文件下载的Servlet

package com.dream.download;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.FileInputStream;
import java.io.IOException;

/**
 * @author ZhangJun
 * @date 2020-04-08
 * @description
 */
@WebServlet("/downloadServlet")
public class DownloadServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        /**
         * 定义Servlet
         *             1. 获取文件名称
         *             2. 使用字节输入流加载文件进内存
         *             3. 指定response的响应头： content-disposition:attachment;filename=xxx
         *             4. 将数据写出到response输出流
         */
        //1.获取文件名称【请求参数】
        String fileName = request.getParameter("fileName");
        //2.使用字节输入流加载文件进入内存
        //2.1 找到文件服务器的路径
        ServletContext servletContext = this.getServletContext();
        //2.2 知道真实路径
        String realPath = servletContext.getRealPath("/img/" + fileName);
        //2.3 用字节输入流
        FileInputStream fileInputStream = new FileInputStream(realPath);
        //2.4设置response的响应头
        //2.4.1 设置响应头类型
        String mimeType = servletContext.getMimeType(fileName);//获取文件的mime类型
        response.setHeader("content-type",mimeType);
        //2.4.2 设置响应头得打开方式
        response.setHeader("content-disposition","attachment;filename="+fileName);
        //3将输入流的数据写出到输出流中
        ServletOutputStream outputStream = response.getOutputStream();
        //缓冲区
        byte[] bytes = new byte[1024];
        //设置读取的个数
        int len =0 ;
        while(fileInputStream.read(bytes)!=-1){
            outputStream.write(bytes,0,len);
        }
        fileInputStream.close();
    }
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request, response);
    }
}