the out.close() is redundant at this point. the stream will be closed automatically since Java 7 (not 9) new feature try-with-resource

src/main/java/org/alfresco/opencmis/CMISUtils.java

@@ -43,7 +43,6 @@ public class CMISUtils
         {
             out.writeObject(source);
             out.flush();
-            out.close();
 
             ObjectInputStream in = new ObjectInputStream(cos.getInputStream());
             target = (T) in.readObject();