Java中的时钟线程

Question

以下是上述应用程序的代码（包括2个类：Main和ClockPanel）

public class Main {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            JFrame frame = new JFrame("Clock");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.setSize(300, 300);
            frame.setLayout(new GridLayout(1, 1));

            ClockPanel clock = new ClockPanel();
            frame.add(clock);

            frame.setVisible(true);
        });
    }
}

public class ClockPanel extends JPanel implements ActionListener {

    private JLabel timeValue;
    private JButton runButton;
    private JButton pauseButton;
    private JButton resetButton;

    private boolean isRunning;
    private int seconds;


    public ClockPanel() {
        timeValue = new JLabel("0");

        setBorder(BorderFactory.createLineBorder(Color.BLACK));
        setAlignmentX(Component.CENTER_ALIGNMENT);
        setLayout(new BoxLayout(this, BoxLayout.Y_AXIS));

        initButtons();

        // create panel to display the clocks
        JPanel timePanel = new JPanel();
        timePanel.setLayout(new BoxLayout(timePanel, BoxLayout.X_AXIS));
        timePanel.setAlignmentX(Component.CENTER_ALIGNMENT);
        JLabel timeLabel = new JLabel("Value");
        timePanel.add(timeLabel);
        timePanel.add(Box.createRigidArea(new Dimension(30, 0)));
        timePanel.add(timeValue);
        add(timePanel);

        add(Box.createRigidArea(new Dimension(0, 20)));

        // create panel to display the buttons
        JPanel buttonPanel = new JPanel();
        buttonPanel.setLayout(new BoxLayout(buttonPanel, BoxLayout.X_AXIS));
        buttonPanel.setAlignmentX(Component.CENTER_ALIGNMENT);
        buttonPanel.add(runButton);
        buttonPanel.add(Box.createRigidArea(new Dimension(30, 0)));
        buttonPanel.add(pauseButton);
        buttonPanel.add(Box.createRigidArea(new Dimension(30, 0)));
        buttonPanel.add(resetButton);
        add(buttonPanel);

        // create thread to run the clock
        Thread clockThread = new Thread(() -> {
            seconds = -1;
            while (true) {
//                System.out.println(); // if this line is removed, the clock will not run
                if (isRunning) {
                    try {
                        seconds++;
                        timeValue.setText(String.valueOf(seconds));
                        Thread.sleep(1000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        });
        clockThread.start();
    }

    private void initButtons() {
        runButton = new JButton("Run");
        pauseButton = new JButton("Pause");
        resetButton = new JButton("Reset");

        runButton.addActionListener(this);
        pauseButton.addActionListener(this);
        resetButton.addActionListener(this);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == runButton) {
            isRunning = true;

            runButton.setEnabled(false);
            pauseButton.setEnabled(true);
            resetButton.setEnabled(false);
        } else if (e.getSource() == pauseButton) {
            isRunning = false;

            runButton.setEnabled(true);
            pauseButton.setEnabled(false);
            resetButton.setEnabled(true);
        } else if (e.getSource() == resetButton) {
            isRunning = false;
            timeValue.setText("0");
            seconds = 0;

            runButton.setEnabled(true);
            pauseButton.setEnabled(false);
            resetButton.setEnabled(false);
        }
    }
}

问题是：如果我删除该行系统。出来println（）（正如我在上面的代码中所评论的那样），时钟将不会运行。我完全不知道为什么会这样。有人能给我解释一下吗？是因为系统。出来println（）影响正在运行的线程或其他什么？

Answer 1

isRun不是易失性，因此每个线程看到的值都可能发散。当值为false时，您有一个循环只会旋转，并且没有理由去检查“真实”值是否发生了变化。

注意，您不应该（直接或间接地）在AWT事件调度线程（EDT）之外使用Swing组件。

简单的修复方法是使isRunning变为volatile，并使用EventQueue。invokeLater调用时间值。setText。

最好用javax完全取代clockThread。摆动计时器（而不是java.util.Timer），但这可能没有抓住重点。至少让时钟线程等待运行更改。