public JsonToken nextToken(JsonParser parser) {
        try {
            return parser.nextValue();
        } catch (IOException e) {
            // Jackson is not able to recover from structural parse errors
            state = END_OF_FEED;
            throw new IllegalArgumentException(e);
        }
    }
Returns the next document operation, or null if we have reached the end
document/src/main/java/com/yahoo/document/json/JsonReader.java