`DeserializationFeature.UNWRAP_SINGLE_VALUE_ARRAYS` only works for POJOs, Maps

[nottmey]

Documentation of DeserializationFeature.UNWRAP_SINGLE_VALUE_ARRAYS only mentiones exceptional behavior for more than one value in the array ("If more than one value is found in the array, a JsonMappingException is thrown."). But trying to parse { "value" : [] } with value as String produces the following Stacktrace: (Parsing as null might be expected instead)

Exception in thread "main" com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.lang.String out of END_ARRAY token
 at [Source: { "value" : [] }; line: 1, column: 14] (through reference chain: app.Data["value"])
    at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:148)
    at com.fasterxml.jackson.databind.DeserializationContext.mappingException(DeserializationContext.java:835)
    at com.fasterxml.jackson.databind.deser.std.StdDeserializer._parseString(StdDeserializer.java:809)
    at com.fasterxml.jackson.databind.deser.std.StringDeserializer.deserialize(StringDeserializer.java:35)
    at com.fasterxml.jackson.databind.deser.std.StringDeserializer.deserialize(StringDeserializer.java:12)
    at com.fasterxml.jackson.databind.deser.SettableBeanProperty.deserialize(SettableBeanProperty.java:523)
    at com.fasterxml.jackson.databind.deser.impl.MethodProperty.deserializeAndSet(MethodProperty.java:95)
    at com.fasterxml.jackson.databind.deser.impl.BeanPropertyMap.findDeserializeAndSet(BeanPropertyMap.java:285)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.vanillaDeserialize(BeanDeserializer.java:248)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:136)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3562)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2578)
    at app.MainKt.main(Main.kt:23)

data class Data(var value : String? = null) // valid example container in Kotlin

This shouldn't be problematic when using DeserializationFeature.ACCEPT_EMPTY_ARRAY_AS_NULL_OBJECT, but it does not take precedence over DeserializationFeature.UNWRAP_SINGLE_VALUE_ARRAYS (bug?) and still gives me the error from above!

Are there any workarounds? I still need to map single element arrays, that sometimes appear to be empty. (I am using version 2.5.1, tested also 2.6.0: same behavior)

[nottmey]

BeanDeserializerBase.java (L1239)

if (ctxt.isEnabled(DeserializationFeature.UNWRAP_SINGLE_VALUE_ARRAYS)) {
    JsonToken t = p.nextToken();
    if (t == JsonToken.END_ARRAY && ctxt.isEnabled(DeserializationFeature.ACCEPT_EMPTY_ARRAY_AS_NULL_OBJECT)) {
        return null;
    }
    final Object value = deserialize(p, ctxt);
    if (p.nextToken() != JsonToken.END_ARRAY) {
        throw ctxt.wrongTokenException(p, JsonToken.END_ARRAY, 
                "Attempted to unwrap single value array for single '" + _valueClass.getName() + "' value but there was more than a single value in the array");
    }
    return value;
}

seems to have the right implementation in method deserializeFromArray(...),
but StdDeserializer (L833)

if (t == JsonToken.START_ARRAY && ctxt.isEnabled(DeserializationFeature.UNWRAP_SINGLE_VALUE_ARRAYS)) {
    p.nextToken();
    // CHECK MISSING HERE
    final String parsed = _parseString(p, ctxt);
    if (p.nextToken() != JsonToken.END_ARRAY) {
        throw ctxt.wrongTokenException(p, JsonToken.END_ARRAY, 
                "Attempted to unwrap single value array for single 'String' value but there was more than a single value in the array");
    }            
    return parsed;            
}

and StringDeserializer (L38)

if (curr == JsonToken.START_ARRAY && ctxt.isEnabled(DeserializationFeature.UNWRAP_SINGLE_VALUE_ARRAYS)) {
    jp.nextToken();
    // CHECK MISSING HERE
    final String parsed = _parseString(jp, ctxt);
    if (jp.nextToken() != JsonToken.END_ARRAY) {
        throw ctxt.wrongTokenException(jp, JsonToken.END_ARRAY, 
                "Attempted to unwrap single value array for single 'String' value but there was more than a single value in the array");
    }            
    return parsed;            
}

do not check for

if (token == JsonToken.END_ARRAY && ctxt.isEnabled(DeserializationFeature.ACCEPT_EMPTY_ARRAY_AS_NULL_OBJECT)) {
  return null;
}

in a very similar situation.

This occurs probably for every other primitive type.

[nottmey]

Problem does occur with target types:

• Byte
• Short
• Integer
• Long
• Float
• Double
• Character
• java.util.Date
• Enums
• (Java primitives? - can't check them now)

Problem does NOT occur with target types:

• Custom Pojos
• Collections

( Does not work for other modules like e.g. org.joda.time.DateTime, because they don't support UNWRAP_SINGLE_VALUE_ARRAYS anyway. Such specific cases are solvable with a custom deserializer. )

[cowtowncoder]

Thank you for thorough investigation. Yes, sounds like ACCEPT_EMPTY_ARRAY_AS_NULL_OBJECT should have precedence.

As to whether empty array should result in null or exception is an interesting question: my first inclination was that exception is appropriate, but then again I can see why null would make sense.

[nottmey]

Thank you for the confirmation.

1. Therefore the discussed behavior (null or exception) should be configurable, as long as the user is willing to take the side effect of ACCEPT_EMPTY_ARRAY_AS_NULL_OBJECT (regardless of target type, empty arrays will be null).
2. Additionally I would recommend to have an additional version of UNWRAP_SINGLE_VALUE_ARRAYS where null is the default behavior (maybe call it UNWRAP_OPTIONAL_SINGLE_VALUE_ARRAYS). This would make it explicitly configurable without side effect.

Since both versions are currently bugged (1.) or not implemented (2.), my current workaround is to intercept and preprocess the JSON file, replacing every occurence of [] with null (which is certainly not an ideal solution).

[cowtowncoder]

Ok: just to make sure, OBJECT in there should follow roughly guidelines of ACCEPT_EMPTY_STRING_AS_OBJECT, although since there is no problem with null for String values, it should probably work there too.