其中createLayer()方法是最重要的代码，它创建了一个匿名LayerUI子类的实例，绘制了墙纸和JPanel视图，创建了UI，在实例中包含了UI的面板容器。
绘制操作是由LayerUI's public void paint(Graphics g，JComponent c)方法实现的，第二个参数引用了视图(被装饰的组件)中的JLayer实例，不是引用的视图。
在视图后创建了渐变渲染墙纸后，调用paint()方法确保视图(没有嵌套面板的单一面板)是透明的，它将会隐藏墙纸，然后绘制视图。
paint()对比paintLayer()
JDK文档中除了提到paint()方法外，还提到了paintLayer()方法，我这里之所以选择paint()方法，是因为LayerUI中不存在paintLayer()，此外，文档还错误地引用了paintLayer() doesn't exist in LayerUI. Furthermore，the documentation incorrectly refers to addPropertyChange()，configureGraphics()，processComponentEvent()，processFocusEvent()，processHierarchyBoundsEvent()，processHierarchyEvent()，processKeyEvent()，processMouseEvent()，processMouseMotionEvent()，processMouseWheelEvent()，and repaintLayer()这些在LayerUI中根本不存在的方法，当然这些方法也可能在JDK 7最终发布时会包含进来。
图1显示了有墙纸背景的UI。
图 1 在用户界面后加了一层墙纸背景
ReverseText程序演示了自定义绘制，避开了事件触发，不需要检测事件，因为程序只关心墙纸的绘制效果。相反，清单2显示了一个需要响应鼠标移动事件的程序代码。
清单2. BrandedUI.java
import java.awt.AWTEvent;
import java.awt.Color;
import java.awt.Component;
import java.awt.EventQueue;
import java.awt.Font;
import java.awt.GradientPaint;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridLayout;
import java.awt.Point;
import java.awt.event.MouseEvent;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JLayer;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;
import javax.swing.plaf.LayerUI;
public class BrandedUI
{
private static Color PALE_BLUE = new Color (0.0f, 0.0f, 1.0f, 0.3f);
private static Color PALE_RED = new Color (1.0f, 0.0f, 0.0f, 0.3f);
private static Font BRAND_FONT = new Font ("Arial", Font.BOLD, 18);
private static String MSG = "My brand";
private static JLayer<JPanel> createLayer ()
{
LayerUI<JPanel> layerUI;
layerUI = new LayerUI<JPanel> ()
{
private Color color = PALE_BLUE;
public void installUI (JComponent c)
{
super.installUI (c);
((JLayer) c).setLayerEventMask (AWTEvent.MOUSE_MOTION_EVENT_MASK);
}
public void eventDispatched (AWTEvent e,
JLayer <? extends JPanel> l)
{
MouseEvent me = (MouseEvent) e;
Point pt = SwingUtilities.convertPoint ((Component) me.getSource (),
me.getX (), me.getY (), l);
int cx = l.getWidth ()/2;
int cy = l.getHeight ()/2;
if (pt.x > cx-45 && pt.x < cx+45 && pt.y > cy-10 && pt.y < cy+10)
color = PALE_RED;
else
color = PALE_BLUE;
l.repaint ();
}
public void paint (Graphics g, JComponent c)
{
// Paint the view.
super.paint (g, c);
// Paint the brand.
g.setColor (color);
g.setFont (BRAND_FONT);
int width = g.getFontMetrics ().stringWidth (MSG);
int height = g.getFontMetrics ().getHeight ();
g.drawString (MSG, (c.getWidth ()-width)/2,
c.getHeight ()/2+height/4);
}
public void uninstallUI (JComponent c)
{
super.uninstallUI (c);
((JLayer) c).setLayerEventMask (0);
}
};
// Create a user interface to be decorated.
JPanel pnlMain = new JPanel ();
pnlMain.setLayout (new GridLayout (2, 1));
JPanel pnlTemp = new JPanel ();
JLabel lblName = new JLabel ("Name:");
pnlTemp.add (lblName);
JTextField txtName = new JTextField (20);
pnlTemp.add (txtName);
pnlMain.add (pnlTemp);
pnlTemp = new JPanel ();
JLabel lblAddr = new JLabel ("Address:");
pnlTemp.add (lblAddr);
JTextField txtAddr = new JTextField (20);
pnlTemp.add (txtAddr);
pnlMain.add (pnlTemp);
// Create the layer for the main panel using our custom layerUI.
return new JLayer<JPanel> (pnlMain, layerUI);
}
private static void createAndShowUI ()
{
JFrame frame = new JFrame ("Branded UI");
frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
frame.add (createLayer ());
frame.pack ();
frame.setLocationRelativeTo (null);
frame.setVisible (true);
}
public static void main (String [] args)
{
Runnable r = new Runnable ()
{
public void run ()
{
createAndShowUI ();
}
};
EventQueue.invokeLater (r);
}
}
上面的代码渲染UI上的文本印记，我们通常使用印记提醒用户使用的是试用软件，印记文本是半透明的，以便背景可以全部显示，我们不希望这个印记给用户造成太大的干扰。
另一方面，我们希望用户能注意到这个印记，让他们下定决心购买这款软件，清单2中的代码通过改变印记的颜色(改成淡红色)来达到这个目的，当鼠标移到初始值是绿色的印记面板上时，颜色就变成淡红色。
