Fastjson DateFormat Priority and Circular Reference Issues: Debugging and Solutions

The author, a senior architect, shares experiences with Fastjson issues encountered in a Java backend project, focusing on date format priority problems and circular reference handling.

DateFormat Priority Issue

When configuring a global date format yyyy-MM-dd HH:mmss and annotating a bean with @JSONField(format="yyyy-MM-dd") , the change worked locally but not in the Linux test environment, resulting in timestamps still containing time components.

Investigation involved simulating Linux via VM options, remote debugging, and inspecting JSONSerializer and its writeWithFormat method, eventually discovering that the test environment used an older Fastjson version ( 1.2.53 ) where the bug (issue #1868) was fixed only in version 1.2.72 .

Circular Reference ($ref) Issue

A sample method returns two lists, young and children , where both reference the same Person objects. Fastjson serializes the second reference as {"$ref":"$.result.young[1]"} , indicating circular reference detection.

The problem can be avoided by disabling the feature with SerializerFeature.DisableCircularReferenceDetect , though the author notes that the data does not actually contain a true circular reference.

Other Observations

Fastjson’s default features such as WriteMapNullValue and WriteNullListAsEmpty can affect API contracts, and users should have control over enabling them.

The author concludes that debugging Fastjson issues often starts with searching GitHub issues, and recommends upgrading to Fastjson 2 or newer versions for better maintenance.