Use custom JSON marshaller for World

grails/hello/grails-app/conf/JsonBootStrap.groovy

@@ -0,0 +1,13 @@
+import org.codehaus.groovy.grails.web.converters.Converter;
+
+import grails.converters.JSON
+import hello.World
+
+
+class JsonBootStrap {
+    def init = { servletContext ->
+        JSON.registerObjectMarshaller(World, { World world, Converter converter ->
+            [id: world.id, randomNumber: world.randomNumber]
+        })
+    }
+}