java – 没有print语句就没有执行代码

参见英文答案 > Loop doesn’t see changed value without a print statement1个
我一直在制作倒计时节目,我想出了这个.

package main;

import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.File;
import java.io.IOException;
import java.net.MalformedURLException;

import javax.sound.sampled.AudioInputStream;
import javax.sound.sampled.AudioSystem;
import javax.sound.sampled.Clip;
import javax.sound.sampled.DataLine;
import javax.sound.sampled.LineUnavailableException;
import javax.sound.sampled.UnsupportedAudioFileException;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JTextField;

public class Gatoo extends JFrame implements ActionListener {
    private int sec,min,secTot,since = 999;
    private long lastTime;

    private JTextField mm = new JTextField(2),ss = new JTextField(2);
    private JLabel minLab = new JLabel("Minutes:"),secLab = new JLabel(
            "Seconds:");
    private JButton start = new JButton("Start");

    private Clip done;
    private boolean started = false;

    private static final long serialVersionUID = 4277921337939922028L;

    public static void main(String[] args) {
        Gatoo cake = new Gatoo("Title");
        cake.pack();
        cake.setSize(800,600);
        cake.setLocationRelativeto(null);
        cake.setDefaultCloSEOperation(3);
        cake.setVisible(true);
        cake.run();
    }

    public Gatoo(String s) {
        super(s);
        setLayout(new FlowLayout());

        start.addActionListener(this);

        add(minLab);
        add(mm);
        add(secLab);
        add(ss);
        add(start);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        started = true;
    }

    public void play(File file) throws MalformedURLException,UnsupportedAudioFileException,IOException,LineUnavailableException {
        AudioInputStream ais = AudioSystem.getAudioInputStream(new File(
                "lib/done.wav"));
        DataLine.Info info = new DataLine.Info(Clip.class,ais.getFormat());
        done = (Clip) AudioSystem.getLine(info);
        done.open(ais);
        done.start();
    }

    public void run() {
        while (true) {
            System.out.print("");// needed?
            if (started) {
                try {
                    min = Integer.parseInt(mm.getText());
                    sec = Integer.parseInt(ss.getText());
                    secTot = (min * 60) + sec;
                    lastTime = System.currentTimeMillis();
                    while (secTot > 0) {
                        since = (int) (System.currentTimeMillis() - lastTime);
                        if (since > 998) {
                            lastTime = System.currentTimeMillis();
                            secTot--;
                        }
                    }

                    play(new File("done.wav"));

                } catch (NumberFormatException exception) {
                    System.out.println("Minutes and seconds must be numbers.");
                    return;
                } catch (Exception exception) {
                    exception.printstacktrace();
                }
                started = false;
            }
        }
    }
}

在最后的while循环中,如果没有print / println语句,倒计时代码就不会执行.怎么会？尽管如此,该程序与print语句完美配合.

解决方法

首先,您的程序是线程不安全的,因为boolean started是一个共享变量,但它既不易变,也不在synchronized块中访问.

现在,意外地,PrintStream #print是一种同步方法,并且在任何实际架构上,使用内存屏障cpu指令实现进入和退出同步块,这导致线程本地状态和主内存之间的完全同步.

因此,通过纯事故,添加打印调用允许一个线程(EDT)的启动标志的设置被另一个(主线程)可见.