Java Swing - 如何在JTextPane中粗体选择文本

import java.awt.BorderLayout;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JTextPane;
import javax.swing.text.AttributeSet;
import javax.swing.text.Element;
import javax.swing.text.MutableAttributeSet;
import javax.swing.text.SimpleAttributeSet;
import javax.swing.text.StyleConstants;
import javax.swing.text.StyledDocument;

public class Main {

  public static void main(String[] args) {
    JFrame frame = new TextStyleTestFrame();
    frame.setSize(300, 300);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
  }
}

class TextStyleTestFrame extends JFrame {
  JButton btnStyle = new JButton("Bold");
  JTextPane textPane = new JTextPane();

  public TextStyleTestFrame() {
    textPane.setText("this is a test.");
    add(textPane, BorderLayout.CENTER);
    add(btnStyle, BorderLayout.NORTH);
    btnStyle.addActionListener(e ->changeStyle());
  }
  private void changeStyle() {
    StyledDocument doc = (StyledDocument) textPane.getDocument();
    int selectionEnd = textPane.getSelectionEnd();
    int selectionStart = textPane.getSelectionStart();
    if (selectionStart == selectionEnd) {
      return;
    }
    Element element = doc.getCharacterElement(selectionStart);
    AttributeSet as = element.getAttributes();

    MutableAttributeSet asNew = new SimpleAttributeSet(as.copyAttributes());
    StyleConstants.setBold(asNew, !StyleConstants.isBold(as));
    doc.setCharacterAttributes(selectionStart, textPane.getSelectedText()
        .length(), asNew, true);
    String text = (StyleConstants.isBold(as) ? "Cancel Bold" : "Bold");
    btnStyle.setText(text);
  }
}