javaWEB，通过ServletContext对象返回资源真实路径创建FileInputStream输入流对象为啥改不行

问题遇到的现象和发生背景

问题相关代码，请勿粘贴截图

@WebServlet("/DownLoadServlet")
public class DownLoadServlet extends HttpServlet {

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

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    //1.获取请求参数名  filename
    String filename = request.getParameter("filename");
    System.out.println("参数名:" + filename);

    //2.获取文件真实路径，并封装到字节输入流
    ServletContext context = getServletContext();
    String realPath = context.getRealPath("/download/" + filename);

    System.out.println("真是路径:" + realPath);
    FileInputStream in = new FileInputStream(realPath);
    
    //3.准备response字节输出流
    ServletOutputStream out = response.getOutputStream();
    
    //4.告诉浏览器下载文件的MIME类型
    String mimeType = context.getMimeType(filename);
    response.setContentType(mimeType);

    //5.告知浏览器以附件的形式保存
    response.setHeader("Content-Disposition", "attachment;filename=" + filename);
    
    //6.IO的拷贝
    byte b[] = new byte[1024*4];
    int len;
    while ((len = in.read(b)) != -1) {
        out.write(b,0,len);
    }
    //7.释放资源
    out.close();
    in.close();
}

}

<h2>文件下载</h2>

<a href="/day07_request_response_1/DownLoadServlet?filename=demo.docx">文档</a>
<a href="/day07_request_response_1/DownLoadServlet?filename=test.zip">压缩包</a>

运行结果及报错内容

我的解答思路和尝试过的方法

我想要达到的结果

看out目录下有没有生成。

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