如何找出JPA中列的长度？

@Entity
public class MyEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(length = 50)
    private String name;

    // 其他属性和方法
}

EntityManagerFactory emf = Persistence.createEntityManagerFactory("my-unit");
EntityManager em = emf.createEntityManager();

em.getTransaction().begin();

MyEntity myEntity = em.find(MyEntity.class, 1L);
int length = myEntity.getName().length();

em.getTransaction().commit();
em.close();
emf.close();