servlet实现下载

public void downLoad(HttpServletResponse response, String headName, String fileUrl) throws Exception{
       
        Logger.log("begin download " + headName);       
        if (headName.endsWith(".jpg") || (headName.endsWith(".gif"))) {
            //no need setting content type and header
        } else if (headName.endsWith(".txt")) {
            response.setContentType( "text;charset=GB2312");
            response.setHeader("Content-disposition", 
                              "attachment; filename=" + headName );
        } else {
            response.setContentType( "application/doc;charset=GB2312");
            response.setHeader("Content-disposition", 
                              "attachment; filename=" + headName );
        }

        String fileURL = fileUrl;
        Logger.log(fileURL);
        if(fileURL == null)
            return;
       
        try{
            File file = new File(fileUrl);           
            FileInputStream bis = new FileInputStream(file);           
            OutputStream bos = response.getOutputStream();

            byte[] buff = new byte[1024];
            int readCount = 0;
            int i = 0;
            readCount = bis.read(buff);
            while (readCount != -1){
               bos.write(buff, 0, readCount);
               readCount = bis.read(buff);
            }
            Logger.log("read finished!");
            if (bis!=null)
                bis.close();           
            if (bos!=null)
                bos.close();

        }catch(Exception e){
           e.printStackTrace();          
           throw e;
        }
    }