java中监听document的变化

package com.baorant;

import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.util.LinkedList;

import javax.swing.Box;
import javax.swing.BoxLayout;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;
import javax.swing.text.BadLocationException;
import javax.swing.text.Document;
import javax.swing.undo.UndoableEdit;

public class JavaDemo {
    JFrame mainWin = new JFrame("监听Document对象");
    JTextArea target = new JTextArea(4, 25);
    JTextArea msg = new JTextArea(5, 35);
    JLabel label = new JLabel("文本域的修改信息");
    Document doc = target.getDocument();// 获得区域内的document对象
    LinkedList<UndoableEdit> undoList = new LinkedList<>();
    final int UNDO_COUNT = 20;

    public void init() {
        msg.setEditable(false);
        doc.addDocumentListener(new DocumentListener() {

            @Override
            public void removeUpdate(DocumentEvent arg0) {
                int offset = arg0.getOffset();
                int len = arg0.getLength();
                msg.append("插入文本的长度：" + len + "
");
                msg.append("插入文本的起始位置：" + offset + "
");
            }

            @Override
            public void insertUpdate(DocumentEvent arg0) {
                int offset = arg0.getOffset();
                int len = arg0.getLength();
                msg.append("插入文本的长度：" + len + "
");
                msg.append("插入文本的起始位置：" + offset + "
");
                try {
                    msg.append("插入文本内容：" + doc.getText(offset, len));
                } catch (BadLocationException evt) {
                    evt.printStackTrace();
                }
            }

            @Override
            public void changedUpdate(DocumentEvent arg0) {
                // TODO Auto-generated method stub

            }
        });
        doc.addUndoableEditListener(e -> {
            UndoableEdit edit = e.getEdit();
            if (edit.canUndo() && undoList.size() < UNDO_COUNT) {
                undoList.add(edit);
            } else if (edit.canUndo() && undoList.size() >= UNDO_COUNT) {
                undoList.pop();
                undoList.add(edit);
            }
        });
        target.addKeyListener(new KeyAdapter() {
            @Override
            public void keyTyped(KeyEvent e) {
                if (e.getKeyChar() == 26) {
                    if (undoList.size() > 0) {
                        undoList.removeLast().undo();
                    }
                }
            }
        });
        Box box = new Box(BoxLayout.Y_AXIS);
        box.add(new JScrollPane(target));
        JPanel panel = new JPanel();
        panel.add(label);
        box.add(panel);
        box.add(new JScrollPane(msg));
        mainWin.add(box);
        mainWin.pack();
        mainWin.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        mainWin.setVisible(true);
    }

    public static void main(String[] args) {
        new JavaDemo().init();
    }
}