1. Overview
A common feature of web applications is the ability to expose a file for download.
In this tutorial, we’ll cover a simple example of creating a downloadable file and serving it from a Java Servlet application.
The file we are using will be from the webapp resources.
2. Maven Dependencies
If using Java EE, then we wouldn’t need to add any dependencies. However, if we’re using Java SE, we’ll need the javax.servlet-api dependency:
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>javax.servlet-api</artifactId>
<version>4.0.1</version>
<scope>provided</scope>
</dependency>
The latest version of the dependency can be found here.
3. Servlet
Lets have a look at the code first and then find out what’s going on:
@WebServlet("/download")
public class DownloadServlet extends HttpServlet {
private final int ARBITARY_SIZE = 1048;
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
resp.setContentType("text/plain");
resp.setHeader("Content-disposition", "attachment; filename=sample.txt");
InputStream in = req.getServletContext().getResourceAsStream("/WEB-INF/sample.txt");
OutputStream out = resp.getOutputStream();
byte[] buffer = new byte[ARBITARY_SIZE];
int numBytesRead;
while ((numBytesRead = in.read(buffer)) > 0) {
out.write(buffer, 0, numBytesRead);
}
in.close();
out.flush();
}
}
3.1. Request End Point
@WebServlet(“/download”) annotation marks the DownloadServlet class to serve requests directed at the “/download” end-point.
Alternatively, we can do this by describing the mapping in the web.xml file.
3.2. Response Content-Type
The HttpServletResponse object has a method called as setContentType which we can use to set the Content-Type header of the HTTP response.
Content-Type is the historical name of the header property. Another name was the MIME type (Multipurpose Internet Mail Extensions). We now simply refer to the value as the Media Type.
This value could be “application/pdf”, “text/plain”, “text/html”, “image/jpg”, etc., the official list is maintained by the Internet Assigned Numbers Authority (IANA) and can be found here.
For our example, we are using a simple text file. The Content-Type for a text file is “text/plain”.
3.3. Response Content-Disposition
Setting the Content-Disposition header in the response object tells the browser how to handle the file it is accessing.
Browsers understand the use of Content-Disposition as a convention but it’s not actually a part of the HTTP standard. W3 has a memo on the use of Content-Disposition available to read here.
The Content-Disposition values for the main body of a response will be either “inline” (for webpage content to be rendered) or “attachment” (for a downloadable file).
If not specified, the default Content-Disposition is “inline”.
Using an optional header parameter, we can specify the filename “sample.txt”.
Some browsers will immediately download the file using the given filename and others will show a download dialog containing our predefined value.
The exact action taken will depend on the browser.
3.4. Reading From File and Writing to Output Stream
In the remaining lines of code, we take the ServletContext from the request, and use it to obtain the file at “/WEB-INF/sample.txt”.
Using HttpServletResponse#getOutputStream(), we then read from the input stream of the resource and write to the response’s OutputStream.
The size of the byte array we use is arbitrary. We can decide the size based on the amount of memory is reasonable to allocate for passing the data from the InputStream to the OutputStream; the smaller the number, the more loops; the bigger the number, the higher memory usage.
This cycle continues until numByteRead is 0 as that indicates the end of the file.
3.5. Close and Flush
close will close the Stream and release any resources that it is currently holding.
flush will write any remaining buffered bytes to the OutputStream destination.
We use these two methods to release memory, ensuring that the data we have prepared is sent out from our application.
3.6. Downloading the File
With everything in place, we are now ready to run our Servlet.
Now when we visit the relative end-point “/download”, our browser will attempt to download the file as “simple.txt”.
4. Conclusion
Downloading a file from a Servlet becomes a simple process. Using streams allow us to pass out the data as bytes and the Media Types inform the client browser what type of data to expect.
It is down to the browser to determine how to handle the response, however, we can give some guidelines with the Content-Disposition header.
All code in this article can be found over on GitHub.