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KraftCloud

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 https://github.com/kraftcloud/examples
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
export KRAFTCLOUD_TOKEN=token
# Set metro to Frankfurt, DE
export KRAFTCLOUD_METRO=fra0

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: https://bold-voice-irwjijr5.fra0.kraft.host
 ├───────── 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: 172.16.28.2
 └────────── 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 https://bold-voice-irwjijr5.fra0.kraft.host. They are different for each run.

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

Terminal window
curl https://bold-voice-irwjijr5.fra0.kraft.host
Hello, World!

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

Terminal window
kraft cloud instance list
NAME               FQDN                                      STATE    CREATED AT    IMAGE                                                           MEMORY   ARGS                            BOOT TIME
http-java17-k7eca  holy-waterfall-p6yu0q7s.fra0.kraft.host  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:

  • SimpleHttpServer.java: the actual Java HTTP server
  • Kraftfile: the KraftCloud specification
  • Dockerfile: the Docker-specified application filesystem
import com.sun.net.httpserver.HttpContext;
import com.sun.net.httpserver.HttpExchange;
import com.sun.net.httpserver.HttpServer;
import java.io.IOException;
import java.io.OutputStream;
import java.net.InetSocketAddress;


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("/");
    context.setHandler(SimpleHttpServer::handleRequest);
    System.out.println("Waiting for HTTP connections on port " + listenPort + " ...");
    server.start();
}


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();
    os.write(response.getBytes());
    os.close();
}


}

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 ./SimpleHttpServer.java /src/SimpleHttpServer.java: Copy the server implementation file in the Docker filesystem.

  • RUN javac SimpleHttpServer.java: Compifile the Java source file.

The following options are available for customizing the application:

  • If only updating the implementation in the SimpleHttpServer.java 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 SimpleHttpServer.java 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.