This is not JButton overriden, but it might give you some ideas.
This is a nice looking hyperlink component derived from JComponent.

The code is mine and it is just released under BSD license.

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

class ActiveLabel extends JComponent {
public final int XMARGIN=2,YMARGIN=2;
public final int ACTIVE=1,IDLE=0,PRESSED=2;
static final Color[] clr = { Color.blue, Color.red, Color.black };
ActionListener lst;
String txt,cmd;
public int state;
int align;
// ctor
public ActiveLabel(String str,int alg)
   {
   align=alg;
   cmd=txt=str;
   state=IDLE;
   enableEvents(AWTEvent.MOUSE_EVENT_MASK );
   lst=null;
   }
public ActiveLabel(String str)
   {
   this(str,Label.LEFT);
   }
public ActiveLabel()
   {
   this("",Label.LEFT);
   }
// paint
public void paint(Graphics g)
   {
   FontMetrics fm=getFontMetrics(getFont());
   int wt=fm.stringWidth(txt);
   int ht=fm.getHeight();
   Rectangle r=getBounds();
   int w=r.width;
   int h=r.height;
   int x=(w-wt) >> 1;
   int y=((h+ht) >> 1)-2;
   if(align == Label.LEFT) x=0;
   else if(align == Label.RIGHT) x=w-wt;
   g.setColor(clr[state]);
   g.drawString(txt,x,y);
   g.drawLine(x,y+1,x+wt,y+1);
   }
// getMinimumSize
public Dimension getMinimumSize()
   {
   FontMetrics fm=getFontMetrics(getFont());
   int h=fm.getHeight() + YMARGIN + YMARGIN;
   int w=fm.stringWidth(txt) + XMARGIN + XMARGIN;
   return new Dimension(w,h);
   }
// minimumSize
public Dimension minimumSize()
   {
   return getMinimumSize();
   }
// preferredSize
public Dimension preferredSize()
   {
   return getMinimumSize();
   }
// getPreferredSize
public Dimension getPreferredSize()
   {
   return getMinimumSize();
   }
//
public void released()
   {
   int a=state;
   state=ACTIVE;
   if(a != state) repaint();
   postAction();
   }
//
public void pressed()
   {
   int a=state;
   state=PRESSED;
   if(a != state) repaint();
   }
//
public void entered()
   {
   int a=state;
   state=ACTIVE;
   if(a != state) repaint();
   }
//
public void exited()
   {
   int a=state;
   state=IDLE;
   if(a != state) repaint();
   }
//
public void processEvent(AWTEvent e)
   {
   if(e.getID() == MouseEvent.MOUSE_PRESSED) pressed();
   else if(e.getID() == MouseEvent.MOUSE_ENTERED) entered();
   else if(e.getID() == MouseEvent.MOUSE_EXITED) exited();
   else if(e.getID() == MouseEvent.MOUSE_RELEASED) released();
   super.processEvent(e);
   }
//
public void setActionCommand(String c)
   {
   cmd=c;
   }
//
public void addActionListener(ActionListener l)
   {
   lst=AWTEventMulticaster.add(lst,l);
   }
//
public void removeActionListener(ActionListener l)
   {
   lst=AWTEventMulticaster.remove(lst,l);
   }
//
private void postAction()
   {
   if(lst != null)
      {
      ActionEvent event = new
ActionEvent(this,ActionEvent.ACTION_PERFORMED,cmd);
      lst.actionPerformed(event);
      }
   }
}