java - GridBagLayout 布局 的行列概念以及gridx，gridy失效？

Question

大家好，本人java初学者。
在做一个GUI界面的时候使用了GridBagLayout布局，代码如下。
我的理解是gridx 和gridy是将组件设置在 某一行某一列 上，所以我用不同的数值尝试性修改两个button的位置，但是不管怎么设置其位置都没有变化。
那么GridBagLayout中一行一列到底有几个单元格？gridxgridy 应该怎么用？

import java.awt.*;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;

import javax.swing.*;



public class LoginPanel extends JFrame implements MouseListener{
    public JLabel lb1,lb2,lb3;
    public JTextField tf1;
    public JPasswordField pw1;
    public JPanel p1;
    public JButton b1,b2;
    public LoginPanel(){
        lb1=new JLabel("请输入用户名:");
        lb2=new JLabel("请输入密码:");
        lb3=new JLabel(" ");
        tf1=new JTextField (15);
        pw1=new JPasswordField (15);
        p1=new JPanel();
        b1=new JButton("确定");
        b2=new JButton("重置");
        this.setSize(600,600);
        p1.setBackground(Color.yellow);
        GridBagLayout playout=new GridBagLayout();
        GridBagConstraints s= new GridBagConstraints();
         
        s.gridwidth=2;
        s.weightx = 0;
        s.weighty=0;
        playout.setConstraints(lb1, s);
        s.gridwidth=0;
        s.weightx = 0;
        s.weighty=0;
        playout.setConstraints(tf1, s);
        s.insets=new Insets(20,0,0,0);
        s.gridwidth=2;
        s.weightx = 0;
        s.weighty=0;
        playout.setConstraints(lb2, s);
        s.gridwidth=0;
        s.weightx = 0;
        s.weighty=0;
        playout.setConstraints(pw1, s);
        
        
        /*********这里的gridxgridy似乎没有生效********************/
        
        s.gridx=3;
        s.gridy=8;
        s.gridwidth=1;
        s.weightx = 0;
        s.weighty=0;
        playout.setConstraints(b1, s);
        s.gridx=6;
        s.gridy=8;
        s.gridwidth=0;
        s.weightx = 0;
        s.weighty=0;
        playout.setConstraints(b2, s);
        
        
        /*****************************/
        
        this.setLayout(playout);
        this.getContentPane().setBackground(Color.gray);
        
        lb1.setPreferredSize(new Dimension(100,30));
         lb2.setPreferredSize(new Dimension(100,30));
        tf1.setPreferredSize(new Dimension(100,30));
          pw1.setPreferredSize(new Dimension(100,30));
        
        //this.setLayout(new BorderLayout());
        add(lb1);
        add(tf1);
        add(lb2);
        add(pw1);
        add(b1);
        add(b2);
        
        //this.pack();
        setVisible(true);
    }
    public static void main( String [] args){
        new LoginPanel();
    }
  
        
    
    
    
    
    
    
    
    
    
    @Override
    public void mouseClicked(MouseEvent e) {
        // TODO Auto-generated method stub
        
    }

    @Override
    public void mouseEntered(MouseEvent e) {
        // TODO Auto-generated method stub
        
    }

    @Override
    public void mouseExited(MouseEvent e) {
        // TODO Auto-generated method stub
        
    }

    @Override
    public void mousePressed(MouseEvent e) {
        // TODO Auto-generated method stub
        
    }

    @Override
    public void mouseReleased(MouseEvent e) {
        // TODO Auto-generated method stub
        
    }
    
        
    }