Java—使用运行时更改约束JScrollPanes下面的组件

我有一个JFrame和一个包含JTable的JScrollPane。在JTable下面，我放置了一个带有SpringLayout的JButton:

JButton向JTable添加一行，并更新JScrollPane的大小。由于它们相互约束，我希望按钮更新其位置并向下移动。然而，这并没有发生
当我用一个普通的JTable尝试同样的方法时，它工作得很好，但是我需要这个JScrollPane来避免JTable扩展到框架之外。 以下是我的完整测试代码：

package tests;

import java.awt.Dimension;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.SpringLayout;
import javax.swing.table.DefaultTableModel;

public class StackOverflowQRuntimeAlignment extends JFrame{

    private JScrollPane sP;
    private JTable tab;
    private JButton but;
    private DefaultTableModel dtm;

    public static void main(String[] args){
        StackOverflowQRuntimeAlignment frame = new StackOverflowQRuntimeAlignment("Test");
        frame.setSize(new Dimension(1280, 720));
        frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
        frame.setVisible(true);
    }

    StackOverflowQRuntimeAlignment(String title){
        super(title);
        //DefaultTabelModel to create the Table
        dtm = new DefaultTableModel(new String[][]{{"data1", "data2"}}, new String[]{"Column1", "Column2"});
        tab = new JTable(dtm);
        //set TableHeaderHeight to RowHeight
        tab.getTableHeader().setPreferredSize(new Dimension(tab.getWidth(), tab.getRowHeight()));
        //create ScrollPane containing the Table
        sP = new JScrollPane(tab);
        //set the size of the ScrollPane
        sP.setPreferredSize(new Dimension(500, (dtm.getRowCount()+1)*16+3));//+1 for the Header, +3 to hide ScrollBars if they are not neccesary
        sP.setMaximumSize(new Dimension(tab.getWidth(), 360));

        //create the button to add an row to the table
        but = new JButton("Hit me");
        but.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                dtm.addRow(new String[]{"Banana", "Apple"});
                //updating the size of the ScrollPane
                sP.setPreferredSize(new Dimension(500, (dtm.getRowCount()+1)*16+3));
                sP.setSize(sP.getPreferredSize());
            }
        });

        //constraining the Components on the GUI
        SpringLayout l = new SpringLayout();
        this.setLayout(l);
        l.putConstraint(SpringLayout.HORIZONTAL_CENTER, sP, 0, SpringLayout.HORIZONTAL_CENTER, this.getContentPane());
        l.putConstraint(SpringLayout.NORTH, sP, 50, SpringLayout.NORTH, this.getContentPane());

        l.putConstraint(SpringLayout.HORIZONTAL_CENTER, but, 0, SpringLayout.HORIZONTAL_CENTER, sP);
        l.putConstraint(SpringLayout.NORTH, but, 50, SpringLayout.SOUTH, sP);

        //adding the Components to the GUI
        this.add(sP);
        this.add(but);
    }
}

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使用JScrollPane的要点是，您不必一直调整它的大小。随着添加到滚动窗格的组件的首选大小增加，滚动条将出现在滚动窗格上。因此，使用滚动窗格的默认首选大小创建GUI，并让Swing布局管理器工作

通常，滚动窗格将添加到边框布局的中心，以便用户调整边框的大小时，空间分配给滚动窗格

但是，从可见GUI添加（或删除）组件的一般规则是：

panel.add(...);
panel.revalidate(); // to invoke the layout manager
panel.repaint(); // to paint the components

panel.add(...);
panel.revalidate(); // to invoke the layout manager
panel.repaint(); // to paint the components