Recursively extracting JSON field values in Groovy - recursion

Question

I need to implement a method that will scan a string of JSON for a particular targetField and either return the value of that field (if it exists), or null (if it doesn't):
// Ex: extractFieldValue(/{ "fizz" : "buzz" }/, 'fizz') => 'buzz'
// Ex: extractFieldValue(/{ "fizz" : "buzz" }/, 'foo') => null
String extractFieldValue(String json, String targetField) {
// ...
}
This solution has to be recursive and work at any nesting-level in the (hierarchical) JSON string. Also it needs to work for JSON arrays as well.
My best attempt so far:
String extractFieldValue(String json, String targetField) {
def slurper = new JsonSlurper()
def jsonMap = slurper.parseText(json)
jsonMap."${targetField}"
}
This only works on top-level (non-nested) JSON fields. I asked the Google Gods how to use JsonSlurper recursively, but couldn't find anything useful. Any ideas here?

Answer 1

Given this input string in a variable called json:
{
"a":"a",
"b":{"f":"f", "g":"g"},
"c":"c",
"d":"d",
"e":[{"h":"h"}, {"i":{"j":"j"}}],
}
This script:
import groovy.json.JsonSlurper
def mapOrCollection (def it) {
it instanceof Map || it instanceof Collection
}
def findDeep(def tree, String key) {
switch (tree) {
case Map: return tree.findResult { k, v ->
mapOrCollection(v)
? findDeep(v, key)
: k == key
? v
: null
}
case Collection: return tree.findResult { e ->
mapOrCollection(e)
? findDeep(e, key)
: null
}
default: return null
}
}
('a'..'k').each { key ->
def found = findDeep(new JsonSlurper().parseText(json), key)
println "${key}: ${found}"
}
Gives these results:
a: a
b: null
c: c
d: d
e: null
f: f
g: g
h: h
i: null
j: j
k: null