如果UUID是许多人的答案，为什么我们不使用业务层的工厂方法来创建实体并在创建时分配主键呢?

例如:

@ManagedBean
public class MyCarFacade {
  public Car createCar(){
    Car car = new Car();
    em.persist(car);
    return car;
  }
}

通过这种方式，我们可以从持久化提供程序获得实体的默认主键，并且我们的hashCode()和equals()函数可以依赖于它。

我们还可以声明Car的构造函数受保护，然后在业务方法中使用反射来访问它们。这样，开发人员就不会打算用new实例化Car，而是通过factory方法。

来说,如何?

我同意Andrew的回答。我们在应用程序中做同样的事情，但不是将uuid存储为VARCHAR/CHAR，而是将其分割为两个长值。请参阅UUID.getLeastSignificantBits()和UUID.getMostSignificantBits()。

还有一件事需要考虑，对UUID. randomuuid()的调用非常慢，因此您可能希望只在需要时才惰性地生成UUID，例如在持久化期间或调用equals()/hashCode()期间

@MappedSuperclass
public abstract class AbstractJpaEntity extends AbstractMutable implements Identifiable, Modifiable {

    private static final long   serialVersionUID    = 1L;

    @Version
    @Column(name = "version", nullable = false)
    private int                 version             = 0;

    @Column(name = "uuid_least_sig_bits")
    private long                uuidLeastSigBits    = 0;

    @Column(name = "uuid_most_sig_bits")
    private long                uuidMostSigBits     = 0;

    private transient int       hashCode            = 0;

    public AbstractJpaEntity() {
        //
    }

    public abstract Integer getId();

    public abstract void setId(final Integer id);

    public boolean isPersisted() {
        return getId() != null;
    }

    public int getVersion() {
        return version;
    }

    //calling UUID.randomUUID() is pretty expensive, 
    //so this is to lazily initialize uuid bits.
    private void initUUID() {
        final UUID uuid = UUID.randomUUID();
        uuidLeastSigBits = uuid.getLeastSignificantBits();
        uuidMostSigBits = uuid.getMostSignificantBits();
    }

    public long getUuidLeastSigBits() {
        //its safe to assume uuidMostSigBits of a valid UUID is never zero
        if (uuidMostSigBits == 0) {
            initUUID();
        }
        return uuidLeastSigBits;
    }

    public long getUuidMostSigBits() {
        //its safe to assume uuidMostSigBits of a valid UUID is never zero
        if (uuidMostSigBits == 0) {
            initUUID();
        }
        return uuidMostSigBits;
    }

    public UUID getUuid() {
        return new UUID(getUuidMostSigBits(), getUuidLeastSigBits());
    }

    @Override
    public int hashCode() {
        if (hashCode == 0) {
            hashCode = (int) (getUuidMostSigBits() >> 32 ^ getUuidMostSigBits() ^ getUuidLeastSigBits() >> 32 ^ getUuidLeastSigBits());
        }
        return hashCode;
    }

    @Override
    public boolean equals(final Object obj) {
        if (obj == null) {
            return false;
        }
        if (!(obj instanceof AbstractJpaEntity)) {
            return false;
        }
        //UUID guarantees a pretty good uniqueness factor across distributed systems, so we can safely
        //dismiss getClass().equals(obj.getClass()) here since the chance of two different objects (even 
        //if they have different types) having the same UUID is astronomical
        final AbstractJpaEntity entity = (AbstractJpaEntity) obj;
        return getUuidMostSigBits() == entity.getUuidMostSigBits() && getUuidLeastSigBits() == entity.getUuidLeastSigBits();
    }

    @PrePersist
    public void prePersist() {
        // make sure the uuid is set before persisting
        getUuidLeastSigBits();
    }

}

如果UUID是许多人的答案，为什么我们不使用业务层的工厂方法来创建实体并在创建时分配主键呢?

例如:

@ManagedBean
public class MyCarFacade {
  public Car createCar(){
    Car car = new Car();
    em.persist(car);
    return car;
  }
}

通过这种方式，我们可以从持久化提供程序获得实体的默认主键，并且我们的hashCode()和equals()函数可以依赖于它。

我们还可以声明Car的构造函数受保护，然后在业务方法中使用反射来访问它们。这样，开发人员就不会打算用new实例化Car，而是通过factory方法。

来说,如何?

我试着自己回答这个问题，直到我读了这篇文章，尤其是画了一个，我才完全满意找到的解决方案。我喜欢他懒创建UUID和最佳存储它的方式。

但我想增加更多的灵活性，即惰性创建UUID仅当hashCode()/equals()被访问时，第一次持久化实体与每个解决方案的优点:

Equals()表示“对象指向相同的逻辑实体” 尽可能使用数据库ID，因为为什么我要做两次工作(性能问题) 防止在尚未持久的实体上访问hashCode()/equals()时出现问题，并在它确实被持久后保持相同的行为

