Why Upgrading to MyBatis‑Plus Breaks LocalDateTime Mapping and How to Fix It

A senior architect shares a real‑world case where a new developer upgraded an old project from MyBatis 3.5.0 to MyBatis‑Plus 3.1.1, keeping MySQL 5.7.36 and mysql‑connector‑java 5.1.26 unchanged. After the change, running com.qsl.OrderTest#orderListAllTest threw the exception "Conversion not supported for type java.time.LocalDateTime".

Background

The legacy system used MySQL 5.7.36, MyBatis 3.5.0, and mysql‑connector‑java 5.1.26. The newcomer felt MyBatis required too much boilerplate and switched to MyBatis‑Plus, but left all other component versions as‑is.

Root Cause Analysis

Stack traces showed the failure originated from the JDBC driver. MyBatis 3.5.1 stopped handling conversions for LocalDateTime, LocalDate, and LocalTime, delegating the work to the JDBC driver. The driver version 5.1.26 does not support these Java 8 time types, leading to the conversion error.

MySQL‑connector‑java started supporting these types from version 5.1.37 . Upgrading the connector to 5.1.37 (or later 5.1.42) resolves the exception.

Fix Implementation

After upgrading the connector, the same test runs without errors and returns correct query results.

INSERT INTO `tbl_order` VALUES (3, 'asdfgh', NULL, '2024-02-21 20:01:31.111', '2024-02-21 20:02:56.764');

Running com.qsl.OrderTest#orderListAllTest now succeeds.

Additional Insights

The issue mirrors MyBatis‑Plus issue #1114, which also points to the same conversion problem. The problem is independent of the connection pool (HikariCP vs Druid), but different pools may surface slightly different stack traces.

The article also describes a separate bug in a file‑generation validation routine where the logic incorrectly passed if *any* main file existed instead of requiring *all* main files. After correcting the validation, a hidden data inconsistency (a file name stored where an ID was expected) caused another failure, illustrating how a single change can expose latent bugs.

Conclusion

Component upgrades, especially in legacy codebases, can have far‑reaching side effects. Before changing libraries, verify compatibility of dependent components, and perform thorough testing to avoid cascading production incidents.