spring.datasource.url=jdbc:mysql://foo.bar.com:12345
spring.datasource.username=user
spring.datasource.password=password
spring.datasource.driver-class-name=com.mysql.jdbc.Driver

@Service
public class ProjectServiceImpl implements ProjectService {

    @Autowired
    ProjectRepository projectRepository;

    @Override
    @Transactional(isolation = Isolation.SERIALIZABLE)
    public Project save(Project project) {
        // there might be some additional logic here, besides the action on repository
        return projectRepository.save(project);
    }

}

@Repository
public interface ProjectRepository extends CrudRepository<Project, Long> {}

"exception": "org.springframework.transaction.InvalidIsolationLevelException",
"message": "JtaTransactionManager does not support custom isolation levels by default - switch 'allowCustomIsolationLevels' to 'true'",

transactionManager.setAllowCustomIsolationLevels(true);