FastJson Date Formatting and Circular Reference Issues: A Debugging Journey

The author, a senior architect, shares a personal experience with FastJson where a global date format (yyyy-MM-dd HH:mmss) failed to apply locally in a Linux test environment, causing timestamps to retain time components.

After local tests passed, the issue surfaced in production due to differing FastJson versions; the test environment used version 1.2.53 while the development environment used a newer version, leading to the bug.

Investigation of the FastJson source revealed the problem was tracked in several GitHub issues (e.g., #1868) and fixed in version 1.2.72, highlighting gaps in test coverage and reliance on core contributors.

The article also discusses FastJson’s circular reference detection, showing how identical objects in different lists can trigger unwanted $ref entries, and recommends disabling this feature via SerializerFeature.DisableCircularReferenceDetect when appropriate.

Additional FastJson features such as WriteMapNullValue and WriteNullListAsEmpty are examined, emphasizing that default enabled behaviors can cause unexpected production failures and should be configurable by users.

Finally, the author reflects on the maintenance challenges of FastJson, the migration to FastJson2, and the importance of consulting open‑source issue trackers to avoid similar pitfalls.