Java SE (Java Platform, Standard Edition)

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Rectangle;
import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTextPane;
import javax.swing.SwingUtilities;
import javax.swing.event.CaretEvent;
import javax.swing.event.CaretListener;
import javax.swing.plaf.basic.BasicTextPaneUI;
import javax.swing.text.BadLocationException;

public class LineHighlightTextPane {
   
   public LineHighlightTextPane () {
   }
   
   void makeUI () {
      
      JTextPane textPane = new JTextPane ();
      textPane.setUI (new LineHighlightTextPaneUI (textPane));
      
      JScrollPane scrollPane = new JScrollPane (textPane);
      
      JFrame frame = new JFrame ("Line Highlight Text Pane");
      frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
      frame.setSize (300, 300);
      
      frame.add (scrollPane, BorderLayout.CENTER);
      frame.setVisible (true);
   }
   
   public static void main (String[] args) {
      SwingUtilities.invokeLater (new Runnable () {
         public void run () {
            new LineHighlightTextPane ().makeUI ();
         }
      });
   }
}

class LineHighlightTextPaneUI extends BasicTextPaneUI {
   
   JTextPane tc;
   
   LineHighlightTextPaneUI (JTextPane t) {
      
      tc = t;
      tc.addCaretListener (new CaretListener () {
         public void caretUpdate (CaretEvent e) {
            
            tc.repaint ();
         }
      });
   }
   
   @Override
   public void paintBackground (Graphics g) {
      
      super.paintBackground (g);
      
      try {
         Rectangle rect = modelToView(tc, tc.getCaretPosition ());
         int y = rect.y;
         int h = rect.height;
         g.setColor (Color.YELLOW);
         g.fillRect (0, y, tc.getWidth (), h);
      } catch (BadLocationException ex) {
         ex.printStackTrace();
      }
   }
}