This is a simple HTTP server written in the Java programming language.

To run this example, follow these steps:

1. Install the CLI. Use the unikraft CLI or the legacy kraft CLI. You need a BuildKit builder. The easiest way to get one is via Docker. Alternatively, you can also directly set up and use BuildKit, see the quick start.

   Note: The unikraft CLI is the current standard, while kraft is the legacy version. Choose one of the CLIs below and only run the commands associated with it for the rest of this guide.

2. Clone the examples repository and cd into the examples/httpserver-java21 directory:

   git clone https://github.com/unikraft-cloud/examples
   cd examples/httpserver-java21/

Make sure to log into Unikraft Cloud and pick a metro close to you. This guide uses fra (Frankfurt, 🇩🇪):

Using the unikraft CLI (Recommended)

unikraft login

or

Using the legacy kraft CLI

# Set Unikraft Cloud access token
export UKC_TOKEN=token
# Set metro to Frankfurt, DE
export UKC_METRO=fra

When done, invoke the following command to deploy this app on Unikraft Cloud:

Using the unikraft CLI (Recommended)

unikraft build . --output <my-org>/httpserver-java21:latest
unikraft run --metro fra \
  -m 1G \
  -p 443:8080/tls+http \
  --scale-to-zero policy=idle,cooldown-time=1000,stateful=true \
  --image <my-org>/httpserver-java21:latest

or

Using the legacy kraft CLI

kraft cloud deploy \
  -M 1Gi \
  -p 443:8080/tls+http \
  --scale-to-zero idle \
  --scale-to-zero-stateful \
  --scale-to-zero-cooldown 1s \
  .

The output shows the instance address and other details:

Using the unikraft CLI (Recommended)

metro:        fra
name:         httpserver-java21-5xw9m
uuid:         b2c3d4e5-f6a7-8901-bcde-f12345678901
state:        starting
image:        <my-org>/httpserver-java21
resources:
  memory:     1GiB
  vcpus:      1
service:
  uuid:       9d96e042-1601-352b-10c7-4a8c7ececbcb
  name:       gentle-wind-b2x9pkqm
  domains:
  - fqdn:     gentle-wind-b2x9pkqm.fra.unikraft.app
networks:
- uuid:       25185154-a4d5-0da1-e28f-3b3e42190b56
  private-ip: 10.0.3.5
  mac:        12:b0:29:0f:68:f3
timestamps:
  created:    just now

or

Using the legacy kraft CLI

[●] Deployed successfully!
 │
 ├───────── name: httpserver-java21-5xw9m
 ├───────── uuid: b2c3d4e5-f6a7-8901-bcde-f12345678901
 ├──────── metro: https://api.fra.unikraft.cloud/v1
 ├──────── state: starting
 ├─────── domain: https://gentle-wind-b2x9pkqm.fra.unikraft.app
 ├──────── image: oci://unikraft.io/<my-org>/httpserver-java21@sha256:4f8a2c6e1d9b3f7a5c0e2b4d8f6a1c3e7b9d2f4a6c8e0b2d4f6a8c0e2b4d6f
 ├─────── memory: 1 GiB
 ├────── service: gentle-wind-b2x9pkqm
 ├─ private fqdn: httpserver-java21-5xw9m.internal
 └─── private ip: 10.0.3.5

In this case, the instance name is httpserver-java21-5xw9m and the address is https://gentle-wind-b2x9pkqm.fra.unikraft.app. They're different for each run.

Use curl to query the Unikraft Cloud instance of the Java HTTP server:

curl https://gentle-wind-b2x9pkqm.fra.unikraft.app

Hello, World!

You can list information about the instance by running:

Using the unikraft CLI (Recommended)

unikraft instances list

METRO  NAME                     STATE    IMAGE                       ARGS  MEMORY  VCPUS  FQDN                              CREATED
fra    httpserver-java21-5xw9m  running  <my-org>/httpserver-java21        1GiB    1      gentle-wind-b2x9pkqm.fra.unik...  2 minutes ago

or

Using the legacy kraft CLI

kraft cloud instance list

NAME                     FQDN                              STATE    STATUS       IMAGE                                                    MEMORY  VCPUS  ARGS  BOOT TIME
httpserver-java21-5xw9m  gentle-wind-b2x9pkqm.fra.unik...  running  since 3mins  oci://unikraft.io/<my-org>/httpserver-java21@sha256:...  1 GiB   1            421.30 ms

When done, you can remove the instance:

Using the unikraft CLI (Recommended)

unikraft instances delete httpserver-java21-5xw9m

or

Using the legacy kraft CLI

kraft cloud instance remove httpserver-java21-5xw9m

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

• Kraftfile: the Unikraft Cloud specification
• Dockerfile: the Docker-specified app filesystem
• SimpleHttpServer.java: the Java HTTP server implementation

Lines in the Kraftfile have the following roles:

• spec: v0.7: The current Kraftfile specification version is 0.7.

• runtime: base-compat:latest: The kernel to use.

• rootfs: Build the app root filesystem. source: ./Dockerfile means the filesystem is built using the Dockerfile. format: erofs means the filesystem type is EROFS.

• cmd: ["/usr/lib/jvm/java-21-openjdk-amd64/bin/java", "-classpath", "/usr/src/", "SimpleHttpServer"]: Use the Java runtime to run SimpleHttpServer as the starting command of the instance.

Lines in the Dockerfile have the following roles:

• FROM --platform=linux/x86_64 ubuntu:24.04 AS build: Build the app using Ubuntu 24.04, which includes Java 21 in its official repositories.

• RUN apt-get ... install openjdk-21-jdk ...: Install OpenJDK 21.

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

• FROM scratch: Build the runtime filesystem from a minimal base image.

• COPY ...: Copy required files to the app filesystem: system libraries, the Java 21 runtime, and the compiled SimpleHttpServer.class.

The following options are available for customizing the app:

• If you only update the implementation in the SimpleHttpServer.java source file, you don't need to make any other changes.

• If you want to add extra files, you need to copy them into the filesystem using the COPY command in the Dockerfile.

• If you want to replace SimpleHttpServer.java with a different source file, update the cmd line in the Kraftfile and replace SimpleHttpServer with the name of your new class.

• More extensive changes may require extending the Dockerfile (see Dockerfile syntax reference).

• Java's Documentation
• Unikraft Cloud's Documentation
• Building Dockerfile images with Buildkit

Use the --help option for detailed information on using Unikraft Cloud:

Using the unikraft CLI (Recommended)

unikraft --help

or

Using the legacy kraft CLI

kraft cloud --help

Or visit the CLI Reference or the legacy CLI Reference.