Shared JNI library (.so) in Tomcat - UnsatisfiedLinkError

Question

I have a JNI library (.so) shared between two web applications deployed in Tomcat7. I am loading the library using the System.loadLibrary only once in the first web application that is being deployed and then in the second I'm checking if it already was loaded to not load anymore (I tried loading it in both and I got UnsatisfiedLinkError - library was loaded by another classloader). I can make any call to the native library in the first application, but in the second one I get UnsatisfiedLinkError with the method name that I am trying to call.

I am running out of ideas of what I can do. Any ideas? I tried most of the solutions on SO. Thank you.

EDIT Yes, I tried adding the library in the tomcat lib folder and loading it from there. Initially it was in the bin folder and the same issue occurs.

kuhajeyan · Accepted Answer · 2016-10-15T16:05:04.917

2

Yes, this will happen when you try to load the library that has already loaded my another web application. Tomcat, uses separate class loaders for each of the web application, and it wont allow you load a same native library more than once to JVM via another class loader

Move any share jar files if any that consumes JNI from you sharedlib.so. Add the system path to sharedlib ,

export LD_LIBRARY_PATH=/path/to/whereyourlinklibrary

Write a simple class like this which enables you to load your shared library when tomcat starts. Just compile this class and drop it in tomcat lib folder

package msm;
public class DLLBootstrapper {

     static {
      System.loadLibrary("sharedlib");
     }

     public static void main(String args[]) {
      System.out.println("Loaded");
     }

    }

you can now load this class from any of your web application ( probably in startup listener)

Class.forName("msm.DLLBootstrapper");

Good to go!

edited Oct 15 '16 at 16:05

answered Oct 14 '16 at 15:36

kuhajeyan

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And the .class file DLLBootstrapper should be in the tomcat lib folder right? – Ovi Faur Oct 15 '16 at 15:38
Or should I pack the .class file in a jar and put in the tomcat lib foldeR? – Ovi Faur Oct 15 '16 at 15:45
yes. you should put that in tomcat lib folder. Tomcat will pick it up when it starts – kuhajeyan Oct 15 '16 at 16:03
The .class or should i export it as a jar? – Ovi Faur Oct 16 '16 at 09:17
just .class should do – kuhajeyan Oct 16 '16 at 18:13
Everything looks good, until I try and use the methods from the loaded libraries and I still get an unsatisfied link error. The DLLs loaded in the DLL Loader, the classForName returns properly. Just confused. – Ted Herrlich Jul 30 '18 at 03:40
In modern Tomcat versions there is a built-in solutions for this issue. See my answer at https://stackoverflow.com/a/55080587/968244 for details. – isapir Mar 09 '19 at 18:34

score 0 · Answer 2 · answered Oct 14 '16 at 13:05

0

Have you tried putting shared JNI library just inside the lib directory of server.It should be shared by all the web applications deployed.

answered Oct 14 '16 at 13:05

Ankit Tripathi

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And load it from there with System.loadLibrary? Or just leave it there and let Tomcat handle all this? – Ovi Faur Oct 14 '16 at 13:09
System.loadLibrary("libraryName"); – Ankit Tripathi Oct 14 '16 at 13:09
Yes, i did try that. The same thing. Initially it was extracted by a ContextInitializer from a .jar file and put into tomcat/bin folder. – Ovi Faur Oct 14 '16 at 13:10
You not only have to put the library in "lib". You have to put the code that loads it there, so it gets loaded by the tomcat shared class loader and not by a webapp class loader (as you already found out you can't load it twice and another webapp is not able to access it) – user2543253 Oct 14 '16 at 13:15
Let's say I extract the library manually and put it in the "lib" folder. Do I have to call "System.loadLibrary" from myWebApp1 in order to use any function from that libray? Or will Tomcat load it by default on startup? Because I tried to let Tomcat load it and it doesnt work :( – Ovi Faur Oct 14 '16 at 13:19
I'll try this: http://kuhajeyan.blogspot.ro/2015/09/resolving-javalangunsatisfiedlinkerror.html – Ovi Faur Oct 14 '16 at 13:41
Didn't read the blog post but looks good. You have to call `System.loadLibrary()` yourself but have to make sure it is *not* from the webapp but from shared code. – user2543253 Oct 14 '16 at 14:12

score 0 · Answer 3 · answered Oct 14 '16 at 14:59

You need to load any native code from within the server classloader and not the webapp classloader. I recommend to write a Listener which loads the binary image of the shared object into the VM. This will happens only once. Please see the AprLifecycleListener on how to properly do that. It included a JNI compnent which likely represents you case exactly.

The shared object has to reside in ${catalina.home}/lib and LD_LIBRARY_PATH hat to point to it.

score 0 · Answer 4 · answered Mar 09 '19 at 18:32

Tomcat has a built-in solution for this issue as of versions 9.0.13, 8.5.35, and 7.0.92:

1) Use the JniLifecycleListener to load the native library.

2) Use the loadLibrary() or load() from org.apache.tomcat.jni.Library instead of System.

See more details and examples in my answer at java.lang.UnsatisfiedLinkError: Native Library XXX.so already loaded in another classloader

Shared JNI library (.so) in Tomcat - UnsatisfiedLinkError

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