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Saturday, 7 June 2014

Servlets : Download file

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How File Downloading Works in Java Servlets

Allowing users to download files (such as PDFs, images, text files, or zip archives) directly from the server is a common web application requirement. In Java Servlets, downloading a file involves setting the appropriate HTTP response headers so the client browser recognizes the response as an attachment and prompts a save dialog, followed by streaming the file bytes using ServletOutputStream.

Essential HTTP Response Headers for File Download

  • Content-Type: Identifies the MIME type of the file (e.g., application/octet-stream for arbitrary binary files or dynamic detection via ServletContext.getMimeType()).
  • Content-Disposition: Instructs the browser to download the file instead of displaying it inline. Setting attachment; filename="filename.ext" triggers the browser's download prompt.
  • Content-Length: Indicates the exact file size in bytes so the client can display an accurate download progress bar.

Step 1: Create the Download Trigger (HTML)

Provide a link or button on your webpage that directs the user to your download servlet:

<a href="DownloadServlet?fileName=sample.pdf">Download PDF File</a>

Step 2: Implement File Download in Java Servlet

In your servlet's doGet() method, locate the file on the server, set the response headers, and stream the file bytes to the client:

String fileName = request.getParameter("fileName");
ServletContext context = getServletContext();
String filePath = context.getRealPath("/downloads/" + fileName);
File downloadFile = new File(filePath);

String mimeType = context.getMimeType(filePath);
if (mimeType == null) {
  mimeType = "application/octet-stream";
}

response.setContentType(mimeType);
response.setContentLength((int) downloadFile.length());
String headerKey = "Content-Disposition";
String headerValue = String.format("attachment; filename=\"%s\"", downloadFile.getName());
response.setHeader(headerKey, headerValue);

try (FileInputStream inStream = new FileInputStream(downloadFile);
     OutputStream outStream = response.getOutputStream()) {
  byte[] buffer = new byte[4096];
  int bytesRead = -1;
  while ((bytesRead = inStream.read(buffer)) != -1) {
    outStream.write(buffer, 0, bytesRead);
  }
}

Best Practices for Serving Downloads

  • Use Buffered Streams: Always stream binary files in chunks (buffers) rather than reading the entire file into memory at once.
  • Always Use OutputStream: Never use response.getWriter() for binary data; use response.getOutputStream() instead.
  • Validate File Paths: Prevent directory traversal attacks by validating that requested files reside strictly within permitted server directories.

📺 Related Servlets Tutorials

Learn more about request processing and stream handling from our channel:

Click here to watch in Youtube : https://www.youtube.com/watch?v=aofzVypK9gY

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Servlets - Download file
FileDownloadDemo Project Dir Structure
Servlets : Download file

DownloadServlet.java 
import java.io.FileInputStream;
import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class DownloadServlet extends HttpServlet
{
    private static final long serialVersionUID = 1L;

    public void init() throws ServletException
    {
        System.out.println("-----------------------------------------");
        System.out.println(" Init method is called in "
                + this.getClass().getName());
        System.out.println("--------------------------------------");
    }

    public void doGet( HttpServletRequest request, HttpServletResponse response )
            throws ServletException, IOException
    {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        String filename = "RamTest.docx";
        String filepath = "D:\\Tomcat\\";
        response.setContentType("APPLICATION/OCTET-STREAM");
        response.setHeader("Content-Disposition", "attachment; filename=\""
                + filename + "\"");

        FileInputStream fileInputStream = new FileInputStream(filepath
                + filename);

        int i;
        while( (i = fileInputStream.read()) != -1 )
        {
            out.write(i);
        }
        fileInputStream.close();
        out.close();
    }

    public void destroy()
    {
        System.out.println("-----------------------------------------");
        System.out.println(" destroy method is called in "
                + this.getClass().getName());
        System.out.println("-----------------------------------------");
    }

}

web.xml 
<?xml version="1.0" encoding="ISO-8859-1"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" metadata-complete="true" version="3.0">
  <display-name>FileDownloadDemo</display-name>
  <description>
        This is a simple web application with a source code organization
        based on the recommendations of the Application Developer's Guide.
    </description>
  <servlet>
    <servlet-name>DownloadServlet</servlet-name>
    <servlet-class>DownloadServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>DownloadServlet</servlet-name>
    <url-pattern>/downloadfile</url-pattern>
  </servlet-mapping>
</web-app>

index.html 
<html>
<body>
    <a href="downloadfile">Download file</a>
</body>
</html>
Environment Used 

JDK version :1.7.0_51
Tomcat version : 7.0.50 

To Download FileDownloadDemoApp Project Click the below link

https://sites.google.com/site/javaee4321/servlets/FileDownloadDemoApp.zip?attredirects=0&d=1

See also:

  • Servlets Tutorial
  • All Design Patterns Links
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