Select for update skip locked from JPA level

问题

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 where MY_CONDITIONS for update skip locked

I run it throughout native query, but I have to write that query for all required entities.

Is there any way to skip locked records using pure JPA? Can I implement my own locking policy?

I don't mind changing JPA provider but I want to use JPA API.

回答1:

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

回答2:

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");
    }});

回答3:

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.

来源：https://stackoverflow.com/questions/41434169/select-for-update-skip-locked-from-jpa-level

标签

java

Oracle

jpa

locking

eclipselink