package com.learn;

import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

/**
 * Created by Administrator on 2017/09/24.
 */
public class DownLoadServlet extends HttpServlet {
  private String filePath;

  @Override
  public void init(ServletConfig config) throws ServletException {
    super.init(config);
    filePath = config.getInitParameter("filePath");
    //初始化路径
    //filePath = config.getServletContext().getRealPath(filePath);
    System.out.println("初始化文件路径："+filePath);
  }

  @Override
  protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    doPost(req,resp);

  }

  @Override
  protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

    OutputStream out; //输出流
    InputStream in; //输入流

    String fileName = req.getParameter("fileName");
    System.out.println("文件名称："+fileName);
    //如果把文件名为null则提示用户
    if(fileName == null){
      out = resp.getOutputStream();
      out.write("please input fileName".getBytes());
      out.close();
    }
    //获取文件流
    in = getServletContext().getResourceAsStream(filePath+ File.separator+fileName);
    System.out.println(in==null?true:false);
    int length = in.available();
    
    //设置返回消息头部信息
    resp.setContentType("application/force-download");
    resp.setHeader("Content-Length",String.valueOf(length));
    resp.setHeader("content-disposition","attachment;filename=\""+fileName+"\"");
    //输出文件到客户端
    out = resp.getOutputStream();
    int bytesend = 0 ;
    byte[] buff = new byte[512];
    while ((bytesend = in.read(buff))!= -1){
      out.write(buff,0,bytesend);
    }
    in.close();
    out.close();
  }
}

 <servlet>
  <servlet-name>download</servlet-name>
  <servlet-class>com.learn.DownLoadServlet</servlet-class>
  <init-param>
    <param-name>filePath</param-name>
    <param-value>file</param-value>
  </init-param>
 </servlet>
 <servlet-mapping>
  <servlet-name>download</servlet-name>
  <url-pattern>/download</url-pattern>
 </servlet-mapping>