Shared library jni under tomcat

[Joel Hirsh <jo...@gmail.com>]

I am running Tomcat 8.0.32 on LInux, and trying to use a jni library under
tomcat, and am hitting a wall.

First I found that only one instance of the native library can be loaded so
the library needs to live in shared/lib for general use.

Then in order to load the native library I found documentation that I both
needed to load the native library from a bootstrap jar that also lives in
shared/lib and that the jar be loaded with the common clasloader
Ref:
https://wiki.apache.org/tomcat/HowTo#I.27m_encountering_classloader_problems_when_using_JNI_under_Tomcat

So I have my code working to do all that.  And if I make a call from the
bootstrap jar to the first native method it works.
But when I try to access the native method from my application code I get
an error "java.lang.UnsatisfiedLinkError:".

I believe that is because the native library has been loaded with the
URLClassLoader and the application is loaded with the WepappClassLoader.
Although I also read that the classloaders are hierarchical, and should go
up the chain to find classes.

I have verified that the URLClassLoader used to load the bootstrap jar is
the same as the parent loader of the application (with .equals())

I did read an ancient thread (2006) that kind of described the problem I am
having, and the last post there indicated that the solution is "to touch
each class that uses native methods (from the loader that loaded the native
library) , forcing the classes to be loaded".  But I don't know how to
'touch' a java class.  And my classes that use native methods are part of
the application and could not be loaded by the URLClassLoader in any case.
Or even if that post is (still) relevant.

I have run out of ideas on what to do or test next.  I've been through doc
like
https://tomcat.apache.org/tomcat-8.0-doc/class-loader-howto.html
and many posts on stackoverflow on this topic, and cannot find anything to
help.

 Any suggestions would be most welcome.

Re: Shared library jni under tomcat

[Christopher Schultz <ch...@christopherschultz.net>]

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Joel,

On 11/20/18 01:03, Joel Hirsh wrote:
> I am running Tomcat 8.0.32 on LInux, and trying to use a jni
> library under tomcat, and am hitting a wall.
> 
> First I found that only one instance of the native library can be
> loaded so the library needs to live in shared/lib for general use.
> 
> Then in order to load the native library I found documentation that
> I both needed to load the native library from a bootstrap jar that
> also lives in shared/lib and that the jar be loaded with the common
> clasloader Ref: 
> https://wiki.apache.org/tomcat/HowTo#I.27m_encountering_classloader_pr
oblems_when_using_JNI_under_Tomcat

All
> 
you should need is the native library in the right place on the
disk (in java.library.path, which you might have to set in setenv.sh)
and enable the APRLifecycleListener in server.xml. It's right at the top
.

> So I have my code working to do all that.  And if I make a call
> from the bootstrap jar to the first native method it works. But
> when I try to access the native method from my application code I
> get an error "java.lang.UnsatisfiedLinkError:".

What kind of native calls are you trying to make?

> I believe that is because the native library has been loaded with
> the URLClassLoader and the application is loaded with the
> WepappClassLoader. Although I also read that the classloaders are
> hierarchical, and should go up the chain to find classes.
> 
> I have verified that the URLClassLoader used to load the bootstrap
> jar is the same as the parent loader of the application (with
> .equals())
> 
> I did read an ancient thread (2006) that kind of described the
> problem I am having, and the last post there indicated that the
> solution is "to touch each class that uses native methods (from the
> loader that loaded the native library) , forcing the classes to be
> loaded".  But I don't know how to 'touch' a java class.  And my
> classes that use native methods are part of the application and
> could not be loaded by the URLClassLoader in any case. Or even if
> that post is (still) relevant.
> 
> I have run out of ideas on what to do or test next.  I've been
> through doc like 
> https://tomcat.apache.org/tomcat-8.0-doc/class-loader-howto.html 
> and many posts on stackoverflow on this topic, and cannot find
> anything to help.
> 
> Any suggestions would be most welcome.

Where is your libtcnative.so file on the disk? What is your
java.library.path and where/how are you setting it? Did you enable the
APRLifecycleListener? What code are you using to load the library
(which you really should not need)? What code are you using to
actually *call* the native methods?

- -chris
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