An enterprise bean must not use thread synchronization primitives
> to synchronize execution of multiple instances.
>
> [Same reason as above. Synchronization would not work if the EJB
> Container distributed enterprise bean instances across several
JVMs.]
Does this imply that one should not use the synchronization keyword on any bean
methods? At first one might think this seems clear from the above statement.
But note the remainder of the sentence, "to synchonize execution of multiple
instances". I am not just nit picking here. There may be valid reasons to allow
synchoronization primitives, for example even to use the synchronize keyword on a
bean method.
Note also the reason given. "Synchronization would not work if the EJB Container
distributed enterprise bean instances across JVMs". One case I am thinking about
relates to using a stateless session bean as a cache. (Yes I mean stateless,
not stateful). In such a case I may want to use the synchronize keyword.
Suppose for example that you want to cache a database table using a stateless
bean. When the bean instance is created (no relationship to calling the create
method) it loads in the rows of the table into some structure, let's just say a
vector. All updates, deletes or inserts to the table are stored in another table
with a sequence number. Whenever a method is called the "Modify" table is read
by the stateless bean and appropriate changes are made to the vector. I would
need to make sure that updates to the vector are done in a synchonous way so I
might use some synchronization primitives here. But note that I am only
interested in this one bean instance. Each stateless bean would have it's own
copy of the database table and I would only be interested in each stateless bean
in isolation.
Note also that you may define a thread safe data structure(to be used in a bean)
that itself use synchronization primitives and surely that would be OK?
So to summarize it would seem to me that using synchronization primitives is not
strictly prohibited in ejb beans. It is only prohibited to use them to
synchronized execution of multiple instances.
dan
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