请问servlet用getInputStream取得流以后，怎么解决中文问题啊？按照书上的写法始终不行！

//Process the HTTP Post request
public void doPost(HttpServletRequest request, HttpServletResponse response) throws
ServletException, IOException {
ServletInputStream in = request.getInputStream();
StringBuffer buffer = new StringBuffer();
BufferedInputStream bis = new BufferedInputStream(in);
int c = -1;
while ( (c = bis.read()) > -1) {
buffer.append( (char) c);
}
String text = buffer.toString();
String returntext = transCode(text, "GBK");//就这样转化中文的啊，可是不行！！
PrintWriter out = response.getWriter();
out.println(returntext);
bis.close();
}

protected String transCode(String value, String enc) {
try {
if (value == null) {
return null;
} else {
value = value.trim();
value = new String(value.getBytes("ISO8859-1"), enc);
return value;
}
} catch (Exception e) {
return null;
}
}