以附件下载文件的一个sevlet

package cn.itcast.web.dowload;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.FileInputStream;
import java.io.IOException;

@WebServlet(name = "DownloadServlet", value = "/DownloadServlet")
public class DownloadServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // 1.获取请求参数
        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
        // 浏览器会默认将文件打开，这里设置响应头以附件的形式打开
        // 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
        response.setHeader("content-disposition", "attachment;filename="+filename);

        // 4.输入流中的数据写出到输出流中
        ServletOutputStream sos = response.getOutputStream();
        byte[] buff = new byte[1025*8];
        int len = 0;
        while ( (len = fis.read(buff)) == -1){
            sos.write(buff, 0, len);
        }

        fis.close(); // 关闭输入流， 输出流可以不用关

    }

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