There have been some updates in the spec in regards to Singletons.
Since a user pointed out the race condition on lazy instantiation
issue that needs to be fixed, I've been digging in the code to
implement these additions as well.
Some of the EJB 3.1 Singleton changes:
-- Callbacks are transactional (we've actually had one user request
this already)
-- New @AccessTimeout annotation to configure how long clients are
willing to wait for accessing a singleton with container managed
security.
Additionally the xml equivalent for @Lock(READ) and @Lock(WRITE) has
changed.
-David
On Jun 28, 2008, at 11:22 AM, David Blevins wrote:
Since we're thinking of calling the release 3.1, I started looking
at implementing singleton beans.
Started hacking last night and think I might have something working.
Here are some of my "singleton" spec notes. Perhaps too terse to
the point of being useless, but I figured I'd share anyway.
In general, think stateless bean where there is exactly one instance
(no pool) that is created at startup and never destroyed.
Singletons:
- concurrent access
- session bean
- supports web services
- not activated or passivated
- can use javax.ejb.TimerService
- can have EJBHome/EJBLocalHome interfaces
- With @Startup created when app starts
- @DependsOn allows for startup ordering
- does not support remove
- instances not discarded on system exception
- Bean-Managed or Container-Managed Concurrency
- Container-Managed Concurrency method attributes:
- ReadOnly (shared access)
- ReadWrite (exclussive access), the default
Some related new API classes (subject to change):
- @Singleton (class)
- @Startup (class)
- @DependsOn (class)
- @ContainerManagedConcurrency (class)
- @BeanManagedConcurrency (class)
- @ReadOnly (class, method)
- @ReadWrite (class, method)
- ConcurrentAccessTimeoutException
Will post more details as I get them.
-David
