package servlet;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.URLEncoder;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import Utils.DownloadUtil;

/**
 * Servlet implementation class demo3
 */
public class demo3 extends HttpServlet {
  private static final long serialVersionUID = 1L;

  protected void doGet(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {

    // 1. 获取要下载的文件名字 例如aa.jpg --- inputStream
    String fileName = request.getParameter("filename");
    // 解决get请求中有中文
    fileName = new String(fileName.getBytes("ISO-8859-1"), "UTF-8");

    // 2. 获取这个文件在tomcat里面的绝对路径地址
    String path = getServletContext().getRealPath("download/" + fileName);
     /**
     *解决下载文件时显示的中文问题
     * ie 或者google使用 URLEncoder
     * 火狐用 base64
     */
    //读取消息头 判断浏览器类型
    String clienrType = request.getHeader("User-Agent");
    if (clienrType.contains("Firefox")) {
      fileName = DownloadUtil.base64EncodeFileName(fileName);
    } else {
      fileName = URLEncoder.encode(fileName, "UTF-8");
    }

    // 让浏览器收到这份资源的时候， 以下载的方式提醒用户，而不是直接展示。
    response.setHeader("Content-Disposition", "attachment; filename=" + fileName);


    // 3. 转化成输入流
    InputStream is = new FileInputStream(path);
    OutputStream os = response.getOutputStream();

    int len = 0;
    byte[] buffer = new byte[1024];
    while ((len = is.read(buffer)) != -1) {
      os.write(buffer, 0, len);
    }

    os.close();
    is.close();
  }

  protected void doPost(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {
    // TODO Auto-generated method stub
    doGet(request, response);
  }
}

配套火狐解析工具类

package Utils;

import java.io.UnsupportedEncodingException;
import sun.misc.BASE64Encoder;

public class DownloadUtil {
  // 设置火狐下载 中文名称工具类
  public static String base64EncodeFileName(String fileName) {
    BASE64Encoder base64Encoder = new BASE64Encoder();
    try {
      return "=?UTF-8?B?" + new String(base64Encoder.encode(fileName.getBytes("UTF-8"))) + "?=";
    } catch (UnsupportedEncodingException e) {
      e.printStackTrace();
      throw new RuntimeException(e);
    }
  }
}
解决 
import sun.misc.BASE64Encoder;

不能导入问题