Code:
AuthenticateUser user = new AuthenticateUser(Credentials, Header);

String Stringifiedjson = new Gson().toJson(user);
Log.d("SpringAndroid", Stringifiedjson);

HttpHeaders requestHeaders = new HttpHeaders();
requestHeaders.setContentType(new MediaType("application", "json"));

HttpEntity < AuthenticateUser > requestEntity = new HttpEntity < AuthenticateUser > (user, requestHeaders);

RestTemplate restTemplate = new RestTemplate(true);
restTemplate.getMessageConverters().add(new MappingJacksonHttpMessageConverter());
restTemplate.getMessageConverters().add(new StringHttpMessageConverter());

final String url = getResources().getString(R.string.ProductionURL) + getResources().getString(R.string.AuthenticateUser);
Log.d(this.getClass().getName(), url);

try {
    ResponseEntity < String > responseEntity = restTemplate.exchange(url, HttpMethod.POST, requestEntity, String.class);
    result = responseEntity.getBody();
    Log.d("Result", result);
} catch (HttpClientErrorException e) {
    Log.d(Constants.HTTP_CLIENT_ERROR, e.getLocalizedMessage());
} catch (Exception e) {
    Log.d("It comes to here", e.getLocalizedMessage(), e);
}
StackTrace

Code:
10-23 20:37:59.245: D/It comes to here(3330): Could not write request: no suitable HttpMessageConverter found for request type [com.sample.transaction.AuthenticateUser] and content type [application/json]
10-23 20:37:59.245: D/It comes to here(3330): org.springframework.web.client.RestClientException: Could not write request: no suitable HttpMessageConverter found for request type [com.sample.transaction.AuthenticateUser] and content type [application/json]
10-23 20:37:59.245: D/It comes to here(3330): 	at org.springframework.web.client.RestTemplate$HttpEntityRequestCallback.doWithRequest(RestTemplate.java:632)
10-23 20:37:59.245: D/It comes to here(3330): 	at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:473)
10-23 20:37:59.245: D/It comes to here(3330): 	at org.springframework.web.client.RestTemplate.execute(RestTemplate.java:438)
10-23 20:37:59.245: D/It comes to here(3330): 	at org.springframework.web.client.RestTemplate.exchange(RestTemplate.java:414)
10-23 20:37:59.245: D/It comes to here(3330): 	at com.sample.home.Trial$TrialAsyncTask.doInBackground(Trial.java:168)
10-23 20:37:59.245: D/It comes to here(3330): 	at com.sample.home.Trial$TrialAsyncTask.doInBackground(Trial.java:108)
10-23 20:37:59.245: D/It comes to here(3330): 	at android.os.AsyncTask$2.call(AsyncTask.java:287)
10-23 20:37:59.245: D/It comes to here(3330): 	at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:305)
10-23 20:37:59.245: D/It comes to here(3330): 	at java.util.concurrent.FutureTask.run(FutureTask.java:137)
10-23 20:37:59.245: D/It comes to here(3330): 	at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230)
10-23 20:37:59.245: D/It comes to here(3330): 	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1076)
10-23 20:37:59.245: D/It comes to here(3330): 	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:569)
10-23 20:37:59.245: D/It comes to here(3330): 	at java.lang.Thread.run(Thread.java:856)

1. What is that i am missing here, i am getting this error. I did a lot of google on this from morning and could not reach to a solution.
2. How can is end Stringified JSONObject to my Service without using MappingJacksonHttpMessageConverter.