Can there be a custom Jackson serializer for a standard class? (e.g. List<List<String>>)

Question

I'm trying to create a custom serializer for a class A's instance variable.

The problem is that the variable is of a "standard" built in type ( List<List<String>> )

I found out that you can in theory create a custom serializer for a type to be used ONLY within your class using mix-ins ; so in theory if I could create a custom serializer for List<List<String>> , I could mix-in it into class A that way.

But how do I create a custom serializer for List<List<String>> ?

Answer 1

I think it can be something like this. I don't know the logic you want to use, when serialize, so I wrote simple json array[][]

private static class ListListSerializer extends StdSerializer<List<List<String>>>{
    protected ListListSerializer(Class<List<List<String>>> t) {
        super(t);
    }
    protected ListListSerializer(){
        this(null);
    }


    @Override
    public void serialize(List<List<String>> lists, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
        jsonGenerator.writeStartArray();
        for (List<String> strings : lists) {
            jsonGenerator.writeStartArray();
            for (String string : strings) {
                jsonGenerator.writeString(string);
            }
            jsonGenerator.writeEndArray();
        }
        jsonGenerator.writeEndArray();
    }
}

As example without mixIn

private static class YourObject {

     private List<List<String>> myStrings = new ArrayList<>();

    public YourObject() {
        List<String> a = Arrays.asList("a","b","c");
        List<String> b = Arrays.asList("d","f","g");
        myStrings.add(a);
        myStrings.add(b);
}

    @JsonSerialize(using = ListListSerializer.class)
    public Object getMyStrings(){
       return myStrings;
    }
}


public static void main(String[] args) throws IOException {
    ObjectMapper mapper = new ObjectMapper();
    System.out.println(mapper.writeValueAsString(new YourObject()));
}

The output is

{"myStrings":[["a","b","c"],["d","f","g"]]}

Is that what you are trying to do?