JLabel
by default is horizontally aligned to the left.
You need to set the horizontalAlignment
to be JLabel.CENTER
JPanel panel = new JPanel(new BorderLayout());
JLabel label = new JLabel("L'alliage a bien été enregistré.");
label.setHorizontalAlignment(JLabel.CENTER);
label.setVerticalAlignment(JLabel.CENTER);
panel.add(label, BorderLayout.CENTER);