Java - JPA - @Version 注释

Question

@Version 注解在 JPA 中是如何工作的？

我找到了各种答案，摘录如下:

JPA uses a version field in your entities to detect concurrent modifications to the same datastore record. When the JPA runtime detects an attempt to concurrently modify the same record, it throws an exception to the transaction attempting to commit last.

但我仍然不确定它是如何工作的。

---

同样来自以下几行:

You should consider version fields immutable. Changing the field value has undefined results.

这是否意味着我们应该将我们的版本字段声明为final？

Answer 1

But still I am not sure how it works?

假设一个实体 MyEntity 有一个带注释的 version 属性:

@Entity
public class MyEntity implements Serializable {    

    @Id
    @GeneratedValue
    private Long id;

    private String name;

    @Version
    private Long version;

    //...
}

更新时，带有 @Version 注释的字段将递增并添加到 WHERE 子句中，如下所示:

UPDATE MYENTITY SET ..., VERSION = VERSION + 1 WHERE ((ID = ?) AND (VERSION = ?))

如果 WHERE 子句未能匹配记录(因为同一实体已被另一个线程更新)，则持久性提供程序将抛出 OptimisticLockException。

Does it mean that we should declare our version field as final

不，但您可以考虑让 setter protected ，因为您不应该调用它。