Allow coercion of '[ ]' into empty/null String with `ACCEPT_EMPTY_ARRAY_AS_NULL_OBJECT`

[ulfandersson]

I'm using an API which uses [] as null, therefore I'm using DeserializationFeature.ACCEPT_EMPTY_ARRAY_AS_NULL_OBJECT.

Sometimes arrays can contain null as well, giving a structure like this example:

{
    "test" : [
        "hello",
        []
    ]
}

Jackson fails to parse this when trying to parse it to a List<String> or String[].

Stack trace for the List<String> case:

com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `java.lang.String` out of START_ARRAY token
 at [Source: (String)"{"test":["hello",[]]}"; line: 1, column: 18] (through reference chain: AcceptEmptyArrayAsNullObject$StringListTest["test"]->java.util.ArrayList[1])

	at com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:63)
	at com.fasterxml.jackson.databind.DeserializationContext.reportInputMismatch(DeserializationContext.java:1342)
	at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1138)
	at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1092)
	at com.fasterxml.jackson.databind.deser.std.StdDeserializer._parseString(StdDeserializer.java:569)
	at com.fasterxml.jackson.databind.deser.std.StringCollectionDeserializer.deserialize(StringCollectionDeserializer.java:203)
	at com.fasterxml.jackson.databind.deser.std.StringCollectionDeserializer.deserialize(StringCollectionDeserializer.java:169)
	at com.fasterxml.jackson.databind.deser.std.StringCollectionDeserializer.deserialize(StringCollectionDeserializer.java:21)
	at com.fasterxml.jackson.databind.deser.impl.MethodProperty.deserializeAndSet(MethodProperty.java:127)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.vanillaDeserialize(BeanDeserializer.java:288)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:151)
	at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4001)
	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2992)

Stack trace for the String[] case:

com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `java.lang.String` out of START_ARRAY token
 at [Source: (String)"{"test":["hello",[]]}"; line: 1, column: 18] (through reference chain: AcceptEmptyArrayAsNullObject$StringArrayTest["test"]->java.lang.Object[][1])

	at com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:63)
	at com.fasterxml.jackson.databind.DeserializationContext.reportInputMismatch(DeserializationContext.java:1342)
	at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1138)
	at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1092)
	at com.fasterxml.jackson.databind.deser.std.StdDeserializer._parseString(StdDeserializer.java:569)
	at com.fasterxml.jackson.databind.deser.std.StringArrayDeserializer.deserialize(StringArrayDeserializer.java:157)
	at com.fasterxml.jackson.databind.deser.std.StringArrayDeserializer.deserialize(StringArrayDeserializer.java:21)
	at com.fasterxml.jackson.databind.deser.impl.MethodProperty.deserializeAndSet(MethodProperty.java:127)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.vanillaDeserialize(BeanDeserializer.java:288)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:151)
	at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4001)
	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2992)

However, the following works when parsed to List<Integer> or Integer[]:

{
    "test" : [
        1,
        []
    ]
}

Jackson version used: 2.9.4.

Test case for the above:
JacksonStringArrayTest.zip

[cowtowncoder]

This may be down to naming problem -- it should probably have been named ACCEPT_EMPTY_ARRAY_AS_NULL_POJO, not _OBJECT -- but empty Array JSON will only be considered equivalent to null for POJOs. And not for scalar types such as Strings.

So, at this point this is a limitation of implementation, and there are no plans for changing this particular limitation.

I would be interested in figuring out a way forward, and part of this would be understanding why and how is empty array being used as marker. I was under impression that some languages/platforms (PHP?) used it as a marker for structured values, but did not think it would be used in place of scalar values like numbers, booleans or Strings.

[ulfandersson]

Thank you for your answer!
What confused me is that it is supported for Integer objects (but perhaps this is only because the handling is similar as for POJOs)? The only case I am really interested in (except for POJOs) is for strings.

The source of the data is a couchbase database document. It's probably due to a null/empty string is somewhere converted to an empty array when storing the document.

Maybe ACCEPT_EMPTY_ARRAY_AS_NULL_STRING could be added? :)

[cowtowncoder]

Thinking about this more, I think that coercion into null Java String makes sense also. Considering this for 3.1.

That is, enabling DeserializationFeature.ACCEPT_EMPTY_ARRAY_AS_NULL_OBJECT should allow coercion of empty JSON array ([ ]) into null for java.lang.String valued properties; no need to add a new feature.

[cowtowncoder]

Quick note: "CoercionConfig" system, added in 2.12.0 via #2113, adds properly configurable mechanism for doing this, see tests in CoerceEmptyArray.java.

[cowtowncoder]

Ok, it looks like what is specifically requested is already working: and may have been working since 2.12.0, but it is difficult to pinpoint the exact point.

But deserialization into String values within List<String> was not working so PR will address that edge case and thus this issue will be marked as fixed in 3.1.0.