MainPanel.java

// -*- mode:java; encoding:utf-8 -*-
// vim:set fileencoding=utf-8:
// @homepage@

package example;

import java.awt.*;
import java.util.Objects;
import javax.swing.*;
import javax.swing.plaf.BorderUIResource;

public final class MainPanel extends JPanel {
  private MainPanel() {
    super(new BorderLayout());
    JTree tree = new JTree();
    tree.setComponentPopupMenu(makePopupMenu());
    add(new JScrollPane(tree));
    setPreferredSize(new Dimension(320, 240));
    EventQueue.invokeLater(() -> getRootPane().setJMenuBar(createMenuBar()));
  }

  private static JPopupMenu makePopupMenu() {
    JMenu menu = new TransparentMenu("Test");
    menu.add("Undo");
    menu.add("Redo");
    JPopupMenu popup = new TranslucentPopupMenu();
    popup.add(menu);
    popup.addSeparator();
    popup.add("Cut");
    popup.add("Copy");
    popup.add("Paste");
    popup.add("Delete");
    return popup;
  }

  private JMenuBar createMenuBar() {
    JMenuBar mb = new JMenuBar();
    String[] menuKeys = {"File", "Edit", "Help"};
    for (String key : menuKeys) {
      JMenu m = createMenu(key);
      mb.add(m);
    }
    return mb;
  }

  private static JMenu createMenu(String key) {
    JMenu menu = new TransparentMenu(key);
    // menu.setForeground(new Color(200, 200, 200));
    menu.setOpaque(false); // Motif lnf
    JMenu sub = new TransparentMenu("JMenu");
    sub.add("JMenuItem");
    sub.add("Looooooooooooooooooooong");
    menu.add(sub);
    menu.add("JMenuItem1");
    menu.add("JMenuItem2");
    return menu;
  }

  public static void main(String[] args) {
    EventQueue.invokeLater(MainPanel::createAndShowGui);
  }

  private static void createAndShowGui() {
    try {
      UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
    } catch (UnsupportedLookAndFeelException ignored) {
      Toolkit.getDefaultToolkit().beep();
    } catch (ClassNotFoundException | InstantiationException | IllegalAccessException ex) {
      ex.printStackTrace();
      return;
    }
    PopupFactory.setSharedInstance(new TranslucentPopupFactory());
    JFrame frame = new JFrame("@title@");
    frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    frame.getContentPane().add(new MainPanel());
    frame.pack();
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
  }
}

// https://ateraimemo.com/Swing/TranslucentPopupMenu.html
final class TranslucentPopupMenu extends JPopupMenu {
  private static final Color ALPHA_ZERO = new Color(0x0, true);
  private static final Paint POPUP_BACK = new Color(250, 250, 250, 200);
  private static final Paint POPUP_LEFT = new Color(230, 230, 230, 200);
  private static final int LEFT_WIDTH = 24;

  @Override public boolean isOpaque() {
    return false;
  }

  @Override public void updateUI() {
    super.updateUI();
    if (Objects.isNull(UIManager.getBorder("PopupMenu.border"))) {
      setBorder(new BorderUIResource(BorderFactory.createLineBorder(Color.GRAY)));
    }
  }

  @Override public Component add(Component c) {
    if (c instanceof JComponent) {
      ((JComponent) c).setOpaque(false);
    }
    return c;
  }

  @Override public JMenuItem add(JMenuItem menuItem) {
    menuItem.setOpaque(false);
    return super.add(menuItem);
  }

  @Override public void show(Component c, int x, int y) {
    EventQueue.invokeLater(() -> {
      Container p = getTopLevelAncestor();
      if (p instanceof JWindow && ((JWindow) p).getType() == Window.Type.POPUP) {
        // Heavy weight
        p.setBackground(ALPHA_ZERO);
      }
    });
    super.show(c, x, y);
  }

  @Override protected void paintComponent(Graphics g) {
    Graphics2D g2 = (Graphics2D) g.create();
    g2.setPaint(POPUP_LEFT);
    g2.fillRect(0, 0, LEFT_WIDTH, getHeight());
    g2.setPaint(POPUP_BACK);
    g2.fillRect(LEFT_WIDTH, 0, getWidth(), getHeight());
    g2.dispose();
  }
}

class TransparentMenu extends JMenu {
  private JPopupMenu popupMenu;

  protected TransparentMenu(String title) {
    super(title);
  }

  // [JDK-4688783] JPopupMenu hardcoded i JMenu - Java Bug System
  // https://bugs.openjdk.org/browse/JDK-4688783
  private void ensurePopupMenuCreated2() {
    if (Objects.isNull(popupMenu)) {
      popupMenu = new TranslucentPopupMenu();
      popupMenu.setInvoker(this);
      popupListener = createWinListener(popupMenu);
    }
  }

  // // @see javax/swing/JMenu.java
  // private void ensurePopupMenuCreated() {
  //   if (popupMenu == null) {
  //     final JMenu thisMenu = this;
  //     popupMenu = new JPopupMenu();
  //     popupMenu.setInvoker(this);
  //     popupListener = createWinListener(popupMenu);
  //   }
  // }

  @Override public JPopupMenu getPopupMenu() {
    ensurePopupMenuCreated2();
    return popupMenu;
  }

  @Override public JMenuItem add(JMenuItem menuItem) {
    ensurePopupMenuCreated2();
    menuItem.setOpaque(false);
    return popupMenu.add(menuItem);
  }

  @Override public Component add(Component c) {
    ensurePopupMenuCreated2();
    if (c instanceof JComponent) {
      ((JComponent) c).setOpaque(false);
    }
    popupMenu.add(c);
    return c;
  }

  @Override public void addSeparator() {
    ensurePopupMenuCreated2();
    popupMenu.addSeparator();
  }

  private void checkIndex(int pos) {
    if (pos < 0) {
      throw new IllegalArgumentException("index less than zero.");
    }
  }

  @Override public void insert(String s, int pos) {
    checkIndex(pos);
    ensurePopupMenuCreated2();
    popupMenu.insert(new JMenuItem(s), pos);
  }

  @Override public JMenuItem insert(JMenuItem mi, int pos) {
    checkIndex(pos);
    ensurePopupMenuCreated2();
    popupMenu.insert(mi, pos);
    return mi;
  }

  @Override public void insertSeparator(int index) {
    checkIndex(index);
    ensurePopupMenuCreated2();
    popupMenu.insert(new JPopupMenu.Separator(), index);
  }

  @Override public boolean isPopupMenuVisible() {
    ensurePopupMenuCreated2();
    return popupMenu.isVisible();
  }
}

/*
<a href="http://today.java.net/pub/a/today/2008/03/18/translucent-and-shaped-swing-windows.html">
Translucent and Shaped Swing Windows | Java.net
</a>
*/
class TranslucentPopupFactory extends PopupFactory {
  @Override public Popup getPopup(Component owner, Component contents, int x, int y) { // throws IllegalArgumentException {
    return new TranslucentPopup(owner, contents, x, y);
  }
}

class TranslucentPopup extends Popup {
  private final JWindow popupWindow;

  protected TranslucentPopup(Component owner, Component contents, int ownerX, int ownerY) {
    super(owner, contents, ownerX, ownerY);
    // create a new heavyweight window
    popupWindow = new JWindow();
    // mark the popup with partial opacity
    // AWTUtilities.setWindowOpacity(popupWindow, (contents instanceof JToolTip) ? .8f : .95f);
    // popupWindow.setOpacity(.5f);
    // AWTUtilities.setWindowOpaque(popupWindow, false); // Java 1.6.0_10
    popupWindow.setBackground(new Color(0x0, true)); // Java 1.7.0
    // determine the popup location
    popupWindow.setLocation(ownerX, ownerY);
    // add the contents to the popup
    popupWindow.getContentPane().add(contents);
    contents.invalidate();
    // JComponent parent = (JComponent) contents.getParent();
    // set the shadow border
    // parent.setBorder(new ShadowPopupBorder());
  }

  @Override public void show() {
    // System.out.println("Always Heavy weight!");
    popupWindow.setVisible(true);
    popupWindow.pack();
    // mark the window as non-opaque, so that the
    // shadow border pixels take on the per-pixel
    // translucency
    // AWTUtilities.setWindowOpaque(popupWindow, false);
  }

  @Override public void hide() {
    popupWindow.setVisible(false);
    popupWindow.removeAll();
    popupWindow.dispose();
  }
}