Java Kernel

Install IJava 1.3.0 Kernel for Jupyter (Java 17) on WSL (Ubuntu) & macOS (Homebrew)

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Prerequisites

• Java 17 (OpenJDK 17) installed
• Python and Jupyter Notebook installed
• Git installed

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1. Install OpenJDK 17

WSL (Ubuntu)

sudo apt update
sudo apt install openjdk-17-jdk
java -version

macOS(Homebrew)

brew update
brew install openjdk@17
echo 'export PATH="/usr/local/opt/openjdk@17/bin:$PATH"' >> ~/.zshrc
source ~/.zshrc
java -version

2. Install Git and Gradle

Windows (Ubuntu)

# WSL (Ubuntu)
sudo apt install git

macOS (Homebrew)

# macOS
brew install git

3. Clone the IJava Repository

git clone https://github.com/SpencerPark/IJava.git
cd IJava
git checkout v1.3.0

4. Build and Install IJava Kernel w/ Gradle Wrapper

chmod +x gradlew

./gradlew installKernel

5. Verify Kernel

jupyter kernelspec list

6. Setting up Kernel in Notebook File

• Create New .ipynb file.
• Click on select kernel (Top Right Corner)
• Select Java
• Make a code cell
• Test following Code

System.out.println("Hello from IJava 1.3.0 with Java 17!");

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Why Do We Need Both OpenJDK and IJava for Java in Jupyter?

1. OpenJDK: The Java Development Kit

• OpenJDK is the official open-source implementation of the Java platform.
• It provides the Java compiler (javac) and the Java Virtual Machine (JVM).
• The compiler translates your Java source code into bytecode.
• The JVM runs this bytecode on your computer.
• Without OpenJDK (or any JDK), you cannot compile or run Java programs at all.

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2. IJava: The Jupyter Kernel for Java

• IJava is a special program that acts as a kernel for Jupyter notebooks.
• Jupyter itself doesn’t understand Java; it only provides the notebook interface.
• The kernel is what executes your code behind the scenes.
• IJava lets Jupyter communicate with the JVM and compile/run your Java code interactively.
• It manages compiling code on-the-fly, keeping the Java environment alive, and sending results back to the notebook interface.

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Why Both Are Needed Together?

• OpenJDK provides the essential Java tools (compiler + runtime) but does not have any interface for Jupyter notebooks.
• IJava provides the integration layer that connects Jupyter with Java, making Java interactive inside notebooks.
• Think of it like this:
  • OpenJDK = the engine that actually runs Java programs.
  • IJava = the driver that knows how to talk to Jupyter and control that engine interactively.

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What Happens When You Run Java in Jupyter?

1. You write Java code in a notebook cell.
2. Jupyter sends your code to the IJava kernel.
3. IJava uses OpenJDK’s compiler and JVM to compile and run your code.
4. Output or errors get sent back through IJava to the notebook to display.

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Summary Table

Component	Role
OpenJDK	Provides compiler and runtime to run Java programs.
IJava Kernel	Bridges Jupyter notebook interface and the JVM.
Together	Enable running Java interactively inside Jupyter.

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