posting a JSON using Spring RestTemplate - json

Question

I've converted a Java Object to json String and I am trying to post it to a rest webservice using RestTemplate and I am always getting a 500 error. The following are the details:
User createJSONUser = createUser(); // provides me the java object
//converting the javaobject to json string successfully using jackson in the next two lines
ObjectMapper mapper = new ObjectMapper();
String jsonString = mapper.writeValueAsString(createJSONUser);
//creating the headers
HttpHeaders requestHeaders = new HttpHeaders();
requestHeaders.setContentType(MediaType.APPLICATION_JSON);
requestHeaders.setAccept(Collections.singletonList(new MediaType("application","json")));
//setting the message converter
restTemplate.getMessageConverters().add(new StringHttpMessageConverter());
//posting the json using resttemplate
ResponseEntity<String> response = restTemplate.exchange(sandBoxURL, HttpMethod.POST, new HttpEntity<String>(jsonString, requestHeaders), String.class);
I am obtaining the resttemplate as a bean which is defined in my context-xml as follows:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:util="http://www.springframework.org/schema/util"
xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.1.xsd
http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-3.1.xsd">
<bean id="responseMessageConverter" class = "com.uhg.iam.esso.rest.client.converters.CustomResponseConverter">
<property name="supportedMediaTypes" value="application/xml"/>
</bean>
<bean id="customErrorHandlerForEssoRestServer" class="com.uhg.iam.esso.rest.client.EssoRestClientErrorHandler"/>
<bean id="stringHttpMessageConverter" class="org.springframework.http.converter.StringHttpMessageConverter"/>
<bean id="jaxb2Marshaller" class="org.springframework.oxm.jaxb.Jaxb2Marshaller">
<property name="supportJaxbElementClass" value="true"/>
<property name="classesToBeBound">
<list>
<value>esso.schemas.core._1.Response</value>
</list>
</property>
</bean>
<bean id="marshallingHttpMessageConverter" class="org.springframework.http.converter.xml.MarshallingHttpMessageConverter">
<property name="marshaller" ref="jaxb2Marshaller"/>
<property name="unmarshaller" ref="jaxb2Marshaller"/>
</bean>
<bean id="jsonHttpMessageConverter" class="org.springframework.http.converter.json.MappingJacksonHttpMessageConverter">
<property name="prefixJson" value="false"/>
<property name="supportedMediaTypes" value="application/json"/>
</bean>
<bean id="restTemplateBean" class="org.springframework.web.client.RestTemplate">
<property name="errorHandler" ref="customErrorHandlerForEssoRestServer"/>
<property name="messageConverters">
<list>
<!-- <ref bean="marshallingHttpMessageConverter"/>
<ref bean="responseMessageConverter"/> -->
<ref bean="jsonHttpMessageConverter"/>
<ref bean="stringHttpMessageConverter"/>
</list>
</property>
</bean>
<bean id="essoRestClientDelegate" class="com.uhg.iam.esso.rest.client.EssoRestClientJSONDelegate">
<property name="restTemplate" ref="restTemplateBean"/>
</bean>
</beans>
Can someone please point out where I am making a mistake?