[MNG-7386] Make sure the ModelMerger$MergingList can be serialized

maven-model/src/main/java/org/apache/maven/model/merge/ModelMerger.java

@@ -19,6 +19,7 @@
  * under the License.
  */
 
+import java.io.ObjectStreamException;
 import java.util.AbstractList;
 import java.util.ArrayList;
 import java.util.Collection;
@@ -2870,8 +2871,9 @@ private static <T> List<T> merge( List<T> tgt, List<T> src, KeyComputer<T> compu
      * Merging list
      * @param <V>
      */
-    private static class MergingList<V> extends AbstractList<V>
+    private static class MergingList<V> extends AbstractList<V> implements java.io.Serializable
     {
+
         private final KeyComputer<V> keyComputer;
         private Map<Object, V> map;
         private List<V> list;
@@ -2882,6 +2884,11 @@ private static class MergingList<V> extends AbstractList<V>
             this.keyComputer = keyComputer;
         }
 
+        Object writeReplace() throws ObjectStreamException
+        {
+            return new ArrayList<>( this );
+        }
+
         @Override
         public Iterator<V> iterator()
         {

maven-model/src/test/java/org/apache/maven/model/merge/ModelMergerTest.java

@@ -0,0 +1,49 @@
+package org.apache.maven.model.merge;
+
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+import java.io.ByteArrayOutputStream;
+import java.io.ObjectOutputStream;
+import java.util.ArrayList;
+
+import org.apache.maven.model.License;
+import org.apache.maven.model.Model;
+import org.junit.Test;
+
+public class ModelMergerTest {
+
+    @Test
+    public void testMergedModelSerialization() throws Exception {
+        Model target = new Model();
+        Model source = new Model();
+        target.setLicenses(new ArrayList<License>());
+        License lic1 = new License();
+        License lic2 = new License();
+        target.getLicenses().add(lic1);
+        source.setLicenses(new ArrayList<License>());
+        source.getLicenses().add(lic2);
+
+        new ModelMerger().mergeModel(target, source, false, null);
+
+        ByteArrayOutputStream baos = new ByteArrayOutputStream();
+        ObjectOutputStream oos = new ObjectOutputStream(baos);
+        oos.writeObject(target);
+    }
+}