IllegalArgumentException during SessionFactory initialization after failed upgrade

Direct answers

Close all active sessions before rebuilding the SessionFactory. Rebuilding while sessions linger is unsafe. Those sessions were created against the previous metamodel and hold JDBC connections and first-level state that will never reconcile with the new mappings. Stop accepting work, close every open Session, then rebuild.

SessionFactory.getCache().evictAll() is not sufficient on its own. It clears only the regions Hibernate knows about. With an external provider such as Ehcache or Infinispan, persistent or clustered cache can survive a JVM restart. A provider-level clear or cache-region version bump is required. Treat evictAll as necessary but not sufficient.

Recommended sequence for this case:

1. Halt inbound traffic and stop accepting new requests.
2. Close all active Session instances and rollback uncommitted transactions.
3. Shut down the old SessionFactory if it is still running.
4. Reconcile the database schema with a versioned migration tool such as Flyway or Liquibase, or manually validated DDL. Do not use hbm2ddl.auto=update in production.
5. Clear the second-level cache at the provider level, for example CacheManager.clearAll for Ehcache or restart the Infinispan cluster.
6. Fix the configuration or mapping issue that caused the IllegalArgumentException.
7. Rebuild the SessionFactory.
8. Smoke test by loading representative previously cached entities and confirm no ClassCastException and correct data.

Likely explanation vs confirmed facts

Likely explanation: An IllegalArgumentException during bootstrap most commonly means the new Hibernate version rejects a setting or mapping: a removed or renamed dialect class, a property name that no longer exists, or an annotation or mapping construct whose semantics changed across versions. Schema drift and second-level cache corruption can cause ClassCastException on subsequent reads after an upgrade.

Confirmed facts: The exception message and its caused-by chain name the offending setting or entity. A failed SessionFactory build does not invalidate an already running factory. hbm2ddl.auto=update does not perform reliable migrations and is unsafe in production. Cache eviction is mandatory when entity mappings or the Hibernate version change the serialized form.

Verification

• Capture the complete exception chain from the failed bootstrap.
• Compare effective configuration against the target Hibernate version documentation for renamed or removed settings.
• Run migration tool validate or dry-run against a production-like schema before restart.
• After rebuild, load cached entities and verify no ClassCastException.

Missing diagnostic detail

Provide the full IllegalArgumentException stack trace including the caused-by chain. The exact offending class or property determines whether a dialect change, property rename, or mapping adjustment is required and changes the fix in step 6.