关于GridBagLayout的问题！！

作了一个关于GridBagLayout 布局的小程序，排列四个图片按钮，怎么显示的是普通的按钮呢？？图片显示不出来？？import java.awt.*;
import java.awt.event.*;
import javax.swing.*;public class GridBagLay {
JButton bt_1,bt_2,bt_3,bt_4;
void buildConstraints(GridBagConstraints gbc,int gx,int gy,int gw,int gh,double wx,double wy,int anc,int fil,Insets in,int ipx,int ipy){
gbc.gridx=gx;
gbc.gridy=gy;
gbc.gridwidth=gw;
gbc.gridheight=gh;
gbc.weightx=wx;
gbc.weighty=wy;
gbc.anchor=anc;
gbc.fill=fil;
gbc.insets=in;
gbc.ipadx=ipx;
gbc.ipady=ipy;
}
public GridBagLay(){
    JFrame frame=new JFrame("GridBagLayout()!!");
    Container con=frame.getContentPane();
    GridBagConstraints gbcs=new GridBagConstraints();
    GridBagLayout gbl=new GridBagLayout();
    con.setLayout(gbl);
    Insets inc=new Insets(0,0,0,0);
    
Icon ic1=new ImageIcon("img/001.gif");
Icon ic2=new ImageIcon("img/002.gif");
Icon ic3=new ImageIcon("img/003.gif");
Icon ic4=new ImageIcon("img/004.gif");
bt_1=new JButton("but1",ic1);
bt_2=new JButton("but2",ic2);
bt_3=new JButton("but3",ic3);
bt_4=new JButton("but4",ic4);
bt_1.setToolTipText("Button1");
bt_2.setToolTipText("Button2");
bt_3.setToolTipText("Button3");
bt_4.setToolTipText("Button4");
  /*  bt_1=new JButton("button1");
    bt_2=new JButton("button2");
    bt_3=new JButton("button3");
    bt_4=new JButton("button4");*/
    
    con.add(bt_1);
    con.add(bt_2);
    con.add(bt_3);
    con.add(bt_4);

buildConstraints(gbcs,0,0,1,1,1,1,GridBagConstraints.CENTER,GridBagConstraints.HORIZONTAL,inc,0,0);
gbl.setConstraints(bt_1,gbcs);

buildConstraints(gbcs,1,0,1,1,1,1,GridBagConstraints.CENTER,GridBagConstraints.HORIZONTAL,inc,0,0);
gbl.setConstraints(bt_2,gbcs);

buildConstraints(gbcs,0,1,1,1,1,1,GridBagConstraints.CENTER,GridBagConstraints.HORIZONTAL,inc,0,0);
gbl.setConstraints(bt_3,gbcs);

buildConstraints(gbcs,1,1,1,1,1,1,GridBagConstraints.CENTER,GridBagConstraints.HORIZONTAL,inc,0,0);
gbl.setConstraints(bt_4,gbcs);


frame.setVisible(true);
frame.setSize(300,300);


frame.addWindowListener(
    new WindowAdapter(){
     public void windowClosing(WindowEvent e){System.exit(0);}
    }
);
}
public static void main(String args[]){
new GridBagLay();
}
}
本人新手，请各位高手帮帮忙看看！！