How to maintain JSON's order in Groovy's JsonSlurper?

Question

I am reading a simple JSON....

{"A":0,"B":0,"C":2,"D":0,"F":5}

into a map using JsonSlurper in Groovy...

Map gradeDistributon = jsonSlurper.parseText(jsonString)

But when iterating over this map with a closure..

gradeDistributon.each{ entry ->   
     println "From map got key ${entry.key}"

I am seeing the keys are not in the order they were in the original JSON, for example 'C' comes first. I think this is because Map does not maintain insertion order in Java. Is there a way I can keep the order of the original JSON?

If it means reading the JSON in a different way (instead of into a Map with JsonSlurper) then I am fine with that if you can show me how.

Answer 1

You can set JVM system property jdk.map.althashing.threshold to make JsonSlurper to use a LinkedHashMap instead of TreeMap as the internal Map implementation, e.g. -Djdk.map.althashing.threshold=512.

The reason is in source code of groovy.json.internal.LazyMap used by JsonSlurper.

private static final String JDK_MAP_ALTHASHING_SYSPROP = System.getProperty("jdk.map.althashing.threshold");

private void buildIfNeeded() {  
    if (map == null) {
        /** added to avoid hash collision attack. */
        if (Sys.is1_7OrLater() && JDK_MAP_ALTHASHING_SYSPROP != null) {
            map = new LinkedHashMap<String, Object>(size, 0.01f);
        } else {
            map = new TreeMap<String, Object>();
        }
    }
}

Please note this solution should be used as a hack as it depends on Groovy's internal implementation details. So this behavior may change in future version of Groovy.

See my blog post for details.