Swing自定义ComboBox弹窗无法稳定获取焦点问题求助

因为JComboBox部分行为不符合需求，且需要重写的方法为私有，我基于Swing实现了自定义ComboBox：继承JComponent，内置JTextField，并重写JPopupMenu作为下拉弹窗。测试时通过JTextField的上下键触发弹窗，但弹窗内的JList无法稳定获取焦点——有时选中项呈灰色（无实际焦点）。我尝试了多种show()、setVisible()、requestFocus()等组合及SwingUtilities.invokeLater()，当前代码为最优尝试，但仍有问题。测试用例（含main方法）运行正常，但在复杂的主应用（添加到工具栏、使用其他Look and Feel）中问题复现，现寻求解决思路。

package com.sdi.cmsanalyzer.gui;

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class CustomComboBox extends JComponent {
    private final JTextField textField;
    private final DropdownMenu dropdown;

    public CustomComboBox() {
        setLayout(new BorderLayout());

        textField = new JTextField();

        textField.addKeyListener(new KeyAdapter() {
            @Override
            public void keyPressed(KeyEvent e) {
                int keyCode = e.getKeyCode();
                if (keyCode == KeyEvent.VK_DOWN) {
                    System.out.println("DOWN");
                    showDropdown();
                    dropdown.selectFirstItem();
                } else if (keyCode == KeyEvent.VK_UP) {
                    System.out.println("UP");
                    showDropdown();
                    dropdown.selectLastItem();
                } else if (keyCode == KeyEvent.VK_ENTER) {
                    System.out.println("ENTER");
                }
            }
        });

        add(textField, BorderLayout.CENTER);

        dropdown = new DropdownMenu();
    }

    public JTextField getEditor() {
        return this.textField;
    }

    private void showDropdown() {
        dropdown.updateItems(getDropdownItems());

        dropdown.setSize(new Dimension(textField.getWidth(), dropdown.getPreferredSize().height));

        int x = 0;
        int y = textField.getHeight();

        Container parent = textField.getParent();
        if (parent != null) {
            Point textFieldLocation = textField.getLocation();
            SwingUtilities.convertPointToScreen(textFieldLocation, textField);
            SwingUtilities.convertPointFromScreen(textFieldLocation, parent);
            x = textFieldLocation.x;
            y = textFieldLocation.y + textField.getHeight();
        }

        dropdown.show(textField, x, y);
    }

    private String[] getDropdownItems() {
        // Replace with your logic to retrieve the dropdown items
        return new String[]{"Option 1", "Option 2", "Option 3", "Option 4", "Option 5", "Option 6", "Option 7", "Option 8", "Option 9", "Option 10"};
    }

    private static class DropdownMenu extends JPopupMenu {
        DefaultListModel<String> listModel = new DefaultListModel<>();
        JScrollPane scrollPane;
        JList<String> list;

        public DropdownMenu() {
            // Create a JList with the list model
            list = new JList<>(listModel);

            // Create a scroll pane and add the list to it
            scrollPane = new JScrollPane(list,
                    ScrollPaneConstants.VERTICAL_SCROLLBAR_AS_NEEDED,
                    ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);

            add(scrollPane);
            pack();
        }

        public void updateItems(String[] items) {
            listModel.clear();
            for (String item : items) {
                listModel.addElement(item);
            }
        }

        protected void selectFirstItem() {
            SwingUtilities.invokeLater(() -> {
                list.setSelectedIndex(0);
                list.ensureIndexIsVisible(0);
                list.requestFocus();
            });
        }

        protected void selectLastItem() {
            SwingUtilities.invokeLater(() -> {
                list.setSelectedIndex(listModel.getSize() - 1);
                list.ensureIndexIsVisible(listModel.getSize() - 1);
                list.requestFocus();
            });
        }
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> createAndShowGUI());
    }

    private static void createAndShowGUI() {
        // Create the main frame
        JFrame frame = new JFrame("Custom ComboBox Example");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        // Create a toolbar
        JToolBar toolBar = new JToolBar();

        // Create an instance of your CustomComboBox
        CustomComboBox customComboBox = new CustomComboBox();

        // Add the CustomComboBox to the toolbar
        toolBar.add(customComboBox);

        // Add the toolbar to the frame
        frame.add(toolBar, BorderLayout.NORTH);

        // Create a center panel
        JPanel centerPanel = new JPanel();
        frame.add(centerPanel, BorderLayout.CENTER);

        // Set the frame size and make it visible
        frame.setSize(400, 200);
        frame.setVisible(true);
    }
}

• 替换requestFocus()为requestFocusInWindow()
  直接调用requestFocus()在复杂界面或不同LAF下兼容性差，改用requestFocusInWindow()更符合Swing焦点规则，成功率更高。修改selectFirstItem()和selectLastItem()中的焦点请求代码：

  list.requestFocusInWindow();
  

• 在弹窗显示完成后再请求焦点
  弹窗的show()方法可能存在异步执行逻辑，可重写DropdownMenu的show()方法，确保弹窗完全显示后再触发焦点请求：

  @Override
  public void show(Component invoker, int x, int y) {
      super.show(invoker, x, y);
      SwingUtilities.invokeLater(() -> {
          if (list.getSelectedIndex() != -1) {
              list.requestFocusInWindow();
          }
      });
  }
  

• 消费上下键事件，避免焦点被抢占
  在JTextField的按键处理中，调用e.consume()阻止事件继续传递，防止工具栏或其他组件的焦点逻辑干扰JList：

  if (keyCode == KeyEvent.VK_DOWN) {
      System.out.println("DOWN");
      showDropdown();
      dropdown.selectFirstItem();
      e.consume();
  } else if (keyCode == KeyEvent.VK_UP) {
      System.out.println("UP");
      showDropdown();
      dropdown.selectLastItem();
      e.consume();
  }
  

• 确保JList的焦点可访问性
  显式设置JList的焦点属性，避免LAF或默认配置禁用焦点：

  public DropdownMenu() {
      list = new JList<>(listModel);
      list.setFocusable(true);
      list.setRequestFocusEnabled(true);
      // 其他初始化代码...
  }
  

• 简化弹窗位置计算
  手动计算位置容易出错，直接以textField为调用者，设置相对位置即可：

  private void showDropdown() {
      dropdown.updateItems(getDropdownItems());
      dropdown.setPreferredSize(new Dimension(textField.getWidth(), dropdown.getPreferredSize().height));
      dropdown.show(textField, 0, textField.getHeight());
  }
  

• 处理Look and Feel的影响
  不同LAF可能修改组件行为，可在弹窗显示前同步UI：

  SwingUtilities.updateComponentTreeUI(dropdown);
  

内容的提问来源于stack exchange，提问作者UglyBob