Spring RestTemplate and generic types



Using Spring's RestTemplate is sometimes more convenient than using JAX-RS implementations. Usually you have Jackson2 on classpath, so JSON to object deserialization is working out of box. But what about parameterized collections like List<T>?

For example you want to retrieve List of some DTOs from REST API using method getForObject:

		List<Person> persons = restTemplate.getForObject("http://rest.com/person", List.class);

You are expecting a list of persons, but instead you get a List of LinkedHashMaps that are representing the persons' properties. Well, Spring has a solution for that, but you cannot use convenience methods like getForObject in this case. The trick is to use more generic method exchange and define a ParameterizedTypeReference as a result type:

		ResponseEntity<List<Person>> response = exchange("http://rest.com/person", HttpMethod.GET, HttpEntity.EMPTY, new ParameterizedTypeReference<List<Person>>() {});

ParameterizedTypeReference is an abstract class, but it does not require you to implement any of its methods. This is a bit weird construct, but it works. It will return a ResponseEntity which you can use to retrieve the object (body) if everything is OK.

It's been like this in Spring for a while and I currently don't know about any other nicer solution, unfortunately.