你也许可以使用这种方法。
它修改编辑器窗格以同步读取所有文件。然后是 PropertyChanngeEvent 在i/o完成时生成。

import java.awt.*;
import java.awt.event.*;
import java.beans.*;
import java.io.*;
import java.net.*;
import javax.swing.*;
import javax.swing.text.*;
import javax.swing.text.html.*;
import java.awt.image.*;
public class EditorPaneLoadSynchronously extends JFrame
    implements ActionListener, PropertyChangeListener
{
    private JEditorPane html;
    private JTextField webURL;
    public EditorPaneLoadSynchronously()
    {
        JPanel urlPanel = new JPanel();
        getContentPane().add(urlPanel, BorderLayout.NORTH);
        webURL = new JTextField("https://stackoverflow.com", 15);
        webURL.addActionListener(this);
        urlPanel.add(webURL);
        JButton gotoURL = new JButton("Goto URL");
        gotoURL.addActionListener(this);
        urlPanel.add(gotoURL);
        HTMLEditorKit editorKit = new HTMLEditorKit()
        {
            private final ViewFactory factory = new HTMLFactory()
            {
                public View create(Element elem)
                {
                    View v = super.create(elem);
                    if ((v != null) && (v instanceof ImageView))
                    {
                        ((ImageView)v).setLoadsSynchronously( true );
                    }
                    return v;
                }
            };
            public ViewFactory getViewFactory()
            {
                return factory;
            }
        };
        html = new JEditorPane();
//      html.setEditorKit( editorKit );
//      html.setEditable( false );
        html.addPropertyChangeListener("page", this);
        JScrollPane scrollPane = new JScrollPane(html);
        scrollPane.setPreferredSize( new Dimension(400, 400) );
        getContentPane().add(scrollPane);
    }
    public void actionPerformed(ActionEvent e)
    {
        try
        {
//          html.setDocument( new HTMLDocument() );
            html.setPage( new URL(webURL.getText()) );
            System.out.println("After setPage");
        }
        catch(Exception exc)
        {
            System.out.println(exc);
        }
    }
    public void propertyChange(PropertyChangeEvent e)
    {
        try
        {
            System.out.println("Page Loaded");
//          BufferedImage bi = ScreenImage.createImage(html);
//          ScreenImage.writeImage(bi, "sync.jpg");
        }
        catch(Exception ee) {}
    }
    private static void createAndShowGUI()
    {
        EditorPaneLoadSynchronously frame = new EditorPaneLoadSynchronously();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setLocationByPlatform( true );
        frame.setVisible( true );
        frame.actionPerformed(null);
    }
    public static void main(String[] args) throws Exception
    {
        java.awt.EventQueue.invokeLater( () -> createAndShowGUI() );
    }
}

注意：上面的代码使用屏幕图像便利类。您可以将其替换为创建和编写BuffereImage的唯一代码。

您试图在呈现gui（包括组件）之前绘制一个组件，但这行不通。建议包括：
使用在swing事件线程上加载gui SwingUtilities.invokeLater() 使用swingworker在后台线程中加载任何web数据。或者在创建swing gui之前在主线程中获取web页面，然后将其传递到swing gui。
仅在加载网页并呈现gui（使其可见）后创建图像
除掉所有的 Thread.sleep 电话。这些不是您的朋友，只会阻碍swing gui的呈现。
例如：

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import java.net.URL;
import javax.imageio.ImageIO;
import javax.swing.*;
public class EditorStuff extends JPanel {
    private static final String URL_PATH = "https://docs.oracle.com/javase/tutorial/index.html";
    private JEditorPane editorPane;
    public EditorStuff(URL url) throws IOException {
        int w = 800;
        int h = 650;
        setPreferredSize(new Dimension(w, h));
        editorPane = new JEditorPane(url);
        // editorPane.setPage(url);
        JScrollPane scrollPane = new JScrollPane(editorPane);
        setLayout(new BorderLayout());
        add(scrollPane);
    }
    public void captureImage() throws IOException {
        int w = editorPane.getWidth();
        int h = editorPane.getHeight();
        BufferedImage img = new BufferedImage(w, h, BufferedImage.TYPE_INT_ARGB);
        Graphics2D g2 = img.createGraphics();
        editorPane.paint(g2);
        ImageIO.write(img, "png", new File("test.png"));
        g2.dispose();
    }
    public static void main(String[] args) {
        try {
            URL url = new URL(URL_PATH);
            SwingUtilities.invokeLater(() -> createAndShowGui(url));
        } catch (IOException ioEx) {
            ioEx.printStackTrace();
        }
    }
    private static void createAndShowGui(final URL url) {
        try {
            EditorStuff mainPanel = new EditorStuff(url);
            JFrame frame = new JFrame("EditorStuff");
            frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
            frame.add(mainPanel);
            frame.pack();
            frame.setLocationByPlatform(true);
            frame.setVisible(true);
            mainPanel.captureImage();  // called *after* rendering
        } catch (IOException ioEx) {
            ioEx.printStackTrace();
        }
    }
}