import javax.swing.*;
import java.awt.*;

public class Test {
    
    public static void main(String[] args){
        JFrame frame = new JFrame("flash 窗体");
        JPanel panel = new JPanel();
            
        panel.setBackground(Color.red);
        frame.add(panel);
        panel.add(new JLabel("单击其他的窗口，让本窗口失去焦点，你就会看到有闪动了！"));
        
        frame.setBounds(300, 300, 500, 200);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
        while(true){
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            frame.setVisible(true);  // 其实就是利用了 setVisible(true)
        }
    }
}