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From Kevan Miller <kevan.mil...@gmail.com>
Subject Re: JNI and classloaders on Restart
Date Fri, 16 Jan 2009 13:49:20 GMT

On Jan 16, 2009, at 7:40 AM, Juergen Weber wrote:

>
> A given native library cannot be loaded by more than one classloader.
> (http://java.sun.com/docs/books/jni/html/design.html -> 11.2.4)
>
> So if a Geronimo application, that uses JNI, is restarted, the new
> classloader cannot load the library and gets  
> java.lang.UnsatisfiedLinkError:
> xx (Library is already loaded in another ClassLoader)
>
> But what if you let the JNI be loaded from a different, non  
> restartable
> classloader?
> http://cwiki.apache.org/GMOxDOC22/classloading.html
> "Plugins become parents of the classloader whereas jars become  
> available
> directly in the classloader. "

Right. A plugin dependency means that the given plugin (ClassLoader)  
must exist and will become a ClassLoader parent of the new plugin  
(ClassLoader) being defined. Jar dependencies define the jars which  
will be searched by the new ClassLoader.

>
> Does that mean, that a jar in the repository is within the thrown away
> classloader whereas a jar in a plugin is not? Then loading a DLL  
> from a
> plugin would be the solution to load a native library from a non- 
> restartable
> classloader, wouldn't it?

Correct. Operations on a given plugin (e.g. start/stop) will affect  
the plugin and any children of the plugin (if you stop a plugin, then  
any children of that plugin must also be stopped). Operations on a  
plugin will not affect the parents of that plugin.

So, you want to define a Plugin A with your JNI libraries. And a  
plugin B with a dependency on plugin A. You should now be able to  
restart B to your hearts content...

--kevan
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