java nio buffer读取数据乱码问题

public static void main(String[] args) throws IOException {
        String charsetName = "GBK";
        String fileName = "data.txt";
        bufferToTxt(charsetName, fileName);
        txtToBuffer(charsetName, fileName);
    }

    public static void txtToBuffer(String charsetName, String fileName) {
        try {
            RandomAccessFile file = new RandomAccessFile(fileName, "rw");
            FileChannel fileChannel = file.getChannel();
            ByteBuffer buffer = ByteBuffer.allocate();

            int count = fileChannel.read(buffer);
            while (count != -) {
                buffer.flip();
                CharBuffer charBuffer = CharBuffer.allocate();
                Charset charset = Charset.forName(charsetName);
                CharsetDecoder decoder = charset.newDecoder();
                decoder.decode(buffer, charBuffer, true);
                charBuffer.flip();
                while (charBuffer.hasRemaining()){
                    System.out.print(charBuffer.get());
                }
                buffer.clear();
                count = fileChannel.read(buffer);
            }
            fileChannel.close();
            file.close();
        } catch (Exception e) {
            e.printStackTrace();
        }

    }

    public static void bufferToTxt(String charsetName, String fileName){
        try {
            RandomAccessFile file = new RandomAccessFile(fileName, "rw");
            FileChannel fileChannel = file.getChannel();
            ByteBuffer buffer = ByteBuffer.allocate();
            buffer.put("世界如此美妙！ good luck".getBytes(charsetName));
            buffer.flip();
            fileChannel.write(buffer);
            fileChannel.close();
            file.close();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }