Componentの3列配置、中央幅固定、左右均等引き伸ばしを行うLayoutManagerを作成する

// SpringLayout
SpringLayout layout = new SpringLayout();
JPanel panel = new JPanel(layout);
panel.setBorder(BorderFactory.createEmptyBorder(5, 5, 5, 5));

SpringLayout.Constraints centerConstraints = layout.getConstraints(box);
centerConstraints.setWidth(Spring.constant(box.getPreferredSize().width));

SpringLayout.Constraints leftConstraints = layout.getConstraints(lsp);
SpringLayout.Constraints rightConstraints = layout.getConstraints(rsp);

Spring width = Spring.max(
    leftConstraints.getWidth(), rightConstraints.getWidth());
leftConstraints.setWidth(width);
rightConstraints.setWidth(width);

panel.add(lsp, leftConstraints);
panel.add(box, centerConstraints);
panel.add(rsp, rightConstraints);

Spring height = layout.getConstraint(SpringLayout.HEIGHT, panel);
leftConstraints.setHeight(height);
rightConstraints.setHeight(height);
centerConstraints.setHeight(height);

centerConstraints.setConstraint(
    SpringLayout.WEST, leftConstraints.getConstraint(SpringLayout.EAST));
rightConstraints.setConstraint(
    SpringLayout.WEST, centerConstraints.getConstraint(SpringLayout.EAST));

layout.putConstraint(SpringLayout.EAST, panel, 0, SpringLayout.EAST, rsp);

// override BorderLayout#layoutContainer(...)
JPanel panel = new JPanel(new BorderLayout(0, 0) {
  @Override public void layoutContainer(Container target) {
    synchronized (target.getTreeLock()) {
      Insets insets = target.getInsets();
      int top = insets.top;
      int bottom = target.getHeight() - insets.bottom;
      int left = insets.left;
      int right = target.getWidth() - insets.right;
      int hgap = getHgap();
      int wc = right - left;
      int we = wc / 2;
      int ww = wc - we;
      Component c = getLayoutComponent(CENTER);
      if (c != null) {
        Dimension d = c.getPreferredSize();
        wc -= d.width + hgap + hgap;
        we = wc / 2;
        ww = wc - we;
        c.setBounds(left + hgap + ww, top, wc, bottom - top);
      }
      c = getLayoutComponent(EAST);
      if (c != null) {
        c.setBounds(right - we, top, we, bottom - top);
      }
      c = getLayoutComponent(WEST);
      if (c != null) {
        c.setBounds(left, top, ww, bottom - top);
      }
    }
  }
});
panel.setBorder(BorderFactory.createEmptyBorder(5, 5, 5, 5));
panel.add(lsp, BorderLayout.WEST);
panel.add(box, BorderLayout.CENTER);
panel.add(rsp, BorderLayout.EAST);