panel=new JPanel();
add(panel);
panel.setLayout(new BoxLayout(panel, BoxLayout.Y_AXIS));

JButton button1=new JButton("button1");
JButton button2=new JButton("button2");

buttonsPanel=new JPanel();
buttonsPanel.setLayout(new BoxLayout(buttonsPanel, BoxLayout.X_AXIS));

buttonsPanel.add(button1, CENTER_ALIGNMENT);
buttonsPanel.add(button2, CENTER_ALIGNMENT);
panel.add(buttonsPanel, BorderLayout.CENTER);

buttonsPanel.add(button1, CENTER_ALIGNMENT);

panel.add(buttonsPanel, BorderLayout.CENTER);

JPanel buttonsPanel = new JPanel();
buttonsPanel.add(button1);
buttonsPanel.add(button2);

frame.setLayout( new GridBagLayout() );
frame.add(buttonsPanel, new GridBagConstraints());