JButton 的动作调用了 focusLost 事件。这怎么可能？

Question

我们的一位客户报告了我们的应用程序出现异常。问题是，我完全无法理解如何重现此错误。

代码如下：

btn.addActionListener(new ActionListener() {
    @Override
    public void actionPerformed(ActionEvent e) {
        popup.show(btn, 3, btn.getHeight());
    }
});

注意事项：

• btn 是 JButton 类型的最终局部变量。
• popup 是 JPopupMenu 类型的最终局部变量。

抛出以下异常：

java.awt.IllegalComponentStateException: component must be showing on the screen to determine its location
    at java.awt.Component.getLocationOnScreen_NoTreeLock(Unknown Source)
    at java.awt.Component.getLocationOnScreen(Unknown Source)
    at javax.swing.JPopupMenu.show(Unknown Source)
    at fr.def.iss.vd2.mod_site_watcher_gui.SiteElementPanel$4.actionPerformed(SiteElementPanel.java:117)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    at javax.swing.plaf.basic.BasicButtonListener.focusLost(Unknown Source)
    at java.awt.Component.processFocusEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.KeyboardFocusManager.redispatchEvent(Unknown Source)
    at java.awt.DefaultKeyboardFocusManager.typeAheadAssertions(Unknown Source)
    at java.awt.DefaultKeyboardFocusManager.dispatchEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$000(Unknown Source)
    at java.awt.EventQueue$1.run(Unknown Source)
    at java.awt.EventQueue$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$2.run(Unknown Source)
    at java.awt.EventQueue$2.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

据我了解，show 方法抱怨 btn 没有显示。调用 actionPerformed 方法时，btn 怎么可能没有显示？

这个堆栈跟踪中最奇怪的是actionPerformed 方法似乎在处理FocusEvent 时被触发（实际上是focusLost）。

问题是：您能解释一下这个堆栈跟踪是如何发生的吗？

结语

感谢trashgod的建议，我找到了问题。

在 Windows 上，当按钮在被按下时消失时，就会触发其 ActionListeners，就像单击了按钮一样。这种行为可以在 Windows 上观察到，但在 Linux 上则不行。

我在 Oracle/Sun 错误数据库上提交了一个错误。这是链接：

http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7115421

（此链接将在几天内生效，经过 Java 团队审核后）。

感谢您的帮助。垃圾神和托马斯的回答很有帮助。

Answer 1

一个可能的来源是竞争条件，它允许在接收者可见之前触发事件。验证您的 Swing GUI 对象是否仅在event dispatch thread 上构造和操作。 How to generate exceptions from RepaintManager 中引用的文章 Debugging Swing, the final summary 提到了几种自动化搜索的方法。

Answer 2

•btn 是 JButton 类型的最终局部变量。

也许这就是问题所在。也许您引用了一个在屏幕上不可见的组件。

您应该使用：

JButton button = (JButton)e.getSource();

那么您肯定知道您正在引用生成事件的组件。

还要确保您没有同名的类变量。

Answer 3

查看DefaultButtonModel#setPressed(...)的源码我们看到如下：

if(!isPressed() && isArmed()) {
        ...
        fireActionPerformed(
            new ActionEvent(this, ActionEvent.ACTION_PERFORMED,
                            getActionCommand(),
                            EventQueue.getMostRecentEventTime(),
                            modifiers));
} 

如您所见，ActionEvent 在按钮被“武装”时被触发，即有焦点但没有被按下。这与“FocusLost”事件一致。

Answer 4

改用特定于模式的操作：

final JButton button = new JButton();
button.addMouseListener(new MouseAdapter(){
    public void mouseClicked(MouseEvent e) {
       popup.show(btn, 3, btn.getHeight());
    }
})

Answer 5

我无法抗拒，我只喜欢PopupFactory

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.Timer;
import javax.swing.border.EmptyBorder;

public class UsePopupFactory {

    private JFrame frame = new JFrame("PopupFactory Sample");
    private PopupFactory factory = PopupFactory.getSharedInstance();
    private Popup popup;

    public UsePopupFactory() {
        JPanel btnPanel = new JPanel();
        btnPanel.setBorder(new EmptyBorder(20, 20, 20, 20));
        btnPanel.setLayout(new GridLayout(0, 3));
        ActionListener actionListener = new ShowPopup(frame);
        JButton start3 = new JButton("Pick Me for Popup");
        JButton start = new JButton("Pick Me for Popup");
        JButton start2 = new JButton("Pick Me for Popup");
        btnPanel.add(start3);
        btnPanel.add(start);
        btnPanel.add(start2);
        start3.setVisible(false);
        start2.setVisible(false);
        start.addActionListener(actionListener);
        Dimension d = Toolkit.getDefaultToolkit().getScreenSize();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(btnPanel, BorderLayout.SOUTH);
        frame.setSize(new Dimension(d.width / 4, d.height / 4));
        frame.setVisible(true);
    }

    private class ShowPopup implements ActionListener {

        private Component component;

        ShowPopup(Component component) {
            this.component = component;
        }

        public synchronized void actionPerformed(ActionEvent actionEvent) {
            JPanel pnl = new JPanel();
            JComboBox combo = new JComboBox();
            JButton button = new JButton("any action");
            pnl.add(combo);
            pnl.add(button);
            pnl.setPreferredSize(new Dimension(250, 40));
            popup = factory.getPopup(component, pnl,
                    frame.getWidth() / 2 - pnl.getPreferredSize().width / 2,
                    frame.getHeight() / 2 - pnl.getPreferredSize().height / 2);
            popup.show();
            Timer timer = new Timer(3000, hider);
            timer.start();
        }
    }
    private Action hider = new AbstractAction() {

        private static final long serialVersionUID = 1L;

        @Override
        public void actionPerformed(ActionEvent e) {
            popup.hide();
        }
    };

    public static void main(final String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                UsePopupFactory uPF = new UsePopupFactory();
            }
        });
    }
}