FULL PRODUCT VERSION :
java version "9.0.4"
Java(TM) SE Runtime Environment (build 9.0.4+11)
Java HotSpot(TM) 64-Bit Server VM (build 9.0.4+11, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Linux prod-wgba-005.oslo-no0030.xxx.xxx 3.10.0-693.21.1.el7.x86_64 #1 SMP Wed Mar 7 19:03:37 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

Microsoft Windows [Version 6.1.76.01]

EXTRA RELEVANT SYSTEM CONFIGURATION :
Seems to be independent of operating system.


A DESCRIPTION OF THE PROBLEM :
Seems to be similar to the 1.4.2 bug described here
https://bugs.java.com/bugdatabase/view_bug.do?bug_id=6208166#

1) Create an object graph (for example object tree) with cyclic dependencies:
- Node class with an Long field and a hashCode() method depending on this field.
- A parent reference.
- A set of children (represented by a HashSet) and a parent reference.
2)
- Create some Node instances and chain them together so that they represent a tree (with cyclic dependencies). 
3) 
- Serialize and de-serialize the object graph from 2)

This works with JDK8 but fails with JDK9.



REGRESSION.  Last worked in version 8u172

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the attached code with JDK9.


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
No output.

ACTUAL -
Exception in thread "main" java.lang.RuntimeException: java.lang.NullPointerException
	at wg.crewmanager.scrapbook.ReproduceHashSetSerializationBug.deSerialize(ReproduceHashSetSerializationBug.java:127)
	at wg.crewmanager.scrapbook.ReproduceHashSetSerializationBug.test(ReproduceHashSetSerializationBug.java:89)
	at wg.crewmanager.scrapbook.ReproduceHashSetSerializationBug.main(ReproduceHashSetSerializationBug.java:73)
Caused by: java.lang.NullPointerException
	at wg.crewmanager.scrapbook.ReproduceHashSetSerializationBug$Superclass.hashCode(ReproduceHashSetSerializationBug.java:38)
	at java.base/java.util.HashMap.hash(HashMap.java:339)
	at java.base/java.util.HashMap.put(HashMap.java:612)
	at java.base/java.util.HashSet.readObject(HashSet.java:342)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:564)
	at java.base/java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:1160)
	at java.base/java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:2207)
	at java.base/java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:2078)
	at java.base/java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1585)
	at java.base/java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:2346)
	at java.base/java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:2240)
	at java.base/java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:2078)
	at java.base/java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1585)
	at java.base/java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:2346)
	at java.base/java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:2240)
	at java.base/java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:2078)
	at java.base/java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1585)
	at java.base/java.io.ObjectInputStream.readObject(ObjectInputStream.java:422)
	at java.base/java.util.ArrayList.readObject(ArrayList.java:825)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:564)
	at java.base/java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:1160)
	at java.base/java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:2207)
	at java.base/java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:2078)
	at java.base/java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1585)
	at java.base/java.io.ObjectInputStream.readObject(ObjectInputStream.java:422)
	at wg.crewmanager.scrapbook.ReproduceHashSetSerializationBug.deSerialize(ReproduceHashSetSerializationBug.java:123)
	... 2 more


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
package wg.crewmanager.scrapbook;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Random;
import java.util.Set;

/**
 * Test serialization of {@link HashSet}.
 *
 */
public class ReproduceHashSetSerializationBug
{

   private static final Random RANDOM = new Random();

   public static class Superclass implements Serializable
   {

      private final Long id;

      public Superclass()
      {
         this.id = RANDOM.nextLong();
      }

      public int hashCode()
      {
         return this.id.hashCode();
      }

      /*
      private void readObject(java.io.ObjectInputStream in)
         throws IOException, ClassNotFoundException
      {
         in.defaultReadObject();
      }
      */

   }


   public static class Node extends Superclass implements Serializable
   {

      private final String name;
      private final Node parent;
      private final Set<Node> children = new HashSet<>();

      public Node(String name, Node parent)
      {
         this.name = name;
         this.parent = parent;
         if (parent != null)
         {
            parent.children.add(this);
         }
      }

   }

   public static void main(String[] args)
   {
      new ReproduceHashSetSerializationBug().test();
   }

   @SuppressWarnings("unchecked")
   private void test()
   {

      Node a = new Node("a", null);
      Node b = new Node("b", a);

      List<Node> elements = new ArrayList<>();
      elements.add(b);
      elements.add(a);

      byte[] bytes = serialize((Serializable) elements);

      List<Node> deserialized = (List<Node>) deSerialize(bytes);
   }

   /**
    * Serializes the given object into a byte array.
    *
    * @param serializable not null
    * @return a byte array, not null
    */
   private static byte[] serialize(Serializable serializable)
   {
      try
      {
         ByteArrayOutputStream out = new ByteArrayOutputStream();
         ObjectOutputStream objectOutputStream = new ObjectOutputStream(out);
         objectOutputStream.writeObject(serializable);
         return out.toByteArray();
      }
      catch (Exception e)
      {
         throw new RuntimeException(e);
      }
   }

   /**
    * De-serializes the given byte array into an object.
    *
    * @param bytes the byte array to de-serializes, not null
    * @return the de-serialized object
    */
   private static Serializable deSerialize(byte[] bytes)
   {
      try
      {
         return (Serializable) new ObjectInputStream(new ByteArrayInputStream(bytes)).readObject();
      }
      catch (Exception e)
      {
         throw new RuntimeException(e);
      }
   }

}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Explicit readObject in super class (remove comments and run again).

Not sure why this makes it work.