我真的很感激对我的混合解决方案的反馈如下

public class MyEntity { @Id() @Column(name = "ID", length = 20, nullable = false, unique = true) @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id = null; @Transient private UUID uuid = null; @Column(name = "UUID_MOST", nullable = true, unique = false, updatable = false) private Long uuidMostSignificantBits = null; @Column(name = "UUID_LEAST", nullable = true, unique = false, updatable = false) private Long uuidLeastSignificantBits = null; @Override public final int hashCode() { return this.getUuid().hashCode(); } @Override public final boolean equals(Object toBeCompared) { if(this == toBeCompared) { return true; } if(toBeCompared == null) { return false; } if(!this.getClass().isInstance(toBeCompared)) { return false; } return this.getUuid().equals(((MyEntity)toBeCompared).getUuid()); } public final UUID getUuid() { // UUID already accessed on this physical object if(this.uuid != null) { return this.uuid; } // UUID one day generated on this entity before it was persisted if(this.uuidMostSignificantBits != null) { this.uuid = new UUID(this.uuidMostSignificantBits, this.uuidLeastSignificantBits); // UUID never generated on this entity before it was persisted } else if(this.getId() != null) { this.uuid = new UUID(this.getId(), this.getId()); // UUID never accessed on this not yet persisted entity } else { this.setUuid(UUID.randomUUID()); } return this.uuid; } private void setUuid(UUID uuid) { if(uuid == null) { return; } // For the one hypothetical case where generated UUID could colude with UUID build from IDs if(uuid.getMostSignificantBits() == uuid.getLeastSignificantBits()) { throw new Exception("UUID: " + this.getUuid() + " format is only for internal use"); } this.uuidMostSignificantBits = uuid.getMostSignificantBits(); this.uuidLeastSignificantBits = uuid.getLeastSignificantBits(); this.uuid = uuid; }

我们通常在实体中有两个id:

仅用于持久化层(以便持久化提供程序和数据库能够找出对象之间的关系)。 是为了我们的应用程序需要(特别是equals()和hashCode())

来看看:

@Entity
public class User {

    @Id
    private int id;  // Persistence ID
    private UUID uuid; // Business ID

    // assuming all fields are subject to change
    // If we forbid users change their email or screenName we can use these
    // fields for business ID instead, but generally that's not the case
    private String screenName;
    private String email;

    // I don't put UUID generation in constructor for performance reasons. 
    // I call setUuid() when I create a new entity
    public User() {
    }

    // This method is only called when a brand new entity is added to 
    // persistence context - I add it as a safety net only but it might work 
    // for you. In some cases (say, when I add this entity to some set before 
    // calling em.persist()) setting a UUID might be too late. If I get a log 
    // output it means that I forgot to call setUuid() somewhere.
    @PrePersist
    public void ensureUuid() {
        if (getUuid() == null) {
            log.warn(format("User's UUID wasn't set on time. " 
                + "uuid: %s, name: %s, email: %s",
                getUuid(), getScreenName(), getEmail()));
            setUuid(UUID.randomUUID());
        }
    }

    // equals() and hashCode() rely on non-changing data only. Thus we 
    // guarantee that no matter how field values are changed we won't 
    // lose our entity in hash-based Sets.
    @Override
    public int hashCode() {
        return getUuid().hashCode();
    }

    // Note that I don't use direct field access inside my entity classes and
    // call getters instead. That's because Persistence provider (PP) might
    // want to load entity data lazily. And I don't use 
    //    this.getClass() == other.getClass() 
    // for the same reason. In order to support laziness PP might need to wrap
    // my entity object in some kind of proxy, i.e. subclassing it.
    @Override
    public boolean equals(final Object obj) {
        if (this == obj)
            return true;
        if (!(obj instanceof User))
            return false;
        return getUuid().equals(((User) obj).getUuid());
    }

    // Getters and setters follow
}

编辑:澄清我关于调用setUuid()方法的观点。下面是一个典型的场景:

User user = new User();
// user.setUuid(UUID.randomUUID()); // I should have called it here
user.setName("Master Yoda");
user.setEmail("yoda@jedicouncil.org");

jediSet.add(user); // here's bug - we forgot to set UUID and 
                   //we won't find Yoda in Jedi set

em.persist(user); // ensureUuid() was called and printed the log for me.

jediCouncilSet.add(user); // Ok, we got a UUID now

当我运行测试并看到日志输出时，我解决了这个问题:

User user = new User();
user.setUuid(UUID.randomUUID());

或者，也可以提供一个单独的构造函数:

@Entity
public class User {

    @Id
    private int id;  // Persistence ID
    private UUID uuid; // Business ID

    ... // fields

    // Constructor for Persistence provider to use
    public User() {
    }

    // Constructor I use when creating new entities
    public User(UUID uuid) {
        setUuid(uuid);
    }

    ... // rest of the entity.
}

我的例子是这样的:

User user = new User(UUID.randomUUID());
...
jediSet.add(user); // no bug this time

em.persist(user); // and no log output

我使用默认构造函数和setter，但您可能会发现双构造函数方法更适合您。

显然，这里已经有了非常有用的答案，但我将告诉你我们是怎么做的。

我们什么也不做。

如果我们确实需要= /hashcode来处理集合，则使用uuid。 您只需在构造函数中创建UUID。我们使用http://wiki.fasterxml.com/JugHome作为UUID。UUID的CPU开销稍高，但与序列化和db访问相比便宜。