swing JPanel画渐变的背景色

import java.awt.BorderLayout; import java.awt.Color; import java.awt.GradientPaint; import java.awt.Graphics; import java.awt.Graphics2D; import java.awt.LayoutManager; import javax.swing.JFrame; import javax.swing.JPanel; import javax.swing.WindowConstants; public class GradientPanel extends JPanel { public GradientPanel(LayoutManager lm) { super(lm); } public void paintComponent(Graphics g) { super.paintComponent(g); if (!isOpaque()) { return; } int width = getWidth(); int height = getHeight(); Graphics2D g2 = (Graphics2D) g; GradientPaint gradientPaint =new GradientPaint(width/2, height/2,Color.white, width, height, Color.blue,false); g2.setPaint(gradientPaint); g2.fillRect(0, 0, width, height); } public static void main(String args[]){ JFrame jf=new JFrame("GradientPanelTest"); jf.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE); GradientPanel gp=new GradientPanel(new BorderLayout()); jf.getContentPane().add(gp); jf.setSize(500,400); jf.setVisible(true); } } import java.awt.BorderLayout; import java.awt.C