Hibernate @GeneratedValue VS自定义分配

Question

// Auto generation
public abstract class BaseEntity implements Serializable {
  @Id
  @GeneratedValue(generator = "uuid")
  @GenericGenerator(name = "uuid", strategy = "uuid2")
  @Column(name = "id", unique = true)
  private UUID id;

}

// Custom assignment
public abstract class BaseEntity implements Serializable {
  @Id
  @Column(name = "id", unique = true)
  private UUID id = UUID.randomUUID();

}

public User extends BaseEntity{
   @OneToOne
   private Address address;

   //... Getter/Setter
}

public Address extends BaseEntity{}

My question is if I want to use UUID as primary key, do we need to use the generator to assign the UUID and what is the difference between two method ?

For the Auto generation, the UUID only generated AFTER it saved into database. The following

  User user = new User();
  User newUser = userRepository.save(user);
  newUser.setAddress(new Address());
  User newUserWithAddress = userRepository.save(user);
  return newUserWithAddress;

For the custom assignment, the UUID can generated BEFORE it saved into database. The code is cleaner. Is that any potential problems if I use custom assignment ?

  User user = new User();
  userRepository.save(user);
  user.setAddress(new Address());
  userRepository.save(user);
  return user;

Answer 1

Assigning a random identifier in the constructor has disadvantages:

• it takes time, most of the time, for nothing: every time Hibernate loads an entity from the database, the constructor is invoked, a new UUID is generated, then Hibernate assigns the actual ID of the saved entity to the field
• Hibernate, (and your own code), unless you also use optimistic locking, has no way to know if a given entity is detached or transient. Whereas in the case of IDs generated by Hibernate, if the ID is null, the entity is transient, otherwise the entity is detached.