HashMap to List JSON output

HashMapSerializer.java

public class HashMapSerializer extends JsonSerializer<Map<Long, Object>> {

  ObjectMapper mapper = new ObjectMapper();

  @Override
  public void serialize(Map<Long, Object> userMap, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
    jsonGenerator.writeString(mapper.writeValueAsString(userMap.values()));
  }
}

POJO.java

...
@JsonSerialize(using = HashSerializer.class)
private final Map<Long, Product> products;
...