Json deep merge array processing strategy
Dzmitry Sankouski
{ "level1": { "value": "a", "anotherValue": "z", "array1": [ { "value": "b", "anotherValue": "z" } ] }}
Json 2:
{ "level1": { "value": "c", "array1": [ { "value": "d" } ] }}After merge, I want to get:
{ "level1": { "value": "c", "anotherValue": "z", "array1": [ { "value": "d", "anotherValue": "z" } ] }}{ "level1": { "value": "c", "anotherValue": "z", "array1": [ { "value": "b", "anotherValue": "z" }, { "value": "d", "anotherValue": "z" } ] }}How do I achieve arrays in json being merged, not concatenated?
Tatu Saloranta
and nothing else. This because while there are many possible ways
users might
want to handle merging of two sequence values, it is surprisingly
difficult to figure out declarative way that covers enough without
being very complicated.
So: instead of trying to figure out a complex solution that might not
cover all cases, I went with simple, predictable solution, which does
not cover all cases.
In future it could be possible to extend `@JsonMerge` to perhaps allow
custom handler of some sort that would allow custom logic, but this
does not yet exist.
-+ Tatu +-
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Saurin Joshi
private static void registerDeserializer(ObjectMapper mapper) {
SimpleModule module = new SimpleModule();
module.setDeserializerModifier(new BeanDeserializerModifier() {
@Override
public JsonDeserializer<?> modifyCollectionDeserializer(DeserializationConfig config, CollectionType type, BeanDescription beanDesc, JsonDeserializer<?> deserializer) {
if (List.class.isAssignableFrom(beanDesc.getBeanClass()) && deserializer instanceof CollectionDeserializer) {
CollectionDeserializer collectionDeserializer = (CollectionDeserializer)deserializer;
if (LIST_CLASSES.stream().anyMatch(clazz -> clazz.isAssignableFrom(type.getContentType().getRawClass()))) {
return new CustomCollectionDeserializer(type, null, null, collectionDeserializer.getValueInstantiator());
}
}
return deserializer;
}});
mapper.registerModule(module);
}
public static <T> T mergeObjects(T obj1, String obj2) throws JsonMappingException, JsonProcessingException {
ObjectReader objectReader = mapper.readerForUpdating(obj1);
return objectReader.readValue(obj2);
}
Tatu Saloranta
CollectionDeserializer. Getting that right requires a lot of work.
There is no way to change definition of merging for Arrays currently
and it seems unlikely there will be much support
given that there seem to be multiple things users may want to and no
obvious simple configuration options to cover options
(or at least I have not been able to think of such).
I think most users would read content as JsonNode and implement
merging on those instances with relatively
simple traversal: this allows you to use exact rules you want.
You can then convert (mapper.treeToValue()) resulting tree into actual
value type you want (and vice versa).
-+ Tatu +-
Saurin Joshi
public class CustomCollectionDeserializer extends CollectionDeserializer {
public CustomCollectionDeserializer(JavaType collectionType, JsonDeserializer<Object> valueDeser, TypeDeserializer valueTypeDeser, ValueInstantiator valueInstantiator) {
super(collectionType, valueDeser, valueTypeDeser, valueInstantiator);
}
protected CustomCollectionDeserializer(JavaType collectionType, JsonDeserializer<Object> valueDeser, TypeDeserializer valueTypeDeser, ValueInstantiator valueInstantiator, JsonDeserializer<Object> delegateDeser, NullValueProvider nuller, Boolean unwrapSingle) {
super(collectionType, valueDeser, valueTypeDeser, valueInstantiator, delegateDeser, nuller, unwrapSingle);
}
@Override
protected CollectionDeserializer withResolved(JsonDeserializer<?> dd, JsonDeserializer<?> vd, TypeDeserializer vtd,
NullValueProvider nuller, Boolean unwrapSingle) {
return new CustomCollectionDeserializer(_containerType, (JsonDeserializer<Object>) vd, vtd, _valueInstantiator, (JsonDeserializer<Object>) dd, nuller, unwrapSingle);
}
@Override
public Collection<Object> deserialize(JsonParser p, DeserializationContext ctxt, Collection<Object> intoValue)
throws IOException {
return super.deserialize(p, ctxt, intoValue);
}
}
Tatu Saloranta
>
> Thanks Tatu for prompt response and suggestion.
>
> I could solve the 1st issue by overriding withResolved() method and got CustomCollectionDeserializer working as described below. However, I see the complications it will bring in for maintaining this class. I will instead go with your suggested approach to do manipulation directly on JSON node and deserialize the merged object.
(and inheritance more generally) is a bit fragile,
and over time CollectionDeserializer constructors have and will be
changing when new configuration settings need
to be passed. Old versions are kept as per SemVer, where possible, but
there will be less testing to try to ensure that
things actually fully work as expected with new default/placeholder
values -- tests typically do not cover @Deprecated
methods, fields or constructors
This may be fine when you control the Jackson version in use and can
work around possible complications wrt upgrades.
But I like to mention it as a general suggestion.
Good luck!
-+ Tatu +-
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