public static String extractField(JSONObject jsonObject, String fieldName) {

        String value;
        if (jsonObject != null) {
            value = jsonObject.getString(fieldName);
        } else {
            value = "";
        }
        return value;
    }
for json convert

@param contents
@return
test/test-integration/src/test/java/com/alipay/sofa/rpc/server/tracer/util/TracerChecker.java