如何使用Jackson JSON和JAVA反序列化多态LinkedList？

Question

I´m having some problems when i try to deserializing a Java class. My classes are :

Abstract Class A {
Long ID
String name 
...
}
Class B extends A{ 
String Type
String Value
}
Class C extends A{
List<A> typedList;
}

And my Serialization/Deseralization Methods:

public String serialize(T object) {
        String returnValue = "";

        try {
            returnValue = mapper.writeValueAsString(object);

        } catch (JsonGenerationException 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();
        }

        System.out.println(returnValue);
        return returnValue;
    }

public List<T> unserialize(String data, Class<?> clazz) {
        List<T> returnValue = new LinkedList<T>();
        //JavaType topMost = mapper.getTypeFactory().constructParametricType(MyWrapper.class, ActualClassRuntime.class); 
        try {

            returnValue = (List<T>) mapper.readValue(data,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 returnValue;
    }

The list inside the C class contain B and C elements. Serialization is done without problems but when i tried to deserialize C , Jackson can´t rebuild my typedList correctly. It uses LinkedHashMap instead of LinkedList and don´t use the correct types. Any ideias how to solve this ? Thanks in advance!

Answer 1

If you expect to get a List, you must pass a List type: here you are giving type of clazz but expect type of List<T> . This won't work, even if clazz is of type List . So you need to pass proper type. But for List (and Map) types, you further need to specify contents type: and type variable does not do. What works is something like:

List<MyType> list = mapper.readValue(jsonSource, new TypeReference<List<MyType>>() { });

where TypeReference must be used due to Java type erasure.

You can also construct structured types from components dynamically, if they are passed as arguments:

Class<?> elem = ...;
List<?> list = mapper.readValue(jsonSource,
  mapper.getTypeFactory().constructCollectionType(ArrayList.class, elem);

The reason you get LinkedHashMap is just because of missing type information: if JSON Object is encountered, and requested type is java.lang.Object (as would be the case if you just passed type List.class to bind to), this is the type that would be used.