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jsongrailsgroovyjackson

Serialized JSON with sorted keys, using Jackson

I'm trying to replace a custom JSON (de)serialization in a groovy/grails project with Jackson.

I'm having trouble getting Jackson to output a pretty-printed JSON with keys sorted in a simple 'natural' alphabetic order. I've tried this (and many variations):

mymap = [ ... ] // Some groovy map
def mapper = new ObjectMapper()
mapper.configure(SerializationConfig.Feature.SORT_PROPERTIES_ALPHABETICALLY, true)
def jsonstring = mapper.defaultPrettyPrintingWriter().writeValueAsString(mymap)

But Jackson stubbornly generates a JSON where the keys seem to be in a random order. I've tried changing the type of 'mymap' with a TreeMap, and in that case all keys are properly sorted as expected.

I'm wondering if there is a way to get the keys sorted without changing 'mymap' above to a TreeMap (and recursively all of its map values...).

SORT_PROPERTIES_ALPHABETICALLY seems to be intended to do precisely that, but it's not doing it for some reason. Would you know why that is? Anything I'm doing wrong above?

I've tried with Jackson 1.8.3, 1.8.8 and 1.9.5, same result (random keys).

Solution

  • As stated by @tim_yates, this doesn't work for map keys.

    You could use

    mapper.configure(SerializationConfig.Feature.ORDER_MAP_ENTRIES_BY_KEYS, true)

    With newer version ( >= 2.6.1) the API changed to:

    mapper.configure(SerializationFeature.ORDER_MAP_ENTRIES_BY_KEYS, true);