Groovy JsonBuilder: object properties being serialized in random order - json

Question

I'm currently having a problem with the groovy JsonBuilder: the properties of the objects I am trying to serialize are coming out in seemingly random order.
Here's the objects' class:
class Game {
String title
String gameImg2
String description
}
And this is the code I've been using:
def game = new Game(title: "a game", gameImg2: "an image", description: "desc")
def json = new JsonBuilder(game)
From this I would expect the output to be:
{"title":"a game","gameImg2":"an image", "description":"desc"}
but instead, I'm getting:
{"gameImg2":"gameImg","title":"hello","description":"desc"}.
From looking at the JsonBuilder example code, it seems that order should be maintained, and indeed, it looks like the toJson method iterates over object.properties, which is a LinkedHashMap. I would have thought this would go through the properties in the order they are declared in the class.
My best guess is that this is something to do with my initialisation of the game object - could using Map syntax to assign properties somehow, but again, this seems to create a LinkedHashMap which should preserve order.
Does anyone have an inkling of what I could have done wrong or incorrectly assumed here? Any leads would be a big help!

Answer 1

The problem is that you are serializing an Object to JSON. The object has declared fields that are translated to Java.
In Java it is not (easily) possible to traverse fields in the order of declaration and the JsonBuilder code definitely makes no attempt to allow this behavior.
Looking at the source code for the JsonBuilder one can see that it uses a class called JsonOutput.groovy to serialize the Object you are passing it, like so:
static String toJson(object) {
if (object == null) {
"null"
} else if (object instanceof Collection ||
object.class.isArray() ||
object instanceof Iterator ||
object instanceof Enumeration) {
"[" + object.collect { toJson(it) }.join(',') + "]"
} else if (object instanceof Enum) {
'"' + object.name() + '"'
} else {
def properties = object.properties
properties.remove('class')
properties.remove('declaringClass')
properties.remove('metaClass')
toJson(properties)
}
}
As you can see the code calls the properies member of the object which returns the object members, the order of the fields depends on the JVM and not the order of declaration.
If you want to maintain order you will need to either pass in a map representing the object or build the JSON object in order manually.
The other alternative is writing your own custom builder, which is much more complicated...