如何在JScrollPane中实现位置敏感的缩放？

public class PositionSensitiveScrollPane extends JScrollPane {
    private static final String VERTICAL_SCROLL_POLICY = "verticalScrollBarPolicy";
    private static final String HORIZONTAL_SCROLL_POLICY = "horizontalScrollBarPolicy";

    public PositionSensitiveScrollPane() {
        setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
        setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_AS_NEEDED);
    }

    @Override
    protected JComponent createScrollPane() {
        return super.createScrollPane();
    }

    @Override
    public void setUI(final javax.swing.ScrollPaneConstants uiAdatper) {
        super.setUI(new SimpleScrollPaneUI());
    }

    @Override
    public void changeSelectionToDefault() {
        super.changeSelectionToDefault();
    }

    @Override
    public void setHorizontalScrollBarPolicy(final int horizontalPolicy) {
        super.setHorizontalScrollBarPolicy(horizontalPolicy);
    }

    @Override
    public void setVerticalScrollBarPolicy(final int verticalPolicy) {
        if (verticalPolicy == javax.swing.ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS) {
            super.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
        } else if (verticalPolicy == javax.swing.ScrollPaneConstants.VERTICAL_SCROLLBAR_NEVER) {
            super.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_NEVER);
        } else {
            // if the policy if different from always and never, we do nothing.
        }
    }

    @Override
    protected void createTrackPropertyChangeListener() {
        super.createTrackPropertyChangeListener();
    }
}

JScrollPane scrollPane = new PositionSensitiveScrollPane();

int bottomY = scrollPane.getViewport().getExtentSize().height;
int bottomMargin = scrollPane