没有print语句就不能执行代码

问题描述

我一直在制作倒计时节目，我想出了这个。

i've been making a countdown program, and i came up with this.

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;
            }
        }
    }
}

In如果没有print / println语句，倒计时代码不会执行的while循环。怎么会？该程序与print语句完全正常。

In the while loop at the end the countdown code doesn't execute without a print / println statement inside. How come? The program works perfectly fine with the print statement though.

推荐答案

首先，你的程序线程不安全因为 boolean started 是共享变量，但它既不是 volatile 也不是在同步块中访问。

First and foremost, your program is thread-unsafe because boolean started is a shared variable, but it is neither volatile nor accessed within synchronized blocks.

现在，不小心， PrintStream #print 是一个同步方法，在任何实际架构上，进入和退出同步块是使用内存屏障 CPU指令实现的，这会导致线程本地状态和主内存之间的完全同步。

Now, accidentally, PrintStream#print is a synchronized method and, on any actual architecture, entering and exiting a synchronized block is implemented using memory barrier CPU instructions, which cause a complete synchronization between the thread-local state and main memory.

因此，纯事故，添加打印调用允许设置已启动由一个线程（EDT）标记为另一个（主线程）可见。

Therefore, by pure accident, adding the print call allows the setting of started flag by one thread (the EDT) to be visible by another (the main thread).

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