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https://jira.codehaus.org/browse/XSTR-690?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=292029#comment-292029
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Joerg Schaible commented on XSTR-690:
-------------------------------------
Please use latest XStream version. Without a real unit test, all I can say is,
that the code in AbstractReferenceUnmarshaller looks different now and null
values are cached.
> Broken deserialization of singleton references using custom readResolve
> method returning null
> ---------------------------------------------------------------------------------------------
>
> Key: XSTR-690
> URL: https://jira.codehaus.org/browse/XSTR-690
> Project: XStream
> Issue Type: Bug
> Components: Converters
> Affects Versions: 1.3.1
> Reporter: Christian Danner
> Assignee: Joerg Schaible
> Attachments: corruptSerializationTest.xml
>
>
> We have our own abstract Enum base class implementation for which a custom
> readResolve method has been implemented that ensures that singleton instances
> are returned upon deserialization (with our without XStream) that looks like
> this:
> public Object readResolve() throws java.io.ObjectStreamException {
> try {
> Class clazz = getClassWithFields();
> Field f = clazz.getField(_fieldName);
> return f.get(null);
> } catch (NoSuchFieldException ex) {
> return null;
> }
> }
> Additionally according readObject and writeObject implementations ensure that
> the name of the Enum field is (de)serialized:
> private void readObject(java.io.ObjectInputStream in) throws
> java.io.IOException {
> try {
> _fieldName = (String) in.readObject();
> } catch (ClassNotFoundException ex) {
> throw new java.io.IOException(ex.getMessage(), ex);
> }
> }
> Now instead of throwing an Exception in case a value is being read that is
> not yet available (in case a newer file is being read by an application using
> an older Enum implementation where the field is not available) we want to
> initialize such missing Enum values with null - hence null is returned in
> case the field with the deserialized name cannot be found in readResolve.
> XStream does currently not support this because the
> AbstractReferenceUnmarshaller does not cache null values (line 63):
> result = super.convert(parent, type, converter);
> if (currentReferenceKey != null && result != null) {
> values.put(currentReferenceKey, result);
> }
> What's making things worse is that the AbstractReflectionConverter (line 165)
> calls readResolve after the Object that has been created by default
> deserialization has already been cached:
> public Object unmarshal(final HierarchicalStreamReader reader, final
> UnmarshallingContext context) {
> Object result = instantiateNewInstance(reader, context);
> // after this call the Object returned by readObject has been cached:
> result = doUnmarshal(result, reader, context);
> return serializationMethodInvoker.callReadResolve(result);
> }
> So in case multiple references to the Enum singleton are stored in a file the
> behaviour is as follows:
> * Upon first deserialization null is returned (as desired) because
> readResolve is called
> * For all successive references the cached Object deserialized using
> readObject() is returned
> The attached .xml file contains an ArrayList with multiple Objects
> referencing such a (non-existing) Enum value.
> While this may seem like a very specific use case it makes me wonder if
> missing support for null return values in readResolve implementations is
> really desired.
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