How to run nar file from command line?

Question

I used Eclipse and built a JNI project on my development machine using the nar-maven plugin. That compiles on multiple architectures and leaves me with .nar files.

My problem is that now I want to run the main program in one of the Java classes in the nar. I keep getting an UnsatisfiedLinkError when the Java program tries to load the native library that should be in the nar file. Now I realize that the .so file needs to be on the java.library.path but my problem is that I cannot see where to make that point since the only copy is somehow inside the nar file. I was under the impression that this would be handled behind the scenes because I'm using the auto-generated NarSystem.loadLibrary method in the Java.

Details follow.

The part of my POM configuring the nar plugin:

        <plugin>
            <groupId>com.github.maven-nar</groupId>
            <artifactId>nar-maven-plugin</artifactId>
            <version>3.5.1</version>
            <extensions>true</extensions>
            <configuration>
                <libraries>
                    <library>
                        <type>jni</type>
                        <narSystemPackage>com.myco.package</narSystemPackage>
                    </library>
                </libraries>
            </configuration>
        </plugin>

In one of my Java classes, I have a static initializer that calls the auto-generated method for loading the library:

static {
   com.myco.package.NarSystem.loadLibrary();
}

After running mvn install, I have the following files in the appropriate sub-directory of my local .m2/repository directory:

• maven-metadata-local.xml
• _maven.repositories
• project-0.0.1-arm-Linux-gpp-jni.nar
• project-0.0.1.nar
• project-0.0.1.pom
• project-0.0.1-javadoc.jar

I try to run from the command line with this command:

java -cp /home/myname/.m2/repository/com/myco/project/0.0.1/project-0.0.1.nar com.myco.package.MainClass

That begins to run the appropriate main program, so it found the Java portion. When it gets to the point where it should load the native library - the static block above - it throws the error

Exception in thread "main" java.lang.UnsatisfiedLinkError: no project-0.0.1 in java.library.path

This question, Nar dependency in multi-module maven project, looked like it would be close, but the answer depends on already having the native library "exposed" somehow. I followed the questioner there trying mvn nar:nar-unpack and mvn exec:java, but that did not advance me. (Get the same apparently null result from the first as described in that question, and the same exception that I described here for the second.)

Other questions considered:

• Maven - UnsatisfiedLinkError with java.library.path Seems to be about finding libraries outside of the maven process. Answer given in terms of maven configuration files, which doesn't seem to help once I'm deployed.
• maven-nar-plugin and tests in dependent module This deals with an issue specific to running tests.
• nar-maven-plugin and native-library-loader dont load native lib This one is close but in any case has no answer. It seems to be different since that poster was trying to specify a library path on the command line somehow.
• Using a native Maven artifact (nar) in a webapp This one is about incorporating into a webapp, so it's different. It does seem to offer this clue: "Both of these are produced as NAR artifacts and require the maven-nar plugin in order to be used." But I haven't figured out how the plugin needs to be used!

So how do I run this main program from the command line? If there's some pre-processing step to "extract" the native library what is it? If it's automatic, then what am I missing? Thanks!

Answer 1

I got a tip and finally got it!

This dependency needs to be added to the POM:

    <dependency>
        <groupId>org.scijava</groupId>
        <artifactId>native-lib-loader</artifactId>
        <version>2.2.0</version>
    </dependency>

After that I could run it using

mvn exec:java -Dexec.mainClass=com.myco.package.MainClass

Trying to run it directly with java from the command line still crashed, but in this case it crashed because it wasn't finding the jars for the dependencies. Presumably I could run it now without maven if I were to specify class path entries for all of them with the -cp flag.