Consider the following very simple program.

======================== HtmlEditorKitLeak.java ============
import javax.swing.text.html.*;
import javax.swing.text.*;
import javax.swing.*;

public class HtmlEditorKitLeak {

    public static void main(String[] args) {
        try {
            main2();
        } catch (Throwable t) {
            t.printStackTrace();
        }
    }
   
   private static void main2(){
      while (true) {
          JEditorPane htmlArea = new JEditorPane();
          StyleSheet style = new HTMLEditorKit().getStyleSheet();
          htmlArea.setDocument(new HTMLDocument(style));
          
          htmlArea = null;
          
          System.out.print(".");

          System.gc();
          System.runFinalization();
      }
   }
}
==================================================

This program eventually consumes all allowed memory and fails.

Looking at the heap dump, it's clear that static field defaultStyles has
a reference to ever-growing array of listeners, keeping all created documents in memory forever.

Static reference from javax.swing.text.html.HTMLEditorKit.defaultStyles (from class javax.swing.text.html.HTMLEditorKit) :
--> javax.swing.text.html.StyleSheet@0x8e8a33c8 (57 bytes) (field styles:)
--> javax.swing.text.StyleContext$NamedStyle@0x8e8ad118 (24 bytes) (field listenerList:)
--> javax.swing.event.EventListenerList@0x8e83e7b0 (12 bytes) (field listenerList:)
--> [Ljava.lang.Object;@0x8e354618 (17208 bytes) (Element 2909 of [Ljava.lang.Object;@0x8e354618:)
--> javax.swing.text.DefaultStyledDocument$StyleContextChangeHandler@0x8fcbfc10 (12 bytes) (field this$0:)
--> javax.swing.text.html.HTMLDocument@0x8fcbd710 (88 bytes)