java - Intellij IDEA 线程问题

Question

我的 IDEA 2016.2.2 有问题
我用飞球写了一个线程演示

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

public class Ball extends JFrame implements Runnable{
    private DrawPanel drawPanel = new DrawPanel();
    private int b_x; // ball's x
    private int b_y; // ball's y
    private int b_d; // ball's diameter
    private Thread thread = null;
    private JButton button;
    private boolean flag = false;

    private Ball(){
        initGUI();
        b_x = b_y = b_d = 50;
        thread = new Thread(this);
        thread.start();
    }

    private void initGUI(){
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setTitle("Ball Thread Demo");
        this.setSize(500, 500);
        this.setLocationRelativeTo(null);
        this.setLayout(new BorderLayout());
        this.add(drawPanel, BorderLayout.CENTER);

        button = new JButton("Start");
        this.add(button, BorderLayout.NORTH);

        button.addActionListener(e -> {
            button.setText(flag ? "Start" : "Stop");
            flag = !flag;
        });
    }

    @Override
    public void run() {
        while(true){
            //System.out.println();
            if(flag) {
                try {
                    Thread.sleep(50);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                b_x += 1;
                b_y += 1;
                drawPanel.repaint();
            }
        }
    }

        class DrawPanel extends JPanel{
            @Override
            protected void paintBorder(Graphics g) {
                g.setColor(Color.white);
                g.fillRect(0, 0, this.getWidth(), this.getHeight());
            }

            @Override
            protected void paintChildren(Graphics g) {
                Graphics2D g2d = (Graphics2D) g;
                g2d.setRenderingHint(
                        RenderingHints.KEY_ANTIALIASING,
                        RenderingHints.VALUE_ANTIALIAS_ON);
                g2d.setColor(new Color(247, 123, 40));
                g2d.fillOval(b_x, b_y, b_d, b_d);
            }
        }

    public static void main(String[] args){
        new Ball().setVisible(true);
    }
}

所以，问题是 - 如果我通过 cmd 启动此代码 - 它工作得很好。但在 IDEA 中，它只能与在 run 方法中注释掉的 System.out.println 一起使用，另一种方式什么也没有发生。这是这个 IDE 的问题还是我错过了一些重要的事情？

Answer 1

b_x、b_y 和 flag 需要设为 volatile ，因为它们以不同的方式更新和读取线程。