Java Swing - 如何在添加JToolbar之后创建JSplitPane

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.GridLayout;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JSplitPane;
import javax.swing.JTextArea;
import javax.swing.JTextPane;
import javax.swing.JToolBar;

public class Main extends JFrame {
  public static void main(String[] args) {
    Main frame = new Main();
    frame.build();
    frame.setVisible(true);

  }

  public void build() {
    setSize(600, 600);
    JTextPane textPane = new JTextPane();

    JScrollPane scrollPane = new JScrollPane(textPane);
    scrollPane.setPreferredSize(new Dimension(300, 300));

    JTextArea changeLog = new JTextArea(5, 30);
    changeLog.setEditable(false);
    JScrollPane scrollPaneForLog = new JScrollPane(changeLog);

    JSplitPane splitPane = new JSplitPane(JSplitPane.VERTICAL_SPLIT,
        scrollPane, scrollPaneForLog);
    splitPane.setOneTouchExpandable(true);

    JPanel statusPane = new JPanel(new GridLayout(1, 1));
    JLabel caretListenerLabel = new JLabel("Caret Status");
    statusPane.add(caretListenerLabel);

    JToolBar toolBar = new JToolBar();
    toolBar.add(new JButton("Btn1"));
    toolBar.add(new JButton("Btn2"));
    toolBar.add(new JButton("Btn3"));
    toolBar.add(new JButton("Btn4"));

    getContentPane().add(toolBar, BorderLayout.PAGE_START);
    getContentPane().add(splitPane, BorderLayout.CENTER);
    getContentPane().add(statusPane, BorderLayout.PAGE_END);

    JMenu editMenu = new JMenu("test");
    JMenu styleMenu = new JMenu("test");
    JMenuBar mb = new JMenuBar();
    mb.add(editMenu);
    mb.add(styleMenu);
    setJMenuBar(mb);

  }
}