Issue while loading the library files(".so" or ".sl") using JNI

Question

Hi,
We are loading the c library files using system.load during the init phase of servlet.
While loading the application for the first time everything goes smooth and application behaves as expected.
We are facing the below issue when we try restarting the application through admin console in case of WAS (web sphere) for any patch deployment in the application.
java.lang.UnsatisfiedLinkError: Native Library /users/test1/siva/jnilib.so already loaded in another classloader.
If we restart the complete WAS every thing works fine.
There is no specific System.unload function available in java to remove the loaded library in JVM.
Is there any alternate way to unload the library which is loaded in class loader to resolve this issue which can be called in the destroy phase of servlet?
Any help here is highly apreciated.
TIA,
Siva.

Answer 1

sivabalan wrote:
Hi,
We are loading the c library files using system.load during the init phase of servlet.
You mean a shared library, not C files (which would be source.)
However I am not sure that loading it in a servlet is a good idea. But that is a different issue.
Is there any alternate way to unload the library which is loaded in class loader to resolve this issue which can be called in the destroy phase of servlet?This is how it works on the Sun VM back to about 1.2. And as far as I know there is no other way for it to work on any other VM.
You have a class with native methods that relies on the shared library. The class and the shared library are loaded into a class loader. If the class loader is unloaded by the GC then the native library will be unloaded as well.
A class loader can be collected by the GC if if the classes loaded by it are no longer actively references. So ALL class instances can be collected.
If the above is true then by running System.gc() twice it will collect the classloader and thus the native library.
So in your situation it might work if your app server allows you do GC (full GC is better if that is an option). You could try unloading the app, then doing the GC option several times, then do a load. Try the GC option about 6 times and the reduce it down to see if 2 work.
If that doesn't work then there could be some programmatic solutions using the same idiom.