java - boolean 值无法正确更新，除非您打印它

Question

这个问题已经有答案了:

Loop doesn't see value changed by other thread without a print statement (1 个回答)

已关闭 8 年前。

所以我正在制作这个java小程序，我只想首先确保按键输入正常工作，然后再使其变得更复杂。我不知道为什么，但是当你删除“System.out.print(needUpdating);”时它没有根据按键输入正确移动矩形。谁能告诉我为什么以及如何解决它？这对我来说完全是个谜。

import java.applet.Applet;
import java.awt.Graphics;
import java.awt.Rectangle;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;    
import javax.swing.JApplet;    

public class firstApplet extends JApplet implements KeyListener, Runnable {
    final int MOVEAMOUNT = 1;       
    boolean needUpdating;       
    int x,y,dx,dy;      
    Thread runner = null;

    public void init() {
        this.setFocusable(true);            
        needUpdating = false;           
        this.requestFocusInWindow();            
        x=0;            
        y=0;            
        dx=0;           
        dy=0;           
        addKeyListener(this);
    }

    public void stop() {            
    }

    public void start() {
        runner = new Thread(this);          
        runner.start();
    }

    public void paint(Graphics g) {
        System.out.println("x= "+x+" y = "+y);          
        g.drawRect( x, y, 100, 15 );
    }

    @Override
    public void keyPressed(KeyEvent e) {                        
        int key = e.getKeyCode();           
        if(key==KeyEvent.VK_UP) {
            System.out.println("up");
            dy=MOVEAMOUNT;
        }
        else if (key==KeyEvent.VK_DOWN) {
            dy=-MOVEAMOUNT;
        }
        else if (key==KeyEvent.VK_LEFT) {
            dx=-MOVEAMOUNT;
        }
        else if (key==KeyEvent.VK_RIGHT) {
            dx=MOVEAMOUNT;
        }
        // TODO Auto-generated method stub          
        needUpdating = true;            
        System.out.println("needUpdating listening = " +needUpdating);
    }

    @Override
    public void keyReleased(KeyEvent e) {
        // TODO Auto-generated method stub
        dx=0;
        dy=0;
    }

    @Override
    public void keyTyped(KeyEvent e) {          
    }

    public void processMovement() {
        System.out.println("processing");
        x+=dx;
        y+=dy;
    }

    @Override
    public void run() {
        this.addKeyListener(this);
        while(true) {
            System.out.print(needUpdating);             
            if(needUpdating) {
                processMovement();                  
                repaint();                  
                needUpdating=false;
            }           
        }           
    }
}

Answer 1

您的代码未正确同步。更好地说，它根本不同步。然而，System.out.println 是一个同步方法，在当今典型的 CPU 架构上，进入同步块(synchronized block)恰好是在 native 代码级别实现为内存屏障 。这具有使 boolean 值的更改对其他线程可见的效果。

结论:正确同步你的代码，“神奇”的行为就会消失。