Select for update skip locked from JPA level

Question

In my application - Oracle with JPA (EclipseLink), I\'m using the following expression to lock the subset of the records in some tables:

select * from MY_TABLE w         


        

Answer 1

I know this post is a bit old, but for the record, just as the accepted answer stated, "javax.persistence.lock.timeout" (org.hibernate.cfg.AvailableSettings#JPA_LOCK_TIMEOUT) set to "-2" (org.hibernate.LockOptions#SKIP_LOCKED) with Hibernate results in "SKIP LOCKED". However, this can be done at run-time without having to set any global settings.

Since 2.0 JPA allows to pass hints along like so

entityManager.find(MyType.class, id, LockModeType.PESSIMISTIC_WRITE, new HashMap<String, Object>() {{
        put("javax.persistence.lock.timeout", "-2");
    }});

Answer 2

Oracle doesn't provide a read lock, since it doesn't need one; undo logs make it unnecessary. So, SELECT...FOR UPDATE is really just a WRITE lock for Oracle.

Using JPA, you want to set the LockModeType as a PESSIMISTIC_WRITE.

Answer 3

Hibernate provides the UPGRADE_SKIPLOCKED Lock mode.

Using JPA and Hibernate, to produce a "SKIP_LOCKED" as per Hibernate LockMode documentation, you have to combine the PESSIMISTIC_WRITE JPA LockModeType:

entityManager.find(Department.class, 1, LockModeType.PESSIMISTIC_WRITE);

and the Lock timeout setting, like for example in persistence.xml for your persistence unit:

<properties>
   <property name="javax.persistence.query.timeout" value="-2"/>
</properties>

(Note that you can configure this LockMode for complex query as well)

SKIP LOCKED is not part of ANSI SQL. Some RDBMS such the following provide this as a specific feature:

• MySQL
• Postgresql
• Oracle

So with pure JPA, it is not possible to specify a "SKIP LOCKED" in queries. Indeed, as documented in LockModeType, JPA 2.1 only supports the following:

• NONE
• OPTIMISTIC
• OPTIMISTIC_FORCE_INCREMENT
• PESSIMISTIC_FORCE_INCREMENT
• PESSIMISTIC_READ
• PESSIMISTIC_WRITE
• READ
• WRITE

However, to enable SKIP LOCKED in your query you can use these alternatives:

• Use specific JPA implementation feature, such as Hibernate LockMode which allows to specify the SKIP LOCKED via a JPA query, thanks to a combination of PESSIMISTIC_WRITE LockModeType Lock Timeout specific setting as described above
• Create a native SQL query as you did