How to Implement a Servlet for File Downloads?

Implementing File Downloads Using Servlets

Problem Statement

This question explores how to implement a servlet to facilitate file downloads from a server. The servlet receives a file name as a parameter in a GET request and aims to return that file to the user's browser for download.

Solution

To implement a simple file download servlet, consider the following steps:

1. Configure Servlet in web.xml:

   Add the servlet definition and mapping to the web.xml file to make the servlet available through the web server.

2. Implement DownloadServlet:

   Create a servlet class, extending the HttpServlet class, to handle the file download requests. In the doGet method:

   • Extract the file name from the request parameter.
   • Fetch the file's information (name, type) from a database or other source.
   • Set the appropriate content type in the response header to indicate the file type.
   • Set the Content-disposition header with "attachment; filename=yourcustomfilename.pdf" to prompt the browser to download the file.
   • Open the file as a byte stream and write it to the response output stream in chunks until the file is completely transferred.

Example Code

Below is a sample implementation of the DownloadServlet:

public class DownloadServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String fileName = request.getParameter("id");
        response.setContentType("application/pdf");
        response.setHeader("Content-disposition", "attachment; filename=yourcustomfilename.pdf");
        File my_file = new File(fileName);
        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);
        }
        out.flush();
    }
}

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