Native library NOT thread safe - how to use it via JNI?

Question

Hello,
has anybody ever tried to use a native library from JNI, when the library is not thread safe?
The library (Windows DLL) was up to now used in an MFC App and thus was only used by one user - that meant one thread - at a time.
Now we would like to use the library like a "server": many Java clients connect the same time to the library via JNI. That would mean each client makes its calls to the library in its own thread. Because the library is not thread safe, this would cause problems.
Now we discussed to load the library several times - separately for each client (for each thread).
Is this possible at all? How can we do that?
And do you think we can solve the problem in this way?
Are there other ways to use the library, though it is not thread safe?
Any ideas welcome.
Thanks for any contributions to the discussion, Ina

Answer 1

(1)
has anybody ever tried to use a native library from
JNI, when the library (Windows DLL) is not thread safe?
Now we want many Java clients.
That would mean each client makes its calls
to the library in its own thread. Because the library
is not thread safe, this would cause problems.Right. And therefore you have to encapsulate the DLL behind a properly synchronized interface class.
Now the details of how you have to do that depends: (a) does the DLL contain state information other than TLS? (b) do you know which methods are not thread-safe?
Depending on (a), (b) two extremes are both possible:
One extreme would be to get an instance of the interface to the DLL from a factory method you'll have to write, where the factory method will block until it can give you "the DLL". Every client thread would obtain "the DLL", then use it, then release it. That would make the whole thing a "client-driven" "dedicated" server. If a client forgets to release the DLL, everybody else is going to be locked out. :-(
The other extreme would be just to mirror the DLL methods, and mark the relevant ones as synchronized. That should be doable if (a) is false, and (b) is true.
(2)
Now we discussed to load the library several times -
separately for each client (for each thread).
Is this possible at all? How can we do that?
And do you think we can solve the problem in this
way?The DLL is going to be mapped into the process address space on first usage. More Java threads just means adding more references to the same DLL instance.
That would not result in thread-safe behavior.