您需要将JTextPane添加到JScrollPane，然后将JScrollPane添加到JFrame（或其内容窗格）：

public class Test1 {

    public Test1() {

        JFrame frm = new JFrame("frontend");

        JLabel l1 = new JLabel("Enter name of text file");
        JLabel l2 = new JLabel("Enter name of text file");
        JLabel l3 = new JLabel("SIMILARITY");
        JLabel l4 = new JLabel("DIFFERENCES");

        JTextField t1 = new JTextField(15);
        JTextField t2 = new JTextField(15);

        JTextPane ta1 = new JTextPane();
        JTextPane ta2 = new JTextPane();

        ta1.setContentType("text/html");
        ta2.setContentType("text/html");
        String a = "vdnogregnroei dfnoj";
        ta1.setText(a);
        ta2.setText(a);

        JScrollPane sp1 = new JScrollPane(ta1);
        JScrollPane sp2 = new JScrollPane(ta2);
        JButton b1 = new JButton("COMPARE");

        Container cont = frm.getContentPane();
        cont.setLayout(new GridBagLayout());
        GridBagConstraints cnt = new GridBagConstraints();

        cnt.fill = GridBagConstraints.HORIZONTAL;
        cnt.insets = new Insets(10, 10, 10, 10);
        cnt.gridx = 0;
        cnt.gridy = 0;
        cont.add(l1, cnt);

        cnt.gridx = 1;
        cnt.gridy = 0;
        cont.add(t1, cnt);

        cnt.gridx = 0;
        cnt.gridy = 1;
        cont.add(l2, cnt);

        cnt.gridx = 1;
        cnt.gridy = 1;
        cont.add(t2, cnt);

        cnt.gridx = 0;
        cnt.gridy = 2;
        cont.add(l3, cnt);

        cnt.gridx = 1;
        cnt.gridy = 2;
//      cnt.weightx = 1;
//      cnt.weighty = 1;
        cont.add(sp1, cnt);

        cnt.gridx = 0;
        cnt.gridy = 3;
//      cnt.weightx = 0;
//      cnt.weighty = 0;
        cont.add(l4, cnt);

        cnt.gridx = 1;
        cnt.gridy = 3;
        cont.add(sp2, cnt);

        cnt.gridx = 0;
        cnt.gridy = 4;
        cont.add(b1, cnt);

        frm.pack();
        frm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frm.setVisible(true);
    }

    public static void main(String[] args) {

        new Test1();
    }
}

我建议你仔细看看这段代码和你的代码之间的区别，注意我在哪里设置了布局，添加了什么组件。

备注：

• 我注解掉了一些提示您调整大小行为的行。
• 您可能想在框架上调用setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE)。
• 网格从x=0和y=0开始，而不是x=1和y=1。

import javax.swing.*;
import javax.swing.text.*;

public class JTextPaneDemo extends JFrame {
    private JTextPane textPane;

    public JTextPaneDemo() {
        setTitle("JTextPane Demo");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setSize(400, 300);
        setLocationRelativeTo(null);

        // Create a new JTextPane object
        textPane = new JTextPane();

        // Add some content to the JTextPane
        String text = "Hello World!\nThis is a JTextPane example.";
        textPane.setText(text);

        // Apply some styles to the text
        StyledDocument doc = textPane.getStyledDocument();
        SimpleAttributeSet center = new SimpleAttributeSet();
        StyleConstants.setAlignment(center, StyleConstants.ALIGN_CENTER);
        doc.setParagraphAttributes(0, doc.getLength(), center, false);

        // Add the JTextPane to the JFrame
        add(new JScrollPane(textPane));

        setVisible(true);
    }

    public static void main(String[] args) {
        new JTextPaneDemo();
    }
}