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Run a Spring Boot app

In this guide we create and deploy a Spring Boot 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/java17-springboot3.2.x/ directory:

Terminal window
git clone
cd examples/java17-springboot3.2.x/

Make sure to log into KraftCloud by setting your token and a metro close to you. We use fra0 (Frankfurt, πŸ‡©πŸ‡ͺ) 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 -M 1024 -p 443:8080 .

The output shows the instance URL and other details:

Terminal window
[●] Deployed successfully!
β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€ name: java17-springboot32x-qseeo
β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€ uuid: b081166d-a2a0-43af-982d-1aa17f06b5c4
β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€ state: running
β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€ url:
β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€ image: java17-springboot32x@sha256:cc2f2ad18ce8e36b8e8f4debee096fef7b0bb8b47762575a2ba5a9de8199c64a
β”œβ”€β”€β”€β”€β”€ boot time: 153.97 ms
β”œβ”€β”€β”€β”€β”€β”€β”€β”€ memory: 1024 MiB
β”œβ”€ service group: long-dust-si7xsngk
β”œβ”€β”€ private fqdn: java17-springboot32x-qseeo.internal
β”œβ”€β”€β”€β”€ private ip:
└────────── args: /usr/lib/jvm/java-17-openjdk-amd64/bin/java -jar /usr/src/demo-0.0.1-SNAPSHOT.jar

In this case, the instance name is java17-springboot32x-qseeo and the URL is They are different for each run.

Use curl to querythe Spring Boot server’s hello endpoint:

Terminal window
Hello World!

When done, you can remove the instance:

Terminal window
kraft cloud instance remove

Customize your Application

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

  • the server
  • Kraftfile: the KraftCloud specification
  • Dockerfile: the Docker-specified application filesystem
package com.example.demo;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;
public class DemoApplication {
public static void main(String[] args) {, args);
public String hello(@RequestParam(value = "name", defaultValue = "World") String name) {
return String.format("Hello %s!", name);

Lines in the Kraftfile have the following roles:

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

  • runtime: java:17: The Unikraft runtime kernel to use is Java.

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

  • cmd: ["/usr/lib/jvm/java-17-openjdk-amd64/bin/java", "-jar", "/usr/src/demo-0.0.1-SNAPSHOT.jar"]: 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/main/java/com/example/demo/: Copy the server implementation file in the Docker filesystem.

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.