You need to provide the provider list while creating the webClient object in your rest client. You can use the below code to resolve your issue:
final String url = "http://localhost:10227/someService";
final String uri = "/manageXyz/fetchAllDataTypes";
final String mediaType = "application/json";
Object response = null;
List<Object> providers = new ArrayList<Object>();
providers.add( new JacksonJaxbJsonProvider() );
WebClient client = WebClient.create(url, providers);
client = client.accept(mediaType).type(mediaType).path(uri);
response = (List<Object>)client.post(oemUser, List.class);
If you are using maven, you also need to provide below required jars to resolve maven dependency in your project:
<dependency>
<groupId>org.codehaus.jackson</groupId>
<artifactId>jackson-jaxrs</artifactId>
<version>1.5.4</version>
</dependency>
<dependency>
<groupId>org.codehaus.jackson</groupId>
<artifactId>jackson-core-asl</artifactId>
<version>1.9.13</version>
</dependency>
<dependency>
<groupId>org.codehaus.jackson</groupId>
<artifactId>jackson-xc</artifactId>
<version>1.9.13</version>
</dependency>