作的()，当使用read()方法读取到缓冲区的末尾时，会使用fill()方法再次预先读入一部分字节。
BufferedInputStream类中重要的是fill()函数,下面写一下fill()函数的分析

    private void fill() throws IOException {
        byte[] buffer = getBufIfOpen();
//如果没有标记，将pos直接重置
        if (markpos < 0)
            pos = 0;            /* no mark: throw away the buffer */
//markpos >= 0 &&pos >= buffer.length           
        else if ( pos >= buffer.length)  /* no room left in buffer */
//将数组前移， markpos == 0
            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 if (buffer.length >= MAX_BUFFER_SIZE) {
                throw new OutOfMemoryError("Required array size too large");
            } else {            /* grow buffer */
                int nsz = (pos <= MAX_BUFFER_SIZE - pos) ?
                        pos * 2 : MAX_BUFFER_SIZE;
                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;
    }

http://blog.csdn.net/songwei128/article/details/23355045