If you are manually assigning IDs to entities instead of using @GeneratedValue , you may inadvertently try to reuse an ID that is already present in the table.
The "duplicate key" error is a vital signal that your application’s logic is at odds with your data's integrity rules. While frustrating, it serves as the final line of defense against corrupt data. By understanding the interplay between JPA’s entity lifecycle and the database’s constraint engine, developers can build more resilient, error-aware applications. If you are manually assigning IDs to entities
Wrap the save logic in a try-catch block specifically for DataIntegrityViolationException . This allows the application to return a user-friendly error message (e.g., "Username already taken") instead of a generic 500 Internal Server Error. Passing a detached entity to the save() method
Passing a detached entity to the save() method can sometimes lead JPA to treat it as a new record (attempting an INSERT ) rather than an update, causing a primary key collision. Both see that it doesn’t
In some cases, using a "query-then-update" approach or custom native queries with ON CONFLICT DO UPDATE (in PostgreSQL) can ensure the operation succeeds regardless of whether the record already exists. Conclusion
In a multi-threaded environment, two processes might check if a value (like an email address) exists at the same time. Both see that it doesn’t, both attempt to insert it, and the second one fails.
To handle these violations gracefully, developers typically employ one of three strategies: