将垂直滚动添加到 JPopupMenu？

这是我使用滚动条创建的版本。这只是一个简单的例子，所以请根据您认为合适的方式进行调整：

import java.awt.Component;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Insets;
import java.awt.LayoutManager;
import java.awt.event.AdjustmentEvent;
import java.awt.event.AdjustmentListener;
import java.awt.event.MouseWheelEvent;
import java.awt.event.MouseWheelListener;
import javax.swing.JPopupMenu;
import javax.swing.JScrollBar;

public class JScrollPopupMenu extends JPopupMenu {
    protected int maximumVisibleRows = 10;

    public JScrollPopupMenu() {
        this(null);
    }


    public JScrollPopupMenu(String label) {
        super(label);
        setLayout(new ScrollPopupMenuLayout());

        super.add(getScrollBar());
        addMouseWheelListener(new MouseWheelListener() {
            @Override public void mouseWheelMoved(MouseWheelEvent event) {
                JScrollBar scrollBar = getScrollBar();
                int amount = (event.getScrollType() == MouseWheelEvent.WHEEL_UNIT_SCROLL)
                             ? event.getUnitsToScroll() * scrollBar.getUnitIncrement()
                             : (event.getWheelRotation() < 0 ? -1 : 1) * scrollBar.getBlockIncrement();

                scrollBar.setValue(scrollBar.getValue() + amount);
                event.consume();
            }
        });
    }

    private JScrollBar popupScrollBar;
    protected JScrollBar getScrollBar() {
        if(popupScrollBar == null) {
            popupScrollBar = new JScrollBar(JScrollBar.VERTICAL);
            popupScrollBar.addAdjustmentListener(new AdjustmentListener() {
                @Override public void adjustmentValueChanged(AdjustmentEvent e) {
                    doLayout();
                    repaint();
                }
            });

            popupScrollBar.setVisible(false);
        }

        return popupScrollBar;
    }

    public int getMaximumVisibleRows() {
        return maximumVisibleRows;
    }

    public void setMaximumVisibleRows(int maximumVisibleRows) {
        this.maximumVisibleRows = maximumVisibleRows;
    }

    public void paintChildren(Graphics g){
        Insets insets = getInsets();
        g.clipRect(insets.left, insets.top, getWidth(), getHeight() - insets.top - insets.bottom);
        super.paintChildren(g);
    }

    protected void addImpl(Component comp, Object constraints, int index) {
        super.addImpl(comp, constraints, index);

        if(maximumVisibleRows < getComponentCount()-1) {
            getScrollBar().setVisible(true);
        }
    }

    public void remove(int index) {
        // can't remove the scrollbar
        ++index;

        super.remove(index);

        if(maximumVisibleRows >= getComponentCount()-1) {
            getScrollBar().setVisible(false);
        }
    }

    public void show(Component invoker, int x, int y){
        JScrollBar scrollBar = getScrollBar();
        if(scrollBar.isVisible()){
            int extent = 0;
            int max = 0;
            int i = 0;
            int unit = -1;
            int width = 0;
            for(Component comp : getComponents()) {
                if(!(comp instanceof JScrollBar)) {
                    Dimension preferredSize = comp.getPreferredSize();
                    width = Math.max(width, preferredSize.width);
                    if(unit < 0){
                        unit = preferredSize.height;
                    }
                    if(i++ < maximumVisibleRows) {
                        extent += preferredSize.height;
                    }
                    max += preferredSize.height;
                }
            }

            Insets insets = getInsets();
            int widthMargin = insets.left + insets.right;
            int heightMargin = insets.top + insets.bottom;
            scrollBar.setUnitIncrement(unit);
            scrollBar.setBlockIncrement(extent);
            scrollBar.setValues(0, heightMargin + extent, 0, heightMargin + max);

            width += scrollBar.getPreferredSize().width + widthMargin;
            int height = heightMargin + extent;

            setPopupSize(new Dimension(width, height));
        }

        super.show(invoker, x, y);
    }

    protected static class ScrollPopupMenuLayout implements LayoutManager{
        @Override public void addLayoutComponent(String name, Component comp) {}
        @Override public void removeLayoutComponent(Component comp) {}

