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Java Hello World

View source on GitHub

This guide walks you through building and running the Java reference on Avocado OS. The app compiles a simple Java program into an executable JAR using OpenJDK 17 inside the SDK container.

Prerequisites​

  • macOS 10.12+ or Linux (Ubuntu 22.04+, Fedora 39+)
  • Docker Desktop installed and running
  • The latest version of the Avocado CLI

For hardware targets, you will also need:

  • Your target device and any required accessories (SD card, USB cable, serial console adapter)
  • See the Support Matrix for your target's requirements

Initialize​

Clone the reference or initialize a new project from it:

avocado init --reference java-hello java-hello
cd java-hello

To target specific hardware instead of the default, pass --target:

avocado init --reference java-hello --target raspberrypi5 java-hello
cd java-hello

Install​

Install the SDK toolchain, extension dependencies, and runtime packages:

avocado install -f

This pulls the SDK container image and installs nativesdk-openjdk-17-jdk for compiling Java source files.

Build​

Build the extensions and assemble the runtime image:

avocado build

The build step runs java-compile.sh inside the SDK container, which:

  1. Compiles Java source files with javac
  2. Creates a manifest with the main class entry point
  3. Packages everything into ref-java/build/jar/hello-world.jar

Then java-install.sh copies the JAR to /usr/lib/ref-java/ and creates a wrapper script at /usr/bin/hello-java.

Deploy​

QEMU​

For QEMU targets, provision and boot the VM:

avocado provision -r dev
avocado sdk run -iE vm dev

SD card targets (Raspberry Pi, Seeed reTerminal, NXP, STMicroelectronics)​

Insert your SD card and provision:

avocado provision -r dev --profile sd

Insert the SD card into the device and apply power.

USB flash targets (OnLogic)​

avocado provision -r dev --profile usb

NVIDIA Jetson​

avocado provision -r dev --profile tegraflash

Follow the USB disconnect/reconnect prompts during the flash process.

Verify​

Log in as root with an empty password. Run the application:

hello-java

You should see output like:

Hello from Avocado Linux!
Java version: 17.0.13
Java vendor: N/A
OS: Linux aarch64

Customize​

Modify the application​

Edit ref-java/src/main/java/com/avocado/hello/HelloWorld.java:

package com.avocado.hello;

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("My custom Java app on Avocado OS!");
    }
}

Add more Java source files​

Add new .java files under ref-java/src/main/java/ and include them in the javac command in java-compile.sh:

javac -d build/classes \
    src/main/java/com/avocado/hello/HelloWorld.java \
    src/main/java/com/avocado/hello/MyNewClass.java

Add external JAR dependencies​

Place JARs in a ref-java/lib/ directory and add them to the classpath:

javac -cp "lib/*" -d build/classes src/main/java/com/avocado/hello/*.java

Rebuild after changes​

After any change, rebuild and reprovision:

avocado build
avocado provision -r dev