Shutdown hooks

If your application supports dynamic extension discovery and download as discussed in sections 3.7 and 3.8, then you should know that your application will need to restart in order to install, uninstall, or update any extension jar. This application restart is handled by ExtensionManager and you generally don't need to worry about it.

However! Your application may have some kind of cleanup method that gets invoked when shutting down, to handle cleaning up resources, closing database connections cleanly, and so on. Here's a hypothetical example:

/**
 * Perform normal shutdown tasks before the application exits.
 */
private void cleanupAndShutDown() {
    // Save the size and position of open windows:
    saveUIState();
    
    // Tell all of our extensions that we are shutting down:
    myExtensionManager.deactivateAll();
    
    // Close any open database connections:
    DatabaseManager.getInstance().close();
    
    // Save anything that may not have been persisted:
    myDocumentManager.saveAll();
}

This is all well and good, but the application restarts fired by ExtensionManagerDialog will use System.exit(), which will cut your cleanup method out of the loop. How do we fix this?

Registering a shutdown hook

The UpdateManager class provides an easy mechanism for this:

// Somewhere in your startup code...
myUpdateManager.registerShutdownHook(MainWindow::cleanupAndShutDown);

This registers your cleanupAndShutDown() method with the UpdateManager class, so that the UpdateManager knows to invoke your cleanup method before restarting the application. You can register as many shutdown hooks as you need.

Note that this is not a replacement for your normal cleanup scheme! The shutdown hook is only invoked when UpdateManager restarts your application, not when your application exits normally.

Failing to register a shutdown hook

If you have not registered any shutdown hooks, you will notice a warning issued in the log during an application restart:

[WARNING] No shutdown hooks are registered! Application may not terminate cleanly.

This is a hint that you should register at least one shutdown hook to provide a clean termination of your application during a restart.