How to Implement a File Download Servlet in Java?

Implementing File Download Servlet

Implementing a file download servlet allows users to retrieve files from a web application.

How to Implement File Download:

To enable file download, we can create a servlet that serves as the download endpoint and map it to a specific URL in web.xml.

Example Servlet:

DownloadServlet.java

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class DownloadServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // Get the file ID from the request
        String id = request.getParameter("id");

        // Fetch file metadata from the database
        String fileName = "";
        String fileType = "";

        // Set the response content type based on file type
        response.setContentType(fileType);

        // Set the content disposition header to prompt download
        response.setHeader("Content-disposition", "attachment; filename=yourcustomfilename.pdf");

        // Create a File object using the file path
        File file = new File(fileName);

        // Stream the file to the client
        InputStream in = new FileInputStream(file);
        OutputStream out = response.getOutputStream();

        // Copy file content in chunks
        byte[] buffer = new byte[4096];
        int length = 0;
        while ((length = in.read(buffer)) > 0) {
            out.write(buffer, 0, length);
        }

        in.close();
        out.flush();
    }
}

In the servlet's doGet method, we:

1. Retrieve the file ID from the request.
2. Fetch file metadata from the database (assuming it's stored there).
3. Set the response content type based on the file's MIME type.
4. Set the content disposition header to prompt the user to download the file with a custom filename.
5. Obtain the file using its path and create a File object.
6. Read the file's content into a buffer and stream it to the client's response stream.

Mapping in 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>

This setup allows users to download files by sending a GET request to /download?id= with a Content-Disposition: attachment header.

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