Why Replacing MyBatis with MyBatis‑Plus Can Break Your App—and How to Fix It

MyBatis to MyBatis‑Plus Migration

An old project uses MySQL 5.7, MyBatis 3.5.0 and mysql‑connector‑java 5.1.26. A new developer wants to replace MyBatis with MyBatis‑Plus to simplify code.

Problem

Running the demo test com.qsl.OrderTest#orderListAllTest after the replacement throws Conversion not supported for type java.time.LocalDateTime.

Root Cause

MyBatis 3.5.1 stopped handling LocalDateTime, LocalDate, and LocalTime conversions internally. The JDBC driver (mysql‑connector‑java 5.1.26) does not support these types, so the conversion fails.

Solution

Upgrade mysql‑connector‑java to version 5.1.37 (or later), which adds support for LocalDateTime, LocalDate, and LocalTime. After the upgrade the test runs successfully and returns correct query results.

Additional Insights

MyBatis 3.5.0 performed the conversion itself; 3.5.1 delegates to the JDBC driver, which explains the new exception. The change is referenced in MyBatis‑Plus issue #1114. Using other connection pools such as Druid may surface different symptoms.

Unrelated Bug‑Fix Story

A separate production incident involved a file‑generation validation bug where the logic incorrectly passed when only one master file existed. After correcting the validation to require all master files, a new exception appeared due to a null timestamp, which was resolved by the same driver upgrade.

Conclusion

Component upgrades can have far‑reaching effects. Avoid unnecessary changes; when changes are unavoidable, perform comprehensive testing to catch hidden incompatibilities.