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.
Use a repository method like existsByEmail(String email) before attempting a save. While this doesn't solve high-concurrency race conditions, it eliminates the majority of "honest" mistakes.
In databases like PostgreSQL, the sequence used to generate IDs can sometimes fall behind the actual maximum ID in the table (often after manual data imports), leading the application to propose IDs that are already taken. Strategies for Resolution If you are manually assigning IDs to entities
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.
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. In databases like PostgreSQL, the sequence used to
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.
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
To handle these violations gracefully, developers typically employ one of three strategies: