Json tp AVRO with org.apache.avro.data

Hi,

I read the exchange on this topic on avro-user mailing list archives: September 2012 (http://mail-archives.apache.org/mod_mbox/avro-user/201209.mbox/%3C445E44A6-345F-42FC-B340-48AF3804A93D [ at ] braindump.ms%3E)

I am sorry if that suppose to be obvious, but I still don't understand how to use the org.apache.avro.data.Json, and Json.Writer classes.
 
Particularly, how to map JSON doc. fields to AVRO schema fields.

Suppose I have a simple JSON doc:

"id" : "abc123",
"width" : 1.3
"details":

"id" : "XXX#11"
"len" : 20

Now, suppose I want AVRO schema something like:

{"namespace" : "exam",
  "type": "record",
 "name": "avro-meme",
 "fields" : [ {"name" : "id", "type ": "string"},
                   {"name" : "width", "type": "double"},
   {"name" : "details",
                        "type" : {
"type" : "record",
 "name" : "abra",
                                 "fields" : [  
{"name" : "id", "type" : "string"},
                                         {"name" : "len", "type" : "int" }
  ]
  } 
                    }
       ]

So how I specify json to avro mapping in org.apache.avro.data.Json , what are the rules of the game?
Or, if it is not how you suppose to use org.apache.avro.data.Json, then how?

Thank you
 Alexey