没有打印语句就无法执行代码

Question

我一直在做倒计时计划，我想到了这个。

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while循环的最后，没有内部的print / println语句，倒计时代码将不会执行。怎么会？该程序可以很好地与print语句一起工作。

Answer 1

首先，您的程序是 线程不安全的， 因为它boolean started是一个 共享变量 ，但是volatile在同步块中既不访问也不访问。

现在，偶然地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指令来实现进入和退出同步块，这会导致线程本地状态与主内存之间的完全同步。

因此， 纯属偶然 ，添加print调用允许started一个线程（EDT）对标志的设置对另一线程（主线程）可见。