Duplicate entry 'admin@devinhyderabad.com' for key 'uk_email'

Q: Check for existence before saving

When to use: Use this for simple checks where you want to return a clean 400 error to the user.. Query the database to see if the unique field already exists. If it does, throw a custom exception instead of letting it hit the DB constraint.

Q: Catch DataIntegrityViolationException globally

When to use: Use this as a fallback to prevent 500 errors if the existence check is bypassed (race condition).. Create a global exception handler that catches the constraint violation and returns a 409 Conflict status.

Q: Add @UniqueConstraint to your Entity

When to use: Use this if the database doesn't actually have the unique constraint set up yet.. Explicitly declare the unique constraint on the Entity class so Hibernate generates the proper DB schema.

🔴 The Error You're Seeing

Confirm this matches your console output. If it does, you're in the right place.

ERROR LOG

2026-02-14 15:05:55.800 ERROR 8842 --- [nio-8080-exec-7] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed: org.springframework.dao.DataIntegrityViolationException: could not execute statement; SQL [n/a]] with root cause

java.sql.SQLIntegrityConstraintViolationException: Duplicate entry 'admin@devinhyderabad.com' for key 'uk_email'

⚡ Quick Fix Works 80% of the time

Check if the record exists before saving, or catch the DataIntegrityViolationException globally.

if (userRepository.existsByEmail(email)) {
    throw new ResourceNotFoundException("Email already in use");
}
userRepository.save(user);

🛠️ Solutions (3 Ways to Fix)

Solution 1✓ Most common cause

Check for existence before saving

👉 Use this for simple checks where you want to return a clean 400 error to the user.

Query the database to see if the unique field already exists. If it does, throw a custom exception instead of letting it hit the DB constraint.

@Service
public class UserService {
    public User createUser(UserRequest req) {
        if (repo.existsByEmail(req.getEmail())) {
            throw new EmailAlreadyExistsException("Email is taken");
        }
        return repo.save(new User(req));
    }
}

Solution 2

Catch DataIntegrityViolationException globally

👉 Use this as a fallback to prevent 500 errors if the existence check is bypassed (race condition).

Create a global exception handler that catches the constraint violation and returns a 409 Conflict status.

@RestControllerAdvice
public class GlobalExceptionHandler {
    @ExceptionHandler(DataIntegrityViolationException.class)
    public ResponseEntity<String> handleDuplicateEntry(DataIntegrityViolationException ex) {
        return ResponseEntity.status(HttpStatus.CONFLICT).body("Resource already exists");
    }
}

Solution 3

Add @UniqueConstraint to your Entity

👉 Use this if the database doesn't actually have the unique constraint set up yet.

Explicitly declare the unique constraint on the Entity class so Hibernate generates the proper DB schema.

@Entity
@Table(name = "users", uniqueConstraints = {
    @UniqueConstraint(columnNames = "email")
})
public class User { ... }

📋 Version Notes

Spring Boot 2.x

Throws DataIntegrityViolationException.

Spring Boot 3.x

Throws DataIntegrityViolationException, often wrapping jakarta.persistence.PersistenceException.

🧠 Why this Happens

Tap to expand the deep technical explanation

Your database table has a unique constraint (like a primary key or a unique index on an email column). You tried to INSERT or UPDATE a row with a value that is already used by another row, violating the uniqueness rule.

The HITEC City Parking Spot Analogy:

It is like two people trying to claim the exact same phone number. The phone company (database) rejects the second person because a number can only belong to one account.

🛡️ How to Prevent This Next Time

Implement existence checks in your service layer for all unique fields, and rely on DB constraints as a final safety net.