那要看。如果你可以通过HTTP服务器或Servlet容器公开访问该文件，则只需将其重定向到via即可response.sendRedirect()。

如果不是，则需要手动将其复制到响应输出流：

OutputStream out = response.getOutputStream();
FileInputStream in = new FileInputStream(my_file);
byte[] buffer = new byte[4096];
int length;
while ((length = in.read(buffer)) > 0){
    out.write(buffer, 0, length);
}
in.close();
out.flush();

当然，你需要处理适当的异常。

假设你可以按以下方式访问servlet

http://localhost:8080/myapp/download?id=7

我需要创建一个servlet并将其注册到web.xml

web.xml

<servlet>
     <servlet-name>DownloadServlet</servlet-name>
     <servlet-class>com.myapp.servlet.DownloadServlet</servlet-class>
</servlet>
<servlet-mapping>
     <servlet-name>DownloadServlet</servlet-name>
     <url-pattern>/download</url-pattern>
</servlet-mapping>

下载Servlet.java

public class DownloadServlet extends HttpServlet {


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

         String id = request.getParameter("id");

         String fileName = "";
         String fileType = "";
         // Find this file id in database to get file name, and file type

         // You must tell the browser the file type you are going to send
         // for example application/pdf, text/plain, text/html, image/jpg
         response.setContentType(fileType);

         // Make sure to show the download dialog
         response.setHeader("Content-disposition","attachment; filename=yourcustomfilename.pdf");

         // Assume file name is retrieved from database
         // For example D:\\file\\test.pdf

         File my_file = new File(fileName);

         // This should send the file to browser
         OutputStream out = response.getOutputStream();
         FileInputStream in = new FileInputStream(my_file);
         byte[] buffer = new byte[4096];
         int length;
         while ((length = in.read(buffer)) > 0){
            out.write(buffer, 0, length);
         }
         in.close();
         out.flush();
    }
}