A DESCRIPTION OF THE PROBLEM :
When XMLEncoder.writeObject() a bean containing a javax.swing.border.LineBorder whose lineColor is null (or any non-default combination), the resulting XML omits all of the border’s state. On decode it fall back to the bean’s default LineBorder. Even registering a PersistenceDelegate( "lineColor","thickness","roundedCorners" ) for LineBorder has no effect

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
successfully serialize Bean and its value(a LineBorader).
ACTUAL -
<?xml version="1.0" encoding="UTF-8"?>
<java version="24.0.1" class="java.beans.XMLDecoder">
 <object class="LineBorderXMLSerializationTest$Bean"/>
</java>

Exception in thread "main" java.lang.RuntimeException: Color mismatch: original=null, deserialized=java.awt.Color[r=255,g=0,b=0]
at LineBorderXMLSerializationTest.main(LineBorderXMLSerializationTest.java:63)


---------- BEGIN SOURCE ----------
import javax.swing.border.LineBorder;
import java.awt.Color;
import java.beans.*;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.util.Objects;

public class LineBorderXMLSerializationTest {

    public static class Bean {
        public final static LineBorder DEFAULT = new LineBorder(Color.RED, 2, true);
        private LineBorder value;
        public Bean() {this.value = DEFAULT;}
        public LineBorder getValue() {return value;}
        public void setValue(LineBorder value) {this.value = value;}
    }

    public static void main(String[] args) throws IOException {
        LineBorder originalBorder = new LineBorder(null, 0, false);

        Bean beanToEncode = new Bean();
        beanToEncode.setValue(originalBorder);

        ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
        try (XMLEncoder encoder = new XMLEncoder(outputStream)) {
            encoder.setPersistenceDelegate(
                LineBorder.class,
                new PersistenceDelegate() {
                    @Override
                    protected Expression instantiate(Object oldInstance, Encoder out) {
                        LineBorder lb = (LineBorder) oldInstance;
                        return new Expression(
                                lb,
                                lb.getClass(),
                                "new",
                                new Object[]{lb.getLineColor(),
                                        lb.getThickness(),
                                        lb.getRoundedCorners()}
                        );
                    }
                }
            );
            encoder.writeObject(beanToEncode);
        }

        byte[] xmlData = outputStream.toByteArray();


        System.out.println(new String(xmlData, StandardCharsets.UTF_8));

        Bean deserializedBean;
        ByteArrayInputStream inputStream = new ByteArrayInputStream(xmlData);
        try (XMLDecoder decoder = new XMLDecoder(inputStream)) {
            deserializedBean = (Bean) decoder.readObject();
        }

        LineBorder deserializedBorder = deserializedBean.getValue();
        Color originalColor = originalBorder.getLineColor();
        Color deserializedColor = deserializedBorder.getLineColor();
        if (!Objects.equals(originalColor, deserializedColor)) {
            throw new RuntimeException("Color mismatch: original=" + originalColor + ", deserialized=" + deserializedColor);
        } else {
            System.out.println("\nColors match.");
        }
    }
}
---------- END SOURCE ----------

Workaround
following way can get correct result
encoder.setPersistenceDelegate(
  Bean.class,
  new PersistenceDelegate() {
    @Override
    protected Expression instantiate(Object old, Encoder out) {
      return new Expression(old, old.getClass(), "new", new Object[]{});
    }
    @Override
    protected void initialize(Class<?> type, Object old, Object existing, Encoder out) {
      super.initialize(type, old, existing, out);
      Bean bOld = (Bean) old;
      if (bOld.getValue() != ((Bean) existing).getValue()) {
        out.writeStatement(
          new Statement(old, "setValue", new Object[]{ bOld.getValue() })
        );
      }
    }
  }
);