@WebServlet("/downloadServlet")
public class DownloadServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1.获取请求参数，文件名称
        String filename = request.getParameter("filename");
        //2.使用字节输入流加载文件进内存
        //2.1找到文件服务器路径
        ServletContext servletContext = this.getServletContext();
        String realPath = servletContext.getRealPath("/img/" + filename);
        //2.2用字节流关联
        FileInputStream fis = new FileInputStream(realPath);

        //3.设置response的响应头
        //3.1设置响应头类型：content-type
        String mimeType = servletContext.getMimeType(filename);//获取文件的mime类型
        response.setHeader("content-type",mimeType);
        //3.2设置响应头打开方式:content-disposition

        //解决中文文件名问题
        //1.获取user-agent请求头、
        String agent = request.getHeader("user-agent");
        //2.使用工具类方法编码文件名即可
        filename = DownLoadUtils.getFileName(agent, filename);

        response.setHeader("content-disposition","attachment;filename="+filename);
        //4.将输入流的数据写出到输出流中
        ServletOutputStream sos = response.getOutputStream();
        byte[] buff = new byte[1024 * 8];
        int len = 0;
        while((len = fis.read(buff)) != -1){
            sos.write(buff,0,len);
        }

        fis.close();

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request,response);
    }
}