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:

git clone https://github.com/kraftcloud/examples
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, 🇩🇪) in this guide:

# 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:

kraft cloud deploy -M 1024 -p 443:8080 .

The output shows the instance URL and other details:

[●] Deployed successfully!
 │
 ├────────── name: java17-springboot32x-qseeo
 ├────────── uuid: b081166d-a2a0-43af-982d-1aa17f06b5c4
 ├───────── state: running
 ├─────────── url: https://long-dust-si7xsngk.fra0.kraft.host
 ├───────── image: java17-springboot32x@sha256:cc2f2ad18ce8e36b8e8f4debee096fef7b0bb8b47762575a2ba5a9de8199c64a
 ├───── boot time: 153.97 ms
 ├──────── memory: 1024 MiB
 ├─ service group: long-dust-si7xsngk
 ├── private fqdn: java17-springboot32x-qseeo.internal
 ├──── private ip: 172.16.6.2
 └────────── 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 https://long-dust-si7xsngk.fra0.kraft.host. They are different for each run.

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

curl https://long-dust-si7xsngk.fra0.kraft.host/hello

Hello World!

When done, you can remove the instance:

kraft cloud instance remove

Customize your Application

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

• DemoApplication.java: the server
• Kraftfile: the KraftCloud specification
• Dockerfile: the Docker-specified application filesystem

DemoApplication.java

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;

@SpringBootApplication
@RestController
public class DemoApplication {
public static void main(String[] args) {
  SpringApplication.run(DemoApplication.class, args);
}
@GetMapping("/hello")
public String hello(@RequestParam(value = "name", defaultValue = "World") String name) {
  return String.format("Hello %s!", name);
}
}

Kraftfile

spec: v0.6

runtime: java:17

rootfs: ./Dockerfile

cmd: ["/usr/lib/jvm/java-17-openjdk-amd64/bin/java", "-jar", "/usr/src/demo-0.0.1-SNAPSHOT.jar"]

Dockerfile

FROM  --platform=linux/x86_64 debian:bookworm AS build

RUN set -xe ; \
apt -yqq update ; \
apt -yqq install default-jre default-jdk curl unzip ;

RUN ldconfig /usr/lib/jvm/java-17-openjdk-amd64/lib/

WORKDIR /src

RUN set -xe ; \
curl -G https://start.spring.io/starter.zip \
     -d applicationName=DemoApplication \
     -d artifactId=demo \
     -d bootVersion=3.2.2 \
     -d dependencies=web \
     -d description=com.example \
     -d javaVersion=17 \
     -d language=java \
     -d name=demo \
     -d packageName=com.example.demo \
     -d packaging=jar \
     -d type=maven-project \
     -d version=0.0.1-SNAPSHOT \
     -o demo.zip ; \
unzip demo.zip \
;

COPY DemoApplication.java src/main/java/com/example/demo/

RUN set -xe ; \
./mvnw compile package install

RUN set -xe ; \
mkdir /tmpdir

FROM scratch

COPY --from=build /tmpdir /tmp
COPY --from=build /src/target/demo-0.0.1-SNAPSHOT.jar /usr/src/demo-0.0.1-SNAPSHOT.jar

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 DemoApplication.java 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 DemoApplication.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 DemoApplication.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:

kraft cloud --help

Or visit the CLI Reference.