java - why do we have to use @Modifying annotation for queries in Data Jpa

Question

for example I have a method in my CRUD interface which deletes a user from the database:

public interface CrudUserRepository extends JpaRepository<User, Integer> {

    @Transactional
    @Modifying
    @Query("DELETE FROM User u WHERE u.id=:id")
    int delete(@Param("id") int id, @Param("userId") int userId);
}

This method will work only with the annotation @Modifying. But what is the need for the annotation here? Why cant spring analyze the query and understand that it is a modifying query?

Answer 1

This will trigger the query annotated to the method as updating query instead of a selecting one. As the EntityManager might contain outdated entities after the execution of the modifying query, we automatically clear it (see JavaDoc of EntityManager.clear() for details). This will effectively drop all non-flushed changes still pending in the EntityManager. If you don't wish the EntityManager to be cleared automatically you can set @Modifying annotation's clearAutomatically attribute to false;

for further detail you can follow this link:-

http://docs.spring.io/spring-data/jpa/docs/1.3.4.RELEASE/reference/html/jpa.repositories.html