Implementing a simple file download servlet

That depends. If said file is publicly available via your HTTP server or servlet container you can simply redirect to via response.sendRedirect().

If it's not, you'll need to manually copy it to response output stream:

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();

You'll need to handle the appropriate exceptions, of course.

Assuming you have access to servlet as below

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

I need to create a servlet and register it to 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>

DownloadServlet.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();
    }
}