Java JNI integration: System.load fails

Hey there!

I am currently trying to get Java code running in Matlab using a native library. My problem is not the general setup, but a single specific dll file that constantly causes UnsatisfiedLinkError exceptions. I am not able to load this specific dll file from within Java (I am trying to connect a software called SUMO to Matlab). So to give you more details, see this example code in java:

import org.eclipse.sumo.libsumo.*;

public class LibsumoTest {

    public static void main (String[] args){

        System.out.println("Let's go!");

        try{
            System.out.println("Loading dll: " + args[0] + "\\libsumojni.dll");
            System.load(args[0] + "\\libsumojni.dll");
            System.out.println("Success!");
            
            // Do stuff with native library
            System.out.println("Version: " + libsumo.getCMD_GETVERSION());            
        }
        catch(UnsatisfiedLinkError ex){
            System.out.println(ex.getLocalizedMessage());
            System.out.println(ex.getMessage());
            ex.printStackTrace();
        }
    }
}

Running this on (windows) command line interface using the provided jar file:

cd C:\test
javac -cp "libsumojni.jar;C:\test" LibsumoTest.java
java  -cp "libsumojni.jar;C:\test" LibsumoTest "C:\test"

the output is:

Let's go!
Loading dll: C:\test\libsumojni.dll
Success!
Version: 0

As you can see, the native library is working in this context (of course I tested way more complicated examples, this is just a minimal example).

Now I want to do the same in Matlab, but if I run this code in matlab:

cd C:\test
javaaddpath(pwd);
javaaddpath([pwd '\libsumojni.jar']);

LibsumoTest.main(pwd);

I always get this error:

Let's go!
Loading dll: C:\test\libsumojni.dll
C:\test\libsumojni.dll: Die angegebene Prozedur wurde nicht gefunden
C:\test\libsumojni.dll: Die angegebene Prozedur wurde nicht gefunden
java.lang.UnsatisfiedLinkError: C:\test\libsumojni.dll: Die angegebene Prozedur wurde nicht gefunden
	at java.lang.ClassLoader$NativeLibrary.load(Native Method)
	at java.lang.ClassLoader.loadLibrary0(ClassLoader.java:1941)
	at java.lang.ClassLoader.loadLibrary(ClassLoader.java:1824)
	at java.lang.Runtime.load0(Runtime.java:809)
	at java.lang.System.load(System.java:1086)
	at LibsumoTest.main(LibsumoTest.java:12)

I am using Matlab 2019b (64 bit), my jdk is 64 bit, the dll file is 64 bit. I also tried to use the static classpath instead of the dynamic one with no change. As long as I take another dll file, there is no error while loading. I made so many tests with different dll files which all worked that I come to the conclusion that there must be something special with this dll that prevents Matlab from loading it. Here is a screenshot of dependency walker with the loaded dll:

Even the function names are correct regarding the jni naming convention. However, there seem to be some problems (red font) but I am not really familiar with them. Do you have an idea where the problem can come from and why in the case using java without matlab this is working? I am also confused by MSVCP140.dll and VCRUNTIME140.dll which are displayed as 32 bit instead of 64 bit?

Any ideas?

Thank you very much

Sven

[Sven Mertin]

So I think I found the problem and it's even more challenging:

The libsumojni.dll depends on xerces, to be more specific it is linked to a xerces-c_3_2.dll which is also provided by the software I try to include into matlab.

Xerces is also used by Matlab, within <matlabroot>/R2019b/bin/win64 there is a file named xerces-c_3_2.dll, too.

But these two dll files are different! Their function names are the same, but the function adresses as well the file sizes differ. My guess is that if I am loading the dll from java within Matlab, the dll included in matlab is shadowing the dll given in the library path. Therefore I copied the matlab version of this file to my build folder for verification. Now I am getting the exact same error without using matlab at all (worked before). This error is now perfectly reproducible.

My conclusion now is that the two different versions of xerces-c_3_2.dll and the shadowing of the file within matlab are the reasons for my errors. Unfortunately, matlab won't run with the other xerces-c_3_2.dll and vice versa.

At the moment I don't have a clue how to continue.

Is there a way to load a native library through Java within matlab which depends on dlls that are shadowed by matlab files? I want to ignore them in this case.

Thanks!

Sven

[Sven Mertin]

I have resolved my problem:

1. I renamed my version of the dll file from 'xerces-c_3_2.dll' to 'xerces-c_3_s.dll'
2. I modified my compiled persona dll in a hex editor and changed the dependency acc. to the naming in (1)
3. I loaded the modified dll with the changed dependency using java within matlab. Now it's working without any problems!

So technically it was more a windows library thing rather than a java question.