Cannot parse map if the map contains 0

[MrPowerGamerBR]

val parsedMap = Constants.HOCON_MAPPER.readValue<Map<Int, String>>("""{ 1 = "abc" }""")
println(parsedMap)

Output: {1=abc}

val parsedMap = Constants.HOCON_MAPPER.readValue<Map<Int, String>>("""{ 0 = "abc" }""")
println(parsedMap)

Output:

Exception in thread "main" com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `java.util.LinkedHashMap` out of START_ARRAY token
 at [Source: UNKNOWN; line: -1, column: -1]
	at com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:63)
	at com.fasterxml.jackson.databind.DeserializationContext.reportInputMismatch(DeserializationContext.java:1343)
	at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1139)
	at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1093)
	at com.fasterxml.jackson.databind.deser.std.StdDeserializer._deserializeFromEmpty(StdDeserializer.java:600)
	at com.fasterxml.jackson.databind.deser.std.MapDeserializer.deserialize(MapDeserializer.java:360)
	at com.fasterxml.jackson.databind.deser.std.MapDeserializer.deserialize(MapDeserializer.java:29)
	at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4013)
	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3023)
	at net.perfectdreams.loritta.QuirkyStuffKt.main(QuirkyStuff.kt:63)
	at net.perfectdreams.loritta.QuirkyStuffKt.main(QuirkyStuff.kt)

This happens even if the Map is another type (Example: Map<String, String>)

This does NOT happen when parsing with lightbend/config:

val conf = ConfigFactory.parseString("""{ 0 = "abc" }""")
println(conf)

Output: Config(SimpleConfigObject({"0":"abc"}))

[MrPowerGamerBR]

I guess that this isn't possible to fix, this happens due to type loss, so the parser needs to do heuristics to figure out if it is a map or a array.

The problem happens when you try to use 0 on a map, because it matches the heuristics, it thinks it is a numerically indexed array... causing issues.

[MrPowerGamerBR]

It looks like @jclawson is inactive, so I guess this bug won't ever be fixed.

In the meantime, I made a custom deserializer with a hacky fix for this issue.

class FixedMapDeserializer : JsonDeserializer<Map<*, *>>() {
    override fun deserialize(p0: JsonParser, p1: DeserializationContext): Map<*, *> {
        val any = p0.readValueAsTree<TreeNode>()
        if (any.isArray) {
            val values = mutableMapOf<Any?, Any?>()
            repeat(any.size()) {
                values[it] = any.get(it)
             }
            return values
         } else {
             val values = mutableMapOf<Any?, Any?>()
             any.fieldNames().forEach {
                 values[it] = any.get(it)
             }
             return values
        }
    }
}

Then, on your HOCON Mapper

val module = SimpleModule()

module.setDeserializerModifier(object: BeanDeserializerModifier() {
      override fun modifyMapDeserializer(config: DeserializationConfig, type: MapType, beanDesc: BeanDescription, deserializer: JsonDeserializer<*>): JsonDeserializer<*> {
          return FixedMapDeserializer()
      }
})

mapper.registerModule(module)

The deserializer is a very simple & quick fix: If the JSON input is a array but Jackson expects a class, it converts the JSON to a Map (first array entry = 0 to mapValue on the map), if else, it just parses as a map.

I didn't test it on every possible combination, but on my quick tests: It works pretty well!