话不多说，并不难，看代码就可以，重点是response.setContentType和response.setHeader两个的设置

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
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 com.nxmu.uploadOrdownload.app.beans.fileupload;
import com.nxmu.uploadOrdownload.app.dao.Impl.fileuploadDao;

public class filedowloadServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
    private fileuploadDao dao = new fileuploadDao();
    protected void doGet(HttpServletRequest request, HttpServletResponse response) 
            throws ServletException, IOException {
        int id = Integer.parseInt(request.getParameter("id"));
        fileupload fileUpload = dao.getBean(id);
        response.setContentType("application/x-msdownload");
        response.setHeader("Content-Disposition", "attachment;filename=" 
                + URLEncoder.encode(fileUpload.getFileName(), "utf-8"));
        OutputStream os = response.getOutputStream();
        BufferedOutputStream bos = new BufferedOutputStream(os);
        InputStream is = new FileInputStream(fileUpload.getFilePath());
        BufferedInputStream bis = new BufferedInputStream(is);
        byte[] buffer = new byte[1024];
        int len;
        while((len = bis.read(buffer)) != -1) {
            bos.write(buffer, 0, len);
        }
        if(bos != null) {
            bos.close();
        }
        if(bis != null) {
            bis.close();
        }
        if(is != null) {
            is.close();
        }
        if(os != null) {
            os.close();
        }
    }
}

下载excel文件的格式为：

String encoding = System.getProperty("file.encoding");
response.setContentType("application/vnd.ms-excel;charset="+encoding);     
response.setHeader("Content-Disposition", "attachment;filename="+URLEncoder.encode(fileName,encoding));