Perceived wisdom (spec) is that Singletons are not allowed/possible with
EJB.
I have a number of tables containing 'static' data - mainly lookup tables
but some very complex mappings. Static here implies non-volatile over a
Client session (or longer). These data are loaded into a client when the
client logs on. I don't want to keep all this data hanging around as EB's
(c. 1000 logins/day) but I do want to minimise the startup overhead.
If I have a (normal Java) class with a static method (getStaticDataObject)
which creates the object if it doesn't exist and returns the object (which
can then be queried for the static data), I have a Singleton pattern. The
object would need to be held in a static variable - also a no-no for EJB. I
would expect the object to load/convert the data using JDBC rather than
EB's.
If the client initialisation code (SLSB) calls this static method, the VM
will contain the data (within the class/object). If another client is
initialised in another VM, it will get a different object, but the same
data. If the class is garbage collected, the next time it is used, it will
have to re-initialise itself - and get the same data.
Worst case is that the initialisation is done for each Client login - best
is that it is done once only. Consistency would need to be controlled
'outside' the AppServer - but it could also be coded into the above
mechanism using checks and reloads as required.
Is this a special case, where a Singleton _is_ appropriate - or a) am I
missing something b) is it just 'not allowed'?
Any other suggestions gratefully received.
Thanks
tim
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