Swing problem

The code below adds two panels in the main window. Can somebody tell me why the second (lowest) panel is not immediately visible but I have to manually resize the main window to see it?

import java.awt.*;
import java.awt.event.*;
import java.sql.*;
import com.sun.java.swing.*;
import com.sun.java.swing.event.*;

public class SwingApp extends JFrame implements ActionListener,CaretListener {
  JPanel p1,p2;
  JLabel l1,l2;
  JButton b1,b2;
  JTextField tf1,tf2;
  Connection con;
  public SwingApp() {
    Container contentPane = getContentPane();
    p1=new JPanel();
    p2=new JPanel(new GridLayout(0,1));

    p1.add(l1=new JLabel("Database Name",SwingConstants.LEFT));
    p1.add(tf1=new JTextField(50));
    p1.add(b1=new JButton("Open"));
    p1.add(b2=new JButton("Close"));
    contentPane.add("Center",p1);

    p2.add(l2=new JLabel("",SwingConstants.CENTER));
    contentPane.add("South",p2);

    b1.setEnabled(false);
    b2.setEnabled(false);
    tf1.addCaretListener(this);
    tf1.addActionListener(this);
    b1.addActionListener(this);
    b2.addActionListener(this);
    addWindowListener(new WindowAdapter() {
      public void windowClosing(WindowEvent e) {
        dispose();
        System.exit(0);
      }
    });
  }

  public void actionPerformed(ActionEvent e) {
      if (e.getSource()==b1 || e.getSource()==tf1) {
      try {
         con=DriverManager.getConnection("jdbc:odbc:"+tf1.getText());
         l2.setText("SUCCESS!");
         tf1.setEnabled(false);
         b1.setEnabled(false);
         b2.setEnabled(true);
      } catch(SQLException sqle) {
        l2.setText("Could not open: "+tf1.getText());
        return;
      }
      } else if (e.getSource()==b2) {
      try {
        con.close();
      } catch(SQLException sqle) {
      }
      con=null;
      tf1.setEnabled(true);
      b1.setEnabled(true);
      b2.setEnabled(false);
      l2.setText("");
      }
  }

  public void caretUpdate(CaretEvent e) {
    if (tf1.getText().equals(""))
      b1.setEnabled(false);
    else
      b1.setEnabled(true);
  }

  public static void main(String[] args) {
    SwingApp window = new SwingApp();

    try {
      Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
      DriverManager.setLogStream(null);
    } catch(ClassNotFoundException cnfe) {
      System.err.println("Class not found");
      return;
    }

    window.setTitle("Java Application");
    window.pack();
    window.setVisible(true);
  }
}