Java Practice: Shutdown Hook

今日 Daily Java Practice 如下，請問其中 runTime.addShutdownHook 的作用。

public class Main implements Runnable {
    public void run() {
        System.out.println("Shutting down");
    }

    public static void main(String[] arg) {
        Runtime runTime = Runtime.getRuntime();
        Main hook = new Main();
        runTime.addShutdownHook(new Thread(hook));
    }
}