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

Probably the cause is from the missing annotations in the subclass that are subclassed too or missing annotation @JsonSubTypes in the superclass. Confirmation? I will try to correct — Giovanni Bitliner, Apr 05 '13 at 15:29
The annotations are "inherited", so that's not your problem. What I'm concerned about is the persence of `java.util.*` in your json... Ha! that's the default typing, I think. Have you tried without that? — Pascal Gélinas, Apr 05 '13 at 15:32
I tried, now it throws different errors. I have added annotation to all types, but the error message is "Could not resolve type id 'F' into a subtype of [simple type, class E]" — Giovanni Bitliner, Apr 05 '13 at 16:15

Pascal Gélinas · Answer 1 · 2013-04-05T15:45:47.787

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);

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

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