Can you explain what a Specification is in the context of Spring Data JPA?

Specification in Spring Data JPA is a pattern that allows creating dynamic and reusable query criteria for the database. It is implemented through the Specification<T> interface, where T is the entity type.

The main idea is to describe the selection condition as a separate object, which can be combined with other specifications using logical operations (AND, OR, NOT). This is convenient for building complex queries without writing many methods in the repository.

Example of usage:

public class UserSpecification implements Specification<User> {
    private String username;

    public UserSpecification(String username) {
        this.username = username;
    }

    @Override
    public Predicate toPredicate(Root<User> root, CriteriaQuery<?> query, CriteriaBuilder cb) {
        return cb.equal(root.get("username"), username);
    }
}

// In repository
List<User> users = userRepository.findAll(new UserSpecification("john"));

Thus, Specification allows flexible query formation, improving code readability and maintainability.