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Run a Java app

In this guide we create and deploy a simple Java-based HTTP web server. To run this example, follow these steps:

  1. Install the kraft CLI tool and a container runtime engine, e.g. Docker.

  2. Clone the examples repository and cd into the examples/http-java17/ directory:

Terminal window
git clone
cd examples/http-java17/

Make sure to log into KraftCloud by setting your token and a metro close to you. We use fra0 (Frankfurt, 🇩🇪) in this guide:

Terminal window
# Set KraftCloud access token
# Set metro to Frankfurt, DE

When done, invoke the following command to deploy this application on KraftCloud:

Terminal window
kraft cloud deploy -p 443:8080 -M 512 .

The output shows the instance URL and other details:

Terminal window
[] Deployed successfully!
────────── name: http-java17-xx739
────────── uuid: 15346dbe-6c6e-461a-a998-e6fc9bfa6d89
───────── state: running
─────────── url:
───────── image: http-java17@sha256:554deac043859fc0eb4f8aeebe2daeb76c5d30960157a00746b9fb7b177ef0ca
───── boot time: 157.15 ms
──────── memory: 1024 MiB
service group: bold-voice-irwjijr5
── private fqdn: http-java17-xx739.internal
──── private ip:
────────── args: /usr/lib/jvm/java-17-openjdk-amd64/bin/java -classpath /usr/src/ SimpleHttpServer

In this case, the instance name is http-java17-xx739 and the URL is They are different for each run.

Use curl to query the KraftCloud instance of the Java-based HTTP web server:

Terminal window
Hello, World!

At any point in time, you can list information about the instance:

Terminal window
kraft cloud instance list
http-java17-k7eca running 1 minute ago http-java17@sha256:6f4cb7632ecbab952feb072e37a1a9... 1.0 GiB /usr/lib/jvm/java-17-openjdk... 157154us

When done, you can remove the instance:

Terminal window
kraft cloud instance remove http-java17-xx739

Customize your Application

To customize the application, update the files in the repository, listed below:

  • the actual Java HTTP server
  • Kraftfile: the KraftCloud specification
  • Dockerfile: the Docker-specified application filesystem
public class SimpleHttpServer {
private static final int listenPort = 8080;
public static void main(String[] args) throws IOException {
HttpServer server = HttpServer.create(new InetSocketAddress(listenPort), 0);
HttpContext context = server.createContext("/");
System.out.println("Waiting for HTTP connections on port " + listenPort + " ...");
private static void handleRequest(HttpExchange exchange) throws IOException {
String response = "Hello, World!\n";
exchange.sendResponseHeaders(200, response.getBytes().length); // response code and length
OutputStream os = exchange.getResponseBody();

Lines in the Kraftfile have the following roles:

  • spec: v0.6: The current Kraftfile specification version is 0.6.

  • runtime: java:latest: The Unikraft runtime kernel to use is Java.

  • rootfs: ./Dockerfile: Build the application root filesystem using the Dockerfile.

  • cmd:: Use as the starting command of the instance.

Lines in the Dockerfile have the following roles:

  • FROM scratch: Build the filesystem from the scratch container image, to create a base image.

  • COPY ./ /src/ Copy the server implementation file in the Docker filesystem.

  • RUN javac Compifile the Java source file.

The following options are available for customizing the application:

  • If only updating the implementation in the source file, no other change is required.

  • If new files are added, these have to be copied in the application filesystem, using the COPY command in the Dockerfile.

  • If a new Java source files is added, update the cmd line in the Kraftfile and replace to run that file when creating the instance.

  • More extensive changes may require expanding the Dockerfile with additional Dockerfile commands.

Learn More

Use the --help option for detailed information on using KraftCloud:

Terminal window
kraft cloud --help

Or visit the CLI Reference.