java - How to keep fields sequence in Gson serialization

Question

Seems like Gson.toJson(Object object) generates JSON code with randomly spread fields of the object. Is there way to fix fields order somehow?

public class Foo {
    public String bar;
    public String baz;
    public Foo( String bar, String baz ) {
        this.bar = bar;
        this.baz = baz;
    }
}
Gson gson = new Gson();
String jsonRequest = gson.toJson(new Foo("bar","baz"));

The string jsonRequest can be:

• { "bar":"bar", "baz":"baz" } (correct)
• { "baz":"baz", "bar":"bar" } (wrong sequence)

Answer 1

You'd need to create a custom JSON serializer.

E.g.

public class FooJsonSerializer implements JsonSerializer<Foo> {

    @Override
    public JsonElement serialize(Foo foo, Type type, JsonSerializationContext context) {
        JsonObject object = new JsonObject();
        object.add("bar", context.serialize(foo.getBar());
        object.add("baz", context.serialize(foo.getBaz());
        // ...
        return object;
    }

}

and use it as follows:

Gson gson = new GsonBuilder().registerTypeAdapter(Foo.class, new FooJsonSerializer()).create();
String json = gson.toJson(foo);
// ...

This maintains the order as you've specified in the serializer.

See also:

• Gson User Guide - Custom serializers and deserializers