jsp实现Servlet文件下载的方法

package com;
import java.io.IOException;
import java.io.PrintWriter;
import java.net.URLEncoder;
import java.util.Date;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import com.sun.xml.internal.messaging.saaj.packaging.mime.internet.MimeUtility;
/**
 * 文件下载类。为了防止客户端浏览器直接打开目标文件（例如在装了MS Office套件的Windows中的IE浏览器可能就会直接在IE浏览器中打开你想下载的doc或者xls文件），在响应头里加入强制下载的MIME类型。
 */
public class DownloadFile extends HttpServlet {
  private static final Log log = LogFactory.getLog(DownloadFile.class);
  public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException {
    long timeStart = 0;
    if(log.isDebugEnabled()){
      timeStart=System.currentTimeMillis();
    }
    response.setContentType("application/x-download charset=UTF-8");
    java.io.FileInputStream fis = null;
    String filepath = request.getRealPath("");
    javax.servlet.ServletOutputStream sos = null;
    // System.out.println("DownloadFile filename:" + filename);
    try {
      if(request.getParameter("filename")==null
          ||request.getParameter("showName")==null){
        return;
      }
      String filename = request.getParameter("filename");
      String showName = request.getParameter("showName");
      request.setCharacterEncoding("UTF-8");
      response.setCharacterEncoding("UTF-8");
      java.io.File file = new java.io.File(filepath + filename);
      if (!file.exists()) {
        log.error(file.getAbsolutePath() + " 文件不存在!");
        return;
      }
      // 读取文件流
      fis = new java.io.FileInputStream(file);
      // 设置下载保存的文件名
      sos = response.getOutputStream();
      showName += filename.substring(filename.lastIndexOf("."));
      String contentDisposition = "", browser = getBrowser(request);
      if ("IE".equals(browser)) {
        contentDisposition = "attachment; filename=" + URLEncoder.encode(showName, "UTF-8").replace("+", "%20");
      } else if ("CH".equals(browser)) {
        contentDisposition = "attachment; filename=" + MimeUtility.encodeText(showName, "UTF8", "B");
      } else if ("SF".equals(browser)) {
        contentDisposition = "attachment; filename=" + new String(showName.getBytes("UTF-8"), "ISO8859-1");
      } else {
        contentDisposition = "attachment; filename*=UTF-8''" + URLEncoder.encode(showName, "UTF-8").replace("+", "%20");
      }
      response.setHeader("Content-Disposition", contentDisposition);
      int byteCount = 0; 
      if (fis != null) {
        byte[] buff = new byte[1024];
        int bytesRead;
        while(-1 != (bytesRead = fis.read(buff, 0, buff.length))) {
          sos.write(buff, 0, bytesRead);
          sos.flush();
          byteCount += bytesRead;
        } 
      }
      sos.flush();
      if(log.isDebugEnabled()){
        log.debug("文件下载完成，文件大小："+ byteCount +"，总共用时:"+ (new Date().getTime() - timeStart) +"毫秒。");
      }
    } catch(IOException ioe) {
      ioe.printStackTrace();
    } finally {
      try {
        if(fis!=null){
          fis.close();
        }
      } catch (IOException e) {
      } finally {
        try {
          if(sos!=null){
            sos.close();
          }
        } catch (IOException e) {
        }
      }
    }
  }
  public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    response.setContentType("text/html");
    PrintWriter out = response.getWriter();
    out.println("!DOCTYPE html PUBLIC \"-//W3C//DTD XHTML 1.0 Transitional//EN\" \"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd\">");
    out.println("html>");
    out.println(" head>");
    out.println("  title>文件下载/title>");
    out.println("  meta http-equiv=\"Content-Type\" content=\"text/html; charset=UTF-8\" />");
    out.println(" /head>");
    out.println(" body>");
    out.print(" This is ");
    out.print(this.getClass().getName());
    out.println(", using the POST method");
    out.println(" /body>");
    out.println("/html>");
    out.flush();
    out.close();
  }
  private String getBrowser(HttpServletRequest request) {
    String userAgent = request.getHeader("USER-AGENT").toLowerCase();
    if (userAgent != null) {
      if (userAgent.indexOf("msie") >= 0) {
        return "IE";
      } else if (userAgent.indexOf("mozilla") >= 0) {
        return "FF";
      } else if (userAgent.indexOf("applewebkit") >= 0) {
        return "CH";
      } else if (userAgent.indexOf("safari") >= 0) {
        return "SF";
      } else if (userAgent.indexOf("opera") >= 0) {
        return "OP";
      }
    }
    return null;
  }
}