html中使用a标签的href属性链接到资源路径和url-pattern再加上?key=value

<h1>打开文件，href传输的路径是直接写资源路径的文件路径即可</h1>
<a href="/s5/downloadResource/wolf.jpg">点击会直接打开图片</a>
<a href="/s5/downloadResource/a.txt">直接打开文本文件</a>
<hr/>
<h1>下载文件，href传输的路径是Servlet资源路径加?加key和value</h1>
<a href="/s5/download?filename=wolf.jpg">下载图片</a>
<a href="/s5/download?filename=a.txt">下载文本</a>

服务端Servlet：文件下载案例的分析：

1. 获取请求参数，getParameter()根据参数名获取参数值
2. 获取getServletContext对象，用getServletContext对象的getRealpath()找到真实路径，再用字节输入流输入到内存
3. 设置响应头的content-type为filename(filename是什么类型需要getMimeType去获取)
4. 设置响应头的content-disposition为attachment附件打开

@WebServlet("/download")
public class DownloadServletTest extends HttpServlet {
  protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // 1.获取请求参数，文件名称
    String filename = request.getParameter("filename");

    // 2.找到真实路径，使用字节输入流输入到内存
    ServletContext servletContext = request.getServletContext();
    String realPath = servletContext.getRealPath("/downlaodResource/" + filename); //+ filename动态的获取文件名，实现点拿个下载哪个
    FileInputStream fis = new FileInputStream(realPath);

    // 3. 设置响应头的content-type为filename(filename是什么类型需要getMimeType去获取)
    String mime = servletContext.getMimeType(filename);
    response.setHeader("content-type","mime");

    // 4. 设置响应头的content-disposition为attachment附件打开
    response.setHeader("content-disposition","attachment;filename"+filename);

    // 5.将输入流的数据写出到输出流中,完成对拷
    ServletOutputStream os = response.getOutputStream();
    byte[] buff = new byte[1024];
    int len = 0;
    while ((len = fis.read(buff)) != -1) {
      os.write(buff,0,len);
    }
    fis.close();
  }

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

Servlet实现文件下载