A cross-platform Java application for synchronizing files between two computers over a serial (COM) port connection using the XMODEM-like protocol.

COM Port File Sync enables reliable file transfer between two machines connected via a serial cable (null-modem cable, USB-to-serial adapters, or virtual COM ports). It uses a custom protocol built on top of XMODEM with CRC-16, making it ideal for scenarios where network connectivity is unavailable or restricted.

• Bidirectional File Synchronization - Sync files in either direction (A -> B or B <- A) between two connected machines
• XMODEM Protocol - Reliable file transfer using XMODEM with CRC-16 checksums and automatic retries
• File Change Detection - Compares file manifests using MD5 checksums to identify changed files
• Automatic Compression - Smart GZIP compression for text-based files with entropy analysis to skip already-compressed content

• Standard Sync - Transfers new and modified files from sender to receiver
• Mirror Mode - Additionally deletes files on the receiver that don't exist on the sender
• Fast Mode - Uses file metadata (size + modification time) instead of MD5 hashes for faster manifest generation

Sync Control

• Direction Toggle - Switch between sender (A -> B) and receiver (B <- A) modes with a single click
• Mirror Mode - Automatically deletes files on receiver that don't exist on sender
• Fast Mode - Uses file metadata (size + modification time) instead of MD5 hashes for faster manifest comparison

File Filtering

• .gitignore Support - Respects .gitignore patterns to exclude files from synchronization
• Remembered Folder Mapping - Remote folder mapping is remembered per port; prompts when mapping changes

User Experience

• Folder Mapping Guard - Remembers local-to-remote folder mappings per port and prompts when paths change
• Dry-Run Preview - Review planned file operations in a preview dialog before applying a sync
• Single-File Drag-and-Drop Sending - Drop one file directly onto the main interface for immediate transfer when connected
• Drag-and-Drop File Send - Quickly queue one local file for transfer by dragging it onto the main window

Connection

• Role Negotiation - Automatic sender/receiver role assignment on connection
• Connection Monitoring - Heartbeat-based connection health monitoring with auto-recovery
• Shared Text Area - Real-time text sharing between connected machines (clipboard sync)

• Java 17 or higher
• Maven 3.6+ for building
• A serial connection between two computers (null-modem cable, USB-serial adapters, or virtual COM port software)

# Clone the repository
git clone https://github.com/yourusername/com-file-sync.git
cd com-file-sync

# Build with Maven
mvn clean package

# The executable JAR will be in target/com-file-sync-x.x.x.jar

Windows:

java -jar target/com-file-sync-*.jar

Or use the included run.bat script which automatically finds the latest JAR in the target directory.

Linux/macOS:

java -jar target/com-file-sync-*.jar

1. Connect two machines via a serial cable (null-modem or virtual COM port)
2. Launch the application on both machines
3. Select the COM port from the dropdown on each machine
4. Click "Connect" on both sides - they will auto-negotiate roles
5. Select sync folders on both machines using the folder dropdown for quick-recent picks or the "Browse..." button
6. Click "Preview Sync" to inspect the operation plan (recommended for first-time syncs)
7. Click "Start Sync" on the sender side when ready to apply changes

The folder dropdown keeps up to the 10 most recently used valid folders for quick switching.

Click the Settings button to configure COM port parameters:

The Shared Text area allows real-time text sharing between connected machines:

• Type or paste text into the area
• Text is automatically sent to the other machine after a 2-second debounce
• Double-click to copy the entire text to clipboard

com.filesync/
|-- Main.java                       # Application entry point and wiring
|-- config/
|   |-- SettingsManager.java        # Persists UI and sync preferences
|-- protocol/
|   |-- SyncProtocol.java           # High-level sync protocol commands
|-- serial/
|   |-- SerialPortManager.java      # Serial port discovery and lifecycle
|   |-- XModemTransfer.java         # XMODEM block transfer with CRC-16
|-- sync/
|   |-- FileSyncManager.java        # Orchestrates manifest and transfer pipeline
|   |-- SyncCoordinator.java        # Maintains sender/receiver state machine
|   |-- RoleNegotiationService.java # Auto role negotiation handshake
|   |-- ConnectionService.java      # Heartbeat and reconnect handling
|   |-- SharedTextService.java      # Shared text channel over protocol
|   |-- FileChangeDetector.java     # Manifest generation and delta detection
|   |-- CompressionUtil.java        # GZIP compression with smart detection
|   |-- GitignoreParser.java        # .gitignore pattern matching
|   |-- FileDropService.java        # Drag-and-drop single file queuing
|   |-- SyncPreviewPlan.java        # Sync operation planning and preview
|   |-- SyncEventBus.java           # Event bus interface
|   |-- SimpleSyncEventBus.java     # Event bus implementation
|   |-- SyncEvent.java              # Event data classes
|   |-- SyncEventType.java          # Event type definitions
|   |-- SyncEventListener.java      # Event listener interface
|-- ui/
    |-- MainFrame.java              # Swing GUI and user interactions
    |-- MainFrameComponents.java    # Reusable UI components
    |-- MainFrameState.java         # UI state management
    |-- ConnectionController.java   # Connection UI logic
    |-- FolderController.java       # Folder selection and mapping
    |-- SyncController.java         # Sync operation UI
    |-- DragDropController.java     # Drag-and-drop handling
    |-- SharedTextController.java   # Shared text area UI
    |-- SettingsDialog.java         # Settings configuration dialog
    |-- SyncPreviewRenderer.java    # Sync preview table rendering
    |-- SyncEventBridge.java        # Bridge between sync events and UI
    |-- LogController.java          # Log display controller
    |-- SyncPreviewRow.java         # Preview table row model
    |-- UiFormatting.java           # UI styling utilities

• "No COM ports found": Ensure serial drivers are installed and the cable is connected
• "Connection timeout": Verify both applications are running and connected to the correct ports
• "Connection Lost": Check cable connection; the heartbeat mechanism will auto-detect disconnections

• Files not syncing: Ensure the sender has "Start Sync" button enabled (must be in sender mode)
• Slow transfers: Consider enabling Fast Mode for large directories
• Permission errors: Ensure write permissions on the sync folder

For testing on a single machine, use virtual COM port software:

1. Install com0com or similar
2. Create a virtual COM port pair (e.g., COM10 <-> COM11)
3. Connect one application instance to COM10 and another to COM11

# Clean and build
mvn clean package

# Run tests
mvn test

# Build without tests
mvn clean package -DskipTests