Java Foreign Function & Memory API Workshop

This workshop project demonstrates Java's Foreign Function & Memory API (Project Panama) using Java 25. It focuses on interoperating with native C libraries, specifically POSIX message queues.

Prerequisites

• Java 25 (Temurin 25+36-LTS)
• Maven 3.9.11+
• Task (Taskfile runner) - Installation instructions
• direnv (optional, for automatic environment setup)

Quick Start

1. Environment Setup

If using direnv and SDKMAN:

direnv allow  # Automatically activates Java 25 and Maven 3.9.11

Or manually:

sdk use java 25-tem
sdk use maven 3.9.11

2. Build the Project

task  # Downloads jextract and builds all modules

Running POSIX Queue Examples

The foreign-posix-queue module includes two example programs that demonstrate inter-process communication using POSIX message queues.

Running the Receiver

In one terminal, start the receiver process:

task run-posix-receiver

The receiver will:

• Open/create a POSIX message queue named /queue
• Display queue attributes (max messages, message size)
• Wait for incoming messages
• Print each received message to stdout

Running the Sender

In another terminal, start the sender process:

task run-posix-sender

The sender will:

• Open the same POSIX message queue (/queue)
• Prompt you to type messages
• Send each line you type to the receiver
• Exit when you press Ctrl+D or Ctrl+C

Example Session

Terminal 1 (Receiver):

$ task run-posix-receiver
opened POSIX queue /queue with attributes (max message: 10) (max message size: 8192 bytes)
Message received: Hello from sender!
Message received: Testing POSIX queues
Message received: This is awesome!

Terminal 2 (Sender):

$ task run-posix-sender
Opened POSIX queue /queue for sending messages
Type messages and press Enter to send (Ctrl+D or Ctrl+C to exit):
Hello from sender!
Message sent: Hello from sender!
Testing POSIX queues
Message sent: Testing POSIX queues
This is awesome!
Message sent: This is awesome!
^D
Sender shutting down...

Troubleshooting

jextract not found

If you see errors about jextract not being found:

task setup-jextract  # Downloads jextract to .local/

Queue already exists

POSIX queues persist across process restarts. To clean up:

# Remove the queue manually
rm /dev/mqueue/queue

Permission denied

Ensure you have permissions to create/access POSIX queues on your system.

Learning Resources

• JEP 454: Foreign Function & Memory API
• Project Panama Documentation
• jextract Guide
• POSIX Message Queues Manual