Java Jackson: deserialize complex polymorphic object model: JsonMappingException: Unexpected token (START_OBJECT), expected VALUE_STRING

Question

I have this tree of objects

A

B extends A

C extends B

D extends B

E extends C

F extends A and has one reference to A

A has the following annotation

@JsonTypeInfo(use=JsonTypeInfo.Id.CLASS,include=JsonTypeInfo.As.PROPERTY,property="@class")

If i try to deserialize a JSON array of objects that extends A, it throws the following error

org.codehaus.jackson.map.JsonMappingException: Unexpected token (START_OBJECT), expected VALUE_STRING: need JSON String that contains type id (for subtype of java.util.Collection)

The json string is generated by toString() method of a set and the set is parametric to type A where A is serialized in JSON with the following code:

ObjectMapper objectMapper=new ObjectMapper();
        objectMapper.enableDefaultTyping(ObjectMapper.DefaultTyping.NON_CONCRETE_AND_ARRAYS);
        String res="";
        try {
            res = objectMapper.writerWithDefaultPrettyPrinter().writeValueAsString(t);
        } catch (JsonGenerationException e) {
            e.printStackTrace();
        } catch (JsonMappingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return res;

The code to deserialize the json array (that is the set described above) is:

ObjectMapper mapper = new ObjectMapper(); 

        mapper.enableDefaultTyping(ObjectMapper.DefaultTyping.NON_CONCRETE_AND_ARRAYS);
        Collection<T> results=null;
        try {
            results =  mapper.readValue(json, TypeFactory.defaultInstance().constructParametricType(Collection.class, clazz ) );
        } catch (JsonParseException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (JsonMappingException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }  
        return results;

The json sample that it parses is like:

"[{
  "@class" : "pack1.pack2.MyClass",
  "id" : null,
  "f1" : "",
  "f2" : 0.9933817827,
  "f3" : 6.883261E-4,
  "f4" : 0.001375699,
  "f5" : {
    "@class" : "pack1.pack2.MyClass2",
    "id" : null,
    "f1" : "",
    "f2" : 0.0,
    "f3" : 0.0,
    "f4" : 0.0,
    "f5" : [ "java.util.HashSet", [ 0 ] ],
    "f6" : [ "java.util.HashSet", [ 2 ] ],
    "f7" : [ "java.util.ArrayList", [ "scelta", "brani", "buona" ] ],
    "f8" : [ null, "NOM", null ],
    "f9" : false
  },
  "f10" : [ "java.util.HashMap", {
    "2" : "ADJ"
  } ],
  "f11" : [ "java.util.HashSet", [ 0 ] ],
  "f12" : [ "java.util.HashSet", [ 2 ] ],
  "f13" : [ "java.util.ArrayList", [ "scelta", "brani", "buona" ] ],
  "featureIndicator" : false
}]"

Here the json string includes only some objects of my sample of java Set

Solution

I believe the problem is with the default typing. The start of your JSON is not generated as what Jackson expect with default typing. The begining of the JSON should be:

["java.util.HashSet", [{

and the end should have an extra closing bracket }]].

That's because you generate the JSON using the toString() method of your set. You should instead use the ObjectMapper, that's configured with default typing, like so:

res = objectMapper.writerWithDefaultPrettyPrinter().writeValueAsString(yourSet);