@Entity
@Table(name = "account")
public class Account {

  transient EntityManager entityManager;

  @Id
  @GeneratedValue
  private Long id;

  @Column(name = "username", nullable = false, unique = true)
  private String username;

  @Column(name = "password", nullable = false)
  private String password;

  ... getters and setters

  @Transactional
  public void persist() {
    if (this.entityManager == null) this.entityManager = entityManager();
    this.entityManager.persist(this);
  }

  @Transactional
  public Account merge() {
    if (this.entityManager == null) this.entityManager = entityManager();
    Account merged = this.entityManager.merge(this);
    this.entityManager.flush();
    return merged;
  }

@Configuration
@ComponentScan
@EnableAutoConfiguration
public class Application {

  public static void main(String[] args) throws Exception {
    SpringApplication.run(Application.class, args);
  }

}

spring.application.name: Test Application

spring.datasource.url: jdbc:mysql://localhost/test
spring.datasource.username=root
spring.datasource.password=
spring.datasource.driverClassName=com.mysql.jdbc.Driver
spring.jpa.hibernate.ddl-auto=update

@Repository
public interface AccountRepository extends PagingAndSortingRepository<Account, Long>

@Configuration
@ComponentScan
@EnableAutoConfiguration

@SpringBootApplication

@Entity
@Table(name = "account")
public class Account {

  @Id
  @GeneratedValue
  private Long id;

  @Column(name = "username", nullable = false, unique = true)
  private String username;

  @Column(name = "password", nullable = false)
  private String password;
....

public interface AccountRepository {
   void persist();
   Account merge();
}

@Repository
class AccountRepositoryJpaImpl impelemnts AccountRepository {

  @Autowired
  private EntityManager entityManager;

  ...
  @Transactional
  public void persist() {
    if (this.entityManager == null) this.entityManager = entityManager();
    this.entityManager.persist(this);
  }

  @Transactional
  public Account merge() {
    if (this.entityManager == null) this.entityManager = entityManager();
    Account merged = this.entityManager.merge(this);
    this.entityManager.flush();
    return merged;
  }
....
}

@SpringBootApplication
@EnableTransactionManagement
class ApplicationConfig {

    public static void main(String[] args) {
        SpringApplication.run(ApplicationConfig .class, args);
    }
}