public Item withJSON(String attrName, String json) {
        checkInvalidAttribute(attrName, json);
        attributes.put(attrName,
            valueConformer.transform(Jackson.fromJsonString(json, Object.class)));
        return this;
    }
Sets the value of the specified attribute in the current item to the
given JSON document in the form of a string.
aws-java-sdk-dynamodb/src/main/java/com/amazonaws/services/dynamodbv2/document/Item.java