Jörg Schaible wrote:
> Hi Kay,
>
> Kay Masslow wrote:
>
>> Hi all,
>>
>> The FAQ recommends "implement a custom mapper to ignore unknown fields
>> automatically (see acceptance test
>> customMapperTest.testCanBeUsedToOmitUnexpectedElements())". However
>> using this code seems to cause trouble with handling of collections.
>
> You get trouble with *implicit* collections/arrays.
>
>> Am I doing something wrong here?
>
> Nothing, but the custom mapper interferes with the handling of the
> implicit collection. The problem is, that those elements ("Item" in your
> case) are no fields of Root. The reflection converter will now ask the
> mapper if it should deserialize a field named "Item" and the custom mapper
> says no now, because that field is not declared anywhere in the class
> hierarchy. If the converter is told, not to deserialize this item, it
> stops .. and will not look for a possible implicit element.
>
>> Using XStream 1.4.4 Please see JUnit
>> 4 code below: testFailsWithUnknownElements() notNull-Assertion fails
>> because the List is not initialized.
>
> This is another caveat of implicit collections (covered by FAQ): If no
> element was found, the collection itself is not initialized i.e. the field
> keeping the collection instance is null. Because of the "implicit"
> declaration of the collection, XStream can no longer separate between an
> empty collection and null, because the collection has no representation in
> the XML at all. You may use a readResolve method to initialize it with an
> empty collection (also FAQ).
>
> It is possible to implement the custom mapper for implicit collections
> though, but it costs a bit more runtime:
>
> new MapperWrapper(next) {
> public boolean shouldSerializeMember(Class definedIn, String fieldName)
> {
> if (getImplicitCollectionDefForFieldName(definedIn, fieldName) !=
> null)
> {
> return true;
> }
> return definedIn != Object.class ?
> super.shouldSerializeMember(definedIn, fieldName) : false;
> }
> }
Sorry, this modification does not work either ... :-/
- Jörg
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