我能救一个孩子 Asset 对象，但加载时，我总是 com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBMappingException: could not instantiate class Asset ，下面是我当前的实现。

private DynamoDBMapper mapper;
    public void saveAsset(Asset asset) {
        mapper.save(mediaAsset);  // OK
    }
    public Asset getAsset(int id) {
        return mapper.load(Asset.class, id);  // DynamoDBMappingException
    }
    @Data
    @AllArgsConstructor
    @DynamoDBTable(tableName = "asset")
    public static class Asset {
        @DynamoDBHashKey(attributeName="Id")
        private Integer id;
        @DynamoDBAttribute(attributeName="Mz")
        private Mz mz;
    }
    @Data
    @AllArgsConstructor
    @NoArgsConstructor
    @DynamoDBDocument
    public static class Mz {
        @DynamoDBAttribute(attributeName = "Codec")
        private String codec;
    }