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.print…

Accepted Answer

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

Solution

Problem