Make the map used as singleton read only to avoid side effects. Also the method implementations are much simpler for the empty map.

Author: rbri

src/main/java/org/htmlunit/html/DomElement.java

@@ -1663,19 +1663,10 @@ public void setInnerHtml(final String source) throws SAXException, IOException {
  * The {@link NamedNodeMap} to store the node attributes.
  */
 class NamedAttrNodeMapImpl implements Map<String, DomAttr>, NamedNodeMap, Serializable {
-    protected static final NamedAttrNodeMapImpl EMPTY_MAP = new NamedAttrNodeMapImpl();
-
     private final OrderedFastHashMap<String, DomAttr> map_;
     private final DomElement domNode_;
     private final boolean caseSensitive_;
 
-    private NamedAttrNodeMapImpl() {
-        super();
-        domNode_ = null;
-        caseSensitive_ = true;
-        map_ = new OrderedFastHashMap<>(0);
-    }
-
     NamedAttrNodeMapImpl(final DomElement domNode, final boolean caseSensitive) {
         super();
         if (domNode == null) {

src/main/java/org/htmlunit/html/DomNode.java

@@ -99,6 +99,8 @@ public abstract class DomNode implements Cloneable, Serializable, Node {
     /** The name of the "element" property. Used when watching property change events. */
     public static final String PROPERTY_ELEMENT = "element";
 
+    private static final NamedNodeMap EMPTY_NAMED_NODE_MAP = new ReadOnlyEmptyNamedNodeMapImpl();
+
     /** The owning page of this node. */
     private SgmlPage page_;
 
@@ -662,7 +664,7 @@ public boolean hasAttributes() {
      */
     @Override
     public NamedNodeMap getAttributes() {
-        return NamedAttrNodeMapImpl.EMPTY_MAP;
+        return EMPTY_NAMED_NODE_MAP;
     }
 
     /**
@@ -1938,4 +1940,77 @@ public DomElement closest(final String selectorString) {
             throw new CSSException("Error parsing CSS selectors from '" + selectorString + "': " + e.getMessage(), e);
         }
     }
+
+    /**
+     * An unmodifiable empty {@link NamedNodeMap} implementation.
+     */
+    private static final class ReadOnlyEmptyNamedNodeMapImpl implements NamedNodeMap, Serializable {
+        private ReadOnlyEmptyNamedNodeMapImpl() {
+            super();
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public int getLength() {
+            return 0;
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public DomAttr getNamedItem(final String name) {
+            return null;
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public Node getNamedItemNS(final String namespaceURI, final String localName) {
+            return null;
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public Node item(final int index) {
+            return null;
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public Node removeNamedItem(final String name) throws DOMException {
+            return null;
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public Node removeNamedItemNS(final String namespaceURI, final String localName) {
+            return null;
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public DomAttr setNamedItem(final Node node) {
+            throw new UnsupportedOperationException("ReadOnlyEmptyNamedAttrNodeMapImpl.setNamedItem");
+        }
+
+        /**
+         * {@inheritDoc}
+         */
+        @Override
+        public Node setNamedItemNS(final Node node) throws DOMException {
+            throw new UnsupportedOperationException("ReadOnlyEmptyNamedAttrNodeMapImpl.setNamedItemNS");
+        }
+    }
 }