Unloading Library

Question

Hi there,
i'm building an OSGi application with several services in different packages. One of these services needs to load a dll-file to use some native methods. Starting and stoping the application works fine, but when i restart it without restarting the knoplerfish framework before i get an linker-error that the library is already loaded by another classloader. So i suspect that my framework is not releasing the dll after i stop my application. Is there any way to force the unloading of such a manually loaded library?
Any hint would be very appreciated.
thx in advance, Chris

Answer 1

Hi jschell,
- That isn't a spec.ok, but I treat official jdk-documentation as a reliable source of information
- It says "can", not "must", "should", etc.yes, that's exactly why I introduced this "dirty loop". As I understand if we won't have got an exception the unload must have been completed. After several attempts we should give up anyway.
- It doesn't address how the class loader is actually garbage collectedcorrect, in fact nobody can force the unload of anything, your System.gc()-call just gives the VM a hint to unload resources no longer needed. Classloaders are classes themselves. So, if all classes loaded by the loader and the loader class itself are no longer referenced, the VM (the GC) is allowed unload them. Native libraries loaded by this loader can be unloaded, too.
This leads to an interesting question:
What happens, if a library L has been loaded through class loader A and an attempt to load the same library through class loader B fails (...has been loaded by an other class loader...) but the caller just ignores this exception? Well, first a class X loaded through class loader B needing the library L should be fit for work, because the library L has been loaded and is present in the VM. But what happens, if class loader A is going to be unloaded, because it is no longer referenced?
I didn't check it out yet. But I think, the library L will be unloaded, too, since class loader B has no knowledge (no reference) about L. So, who should prevent the GC from unloading L? Next time when X (loaded by B) needs L an exception should occur. What do you think?
Thanks for this nice "nitpicker"-discussion ;-)
Regards.
Karl