Resteasy converting List to LinkedHashMap

Question

I am using resteasy to send key/value pairs in between some restful web-services.

to post a list of these pairs i use this snippet

List<Pair> pairs = new ArrayList<>();
pairs.add(new Pair("name", "Arnold"));
pairs.add(new Pair("age", "20"));

ResteasyClient resteasyClient = getClient();
ResteasyWebTarget target = resteasyClient.target(targetURI);
Invocation.Builder request = target.request();
request.post(Entity.entity(entity, MediaType.APPLICATION_JSON_TYPE));

Pair is just an unannotated Class with

public String key,
public String value 
default constructor 
(key,value) constructor

The target resource an receive this via:

 @POST
 @Path("/metadata")
 @Consumes(MediaType.APPLICATION_JSON)
 public Response postMetadata(List<Pair> properties) {...

the jax-rs resource can read the List correctly.

Now the other way is the Problem:

The Retrieval Resource is defined as:

@GET
@Path("/getUrl.../{id}")
@Produces(MediaType.APPLICATION_JSON)
public Response getMetadata(@PathParam("id") String id) {
data_from_hibernate = em.find(id);
...
List<Pair> properties = new ArrayList<>();
//fill List from data
return Response.ok().entity(properties).build();

Client side is:

Response response = request.get("http://getUrl.../"+id);
List<Pair> properties = response.readEntity(List.class);

logger.info("got properties != null? " +(properties!=null));
logger.info("size: "+properties.size());
logger.info("[0]" + properties.get(0));
logger.info("type: "+properties.get(0).getClass().getName());

The List is not null;

size is correct;

get(0) does output in a strange format: [0]{"name": "value"};

this is followed by the strange format's reason: an Exception tells me that:

java.util.LinkedHashMap cannot be cast to Pair

How is properties a LinkedHashMap at this point, or rather: a List of LinkedHashMaps?

Answer 1

Without type information (when you use readEntity), Jackson (the underlying deserializer) will convert arbitrary JSON objects to LinkedHashMap.

When using readEntity, you provide generic type information by using GenericType.

List<Pair> properties = response.readEntity(new GenericType<List<Pair>>(){});