        @Override public Dimension preferredLayoutSize(Container parent) {
            int visibleAmount = Integer.MAX_VALUE;
            Dimension dim = new Dimension();
            for(Component comp :parent.getComponents()){
                if(comp.isVisible()) {
                    if(comp instanceof JScrollBar){
                        JScrollBar scrollBar = (JScrollBar) comp;
                        visibleAmount = scrollBar.getVisibleAmount();
                    }
                    else {
                        Dimension pref = comp.getPreferredSize();
                        dim.width = Math.max(dim.width, pref.width);
                        dim.height += pref.height;
                    }
                }
            }

            Insets insets = parent.getInsets();
            dim.height = Math.min(dim.height + insets.top + insets.bottom, visibleAmount);

            return dim;
        }

        @Override public Dimension minimumLayoutSize(Container parent) {
            int visibleAmount = Integer.MAX_VALUE;
            Dimension dim = new Dimension();
            for(Component comp : parent.getComponents()) {
                if(comp.isVisible()){
                    if(comp instanceof JScrollBar) {
                        JScrollBar scrollBar = (JScrollBar) comp;
                        visibleAmount = scrollBar.getVisibleAmount();
                    }
                    else {
                        Dimension min = comp.getMinimumSize();
                        dim.width = Math.max(dim.width, min.width);
                        dim.height += min.height;
                    }
                }
            }

            Insets insets = parent.getInsets();
            dim.height = Math.min(dim.height + insets.top + insets.bottom, visibleAmount);

            return dim;
        }

        @Override public void layoutContainer(Container parent) {
            Insets insets = parent.getInsets();

            int width = parent.getWidth() - insets.left - insets.right;
            int height = parent.getHeight() - insets.top - insets.bottom;

            int x = insets.left;
            int y = insets.top;
            int position = 0;

            for(Component comp : parent.getComponents()) {
                if((comp instanceof JScrollBar) && comp.isVisible()) {
                    JScrollBar scrollBar = (JScrollBar) comp;
                    Dimension dim = scrollBar.getPreferredSize();
                    scrollBar.setBounds(x + width-dim.width, y, dim.width, height);
                    width -= dim.width;
                    position = scrollBar.getValue();
                }
            }

            y -= position;
            for(Component comp : parent.getComponents()) {
                if(!(comp instanceof JScrollBar) && comp.isVisible()) {
                    Dimension pref = comp.getPreferredSize();
                    comp.setBounds(x, y, width, pref.height);
                    y += pref.height;
                }
            }
        }
    }
}

除了上面的JScrollPopupMenu之外，我还需要在子菜单中有一个滚动条（又名“右拉菜单”）。这似乎是一种更常见的情况。所以我改编了一个JMenu来使用JScrollPopupMenu，称为JScrollMenu：

import javax.swing.Action;
import javax.swing.JButton;
import javax.swing.JMenu;
import javax.swing.JMenuItem;
import javax.swing.JPopupMenu;
import javax.swing.MenuElement;
import javax.swing.UIManager;
import javax.swing.plaf.MenuItemUI;
import javax.swing.plaf.PopupMenuUI;
import java.awt.Component;
import java.awt.ComponentOrientation;



public class JScrollMenu extends JMenu {
    // Covers the one in the JMenu because the method that creates it in JMenu is private
    /** The popup menu portion of the menu.*/
    private JPopupMenu popupMenu;


    /**
     * Constructs a new <code>JMenu</code> with no text.
     */
    public JScrollMenu() {
        this("");
    }

    /**
     * Constructs a new <code>JMenu</code> with the supplied string as its text.
     *
     * @param s the text for the menu label
     */
    public JScrollMenu(String s) {
        super(s);
    }

    /**
     * Constructs a menu whose properties are taken from the <code>Action</code> supplied.
     *
     * @param a an <code>Action</code>
     */
    public JScrollMenu(Action a) {
        this();
        setAction(a);
    }


    /**
     * Lazily creates the popup menu. This method will create the popup using the <code>JScrollPopupMenu</code> class. 
     */
    protected void ensurePopupMenuCreated() {
        if(popupMenu == null) {
            this.popupMenu = new JScrollPopupMenu();
            popupMenu.setInvoker(this);
            popupListener = createWinListener(popupMenu);
        }
    }

