Java - JPA - Version annotation

Question

How does  Version annotation work in JPA   I found various answers whose extract is as follows      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    But I am still not sure how it works     Also as from the following lines      You should consider version fields immutable  Changing the field value has undefined results     Does it mean that we should declare our version field as final

User · Answer

Although  Pascal answer is perfectly valid  from my experience I find the code below helpful to accomplish optimistic locking    Entity public class MyEntity implements Serializable                        Version      Column name    optlock   columnDefinition    integer DEFAULT 0   nullable   false      private long version   0L                  Why  Because    Optimistic locking won t work if field annotated with  Version is accidentally set to null  As this special field isn t necessarily a business version of the object  to avoid a misleading  I prefer to name such field to something like optlock rather than version    First point doesn t matter if application uses only JPA for inserting data into the database  as JPA vendor will enforce 0 for  version field at creation time  But almost always plain SQL statements are also in use  at least during unit and integration testing

User · Answer

But still I am not sure how it works    Let s say an entity MyEntity has an annotated version property    Entity public class MyEntity implements Serializable             Id      GeneratedValue     private Long id       private String name        Version     private Long version                 On update  the field annotated with  Version will be incremented and added to the WHERE clause  something like this   UPDATE MYENTITY SET      VERSION   VERSION   1 WHERE   ID      AND  VERSION         If the WHERE clause fails to match a record  because the same entity has already been updated by another thread   then the persistence provider will throw an OptimisticLockException      Does it mean that we should declare our version field as final   No but you could consider making the setter protected as you re not supposed to call it

User · Answer

Every time an entity is updated in the database the version field will be increased by one  Every operation that updates the entity in the database will have appended WHERE version   VERSION THAT WAS LOADED FROM DATABASE to its query   In checking affected rows of your operation the jpa framework can make sure there was no concurrent modification between loading and persisting your entity because the query would not find your entity in the database when it s version number has been increased between load and persist

User · Answer

Version used to ensure that only one update in a time  JPA provider will check the version  if the expected version already increase then someone else already update the entity so an exception will be thrown   So updating entity value would be more secure  more optimist   If the value changes frequent  then you might consider not to use version field  For an example  an entity that has counter field  that will increased everytime a web page accessed

User · Answer

Just adding a little more info   JPA manages the version under the hood for you  however it doesn t do so when you update your record via JPAUpdateClause  in such cases you need to manually add the version increment to the query   Same can be said about updating via JPQL  i e  not a simple change to the entity  but an update command to the database even if that is done by hibernate  Pedro

[java] Java - JPA - @Version annotation

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