Jackson Custom Deserializer - pick up only keys of map and return as list.

jackson-custom-serializer.java

    @JsonDeserialize(contentUsing = Foo.class)
    private static class Permissions extends HashMap<String, List<String>> {

    }

    private static class Foo extends StdDeserializer<Collection<String>> {
        protected Foo() {
            super(Collection.class);
        }

        @Override
        public Collection<String> deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException {
            switch (jp.getCurrentToken()) {
                case START_OBJECT: {
                    JsonDeserializer<Object> deserializer = ctxt.findRootValueDeserializer(ctxt.constructType(Map.class));
                    Map<String, Object> map = (Map<String, Object>) deserializer.deserialize(jp, ctxt);
                    return map.keySet();
                }
                case START_ARRAY: {
                    JsonDeserializer<Object> deserializer = ctxt.findRootValueDeserializer(ctxt.constructType(List.class));
                    List<String> list = (List<String>) deserializer.deserialize(jp, ctxt);
                    return list;
                }
                default:
                    throw ctxt.mappingException("Expected to see an object or array, found " + jp.getCurrentToken());
            }
        }
    }