//////////////////////////////
//// All of these methods are necessary because ensurePopupMenuCreated() is private in JMenu
//////////////////////////////
    @Override
    public void updateUI() {
        setUI((MenuItemUI) UIManager.getUI(this));

        if(popupMenu != null) {
            popupMenu.setUI((PopupMenuUI) UIManager.getUI(popupMenu));
        }
    }


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


    @Override
    public void setMenuLocation(int x, int y) {
        super.setMenuLocation(x, y);
        if(popupMenu != null) {
            popupMenu.setLocation(x, y);
        }
    }

    @Override
    public JMenuItem add(JMenuItem menuItem) {
        ensurePopupMenuCreated();
        return popupMenu.add(menuItem);
    }

    @Override
    public Component add(Component c) {
        ensurePopupMenuCreated();
        popupMenu.add(c);
        return c;
    }

    @Override
    public Component add(Component c, int index) {
        ensurePopupMenuCreated();
        popupMenu.add(c, index);
        return c;
    }


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

    @Override
    public void insert(String s, int pos) {
        if(pos < 0) {
            throw new IllegalArgumentException("index less than zero.");
        }

        ensurePopupMenuCreated();
        popupMenu.insert(new JMenuItem(s), pos);
    }

    @Override
    public JMenuItem insert(JMenuItem mi, int pos) {
        if(pos < 0) {
            throw new IllegalArgumentException("index less than zero.");
        }
        ensurePopupMenuCreated();
        popupMenu.insert(mi, pos);
        return mi;
    }

    @Override
    public JMenuItem insert(Action a, int pos) {
        if(pos < 0) {
            throw new IllegalArgumentException("index less than zero.");
        }

        ensurePopupMenuCreated();
        JMenuItem mi = new JMenuItem(a);
        mi.setHorizontalTextPosition(JButton.TRAILING);
        mi.setVerticalTextPosition(JButton.CENTER);
        popupMenu.insert(mi, pos);
        return mi;
    }

    @Override
    public void insertSeparator(int index) {
        if(index < 0) {
            throw new IllegalArgumentException("index less than zero.");
        }

        ensurePopupMenuCreated();
        popupMenu.insert(new JPopupMenu.Separator(), index);
    }


    @Override
    public void remove(JMenuItem item) {
        if(popupMenu != null){
            popupMenu.remove(item);
        }
    }

    @Override
    public void remove(int pos) {
        if(pos < 0) {
            throw new IllegalArgumentException("index less than zero.");
        }
        if(pos > getItemCount()) {
            throw new IllegalArgumentException("index greater than the number of items.");
        }
        if(popupMenu != null){
            popupMenu.remove(pos);
        }
    }

    @Override
    public void remove(Component c) {
        if(popupMenu != null){
            popupMenu.remove(c);
        }
    }

    @Override
    public void removeAll() {
        if(popupMenu != null){
            popupMenu.removeAll();
        }
    }

    @Override
    public int getMenuComponentCount() {
        return (popupMenu == null) ? 0 : popupMenu.getComponentCount();
    }

    @Override
    public Component getMenuComponent(int n) {
        return (popupMenu == null) ? null : popupMenu.getComponent(n);
    }

    @Override
    public Component[] getMenuComponents() {
        return (popupMenu == null) ? new Component[0] : popupMenu.getComponents();
    }

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

    @Override
    public MenuElement[] getSubElements() {
        return popupMenu == null ? new MenuElement[0] : new MenuElement[]{popupMenu};
    }


    @Override
    public void applyComponentOrientation(ComponentOrientation o) {
        super.applyComponentOrientation(o);

        if(popupMenu != null) {
            int ncomponents = getMenuComponentCount();
            for(int i = 0; i < ncomponents; ++i) {
                getMenuComponent(i).applyComponentOrientation(o);
            }
            popupMenu.setComponentOrientation(o);
        }
    }

    @Override
    public void setComponentOrientation(ComponentOrientation o) {
        super.setComponentOrientation(o);
        if(popupMenu != null) {
            popupMenu.setComponentOrientation(o);
        }
    }
}