Lazy-initialize SerialPort in transport classes

[kevinherron]

Summary

• Defer SerialPort.getCommPort() from constructor to connect/bind time using double-checked locking. Previously, the SerialPort was eagerly created in the constructor, which on Linux/macOS throws immediately for non-existent port descriptors (since jSerialComm validates the filesystem path). This made it impossible to catch and handle the error as a ModbusConnectException. On Windows, getCommPort() never validates — failure is deferred to openPort() — so the eager call silently succeeded but the behavior was inconsistent across platforms.
• Wrap errors in proper exception types — getCommPort failures are now caught and surfaced as ModbusException, and connect/bind failures (both from getCommPort and openPort) are wrapped as ModbusConnectException. Previously these were generic Exception instances.
• Add comprehensive cross-platform tests documenting the platform-specific getCommPort behavior and verifying that both client and server transports handle bogus port names correctly on all OSes.
• Expand CI matrix to build and test on Ubuntu, macOS, and Windows, since the new OS-specific tests exercise platform-dependent behavior.

Motivation

SerialPort.getCommPort(String) behaves differently by platform:

• Linux/macOS: Validates that the port descriptor exists on the filesystem. Throws SerialPortInvalidPortException immediately if the file does not exist.
• Windows: Rewrites the descriptor to \\.\COMx format without validation. The SerialPort object is created successfully; failure is deferred to openPort(), which returns false.

By deferring getCommPort to connect/bind time, the transport classes can catch the exception on Linux/macOS and wrap it as a ModbusConnectException, giving callers a consistent error-handling experience across all platforms.

Test plan

• SerialPortGetCommPortBehaviorTest — OS-gated tests document raw getCommPort behavior on each platform
• ClientTransport / ServerTransport nested tests verify constructor never calls getCommPort, and that connect/bind with bogus ports fails with ModbusConnectException
• CI runs on ubuntu-latest, macos-latest, and windows-latest

[Copilot]

Pull request overview

This PR defers creation of jSerialComm SerialPort instances until connect/bind time to normalize cross-platform behavior (Linux/macOS throwing on invalid descriptors vs Windows deferring failure to openPort()), and adds tests + CI coverage to lock that behavior in.

Changes:

• Lazily initialize SerialPort in client/server transports (double-checked locking) and wrap failures in ModbusException / ModbusConnectException.
• Add OS-gated tests documenting SerialPort.getCommPort() behavior and ensuring bogus ports fail consistently via ModbusConnectException.
• Expand GitHub Actions CI to run Maven build/tests on Ubuntu, macOS, and Windows.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 7 comments.

File	Description
modbus-serial/src/main/java/com/digitalpetri/modbus/serial/client/SerialPortClientTransport.java	Lazy SerialPort creation + improved exception wrapping and connection checks.
modbus-serial/src/main/java/com/digitalpetri/modbus/serial/server/SerialPortServerTransport.java	Lazy SerialPort creation + improved exception wrapping during bind/unbind.
modbus-serial/src/test/java/com/digitalpetri/modbus/serial/SerialPortGetCommPortBehaviorTest.java	New cross-platform behavior tests for getCommPort() and transport failure semantics.
.github/workflows/maven.yml	CI now runs the Maven build on Linux/macOS/Windows to exercise platform-specific tests.

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