JNI：UnsatisfiedLinkError

Question

I'm trying to test out some JNI code integrating a Java class with some ROS functionality and I'm struggling to get the Java methods linked up correctly. I've got the native code compiled against the JNI interface correctly (or so I think) but at runtime I get an UnsatisifiedLinkError on the first native method I have defined. At this point I'm not sure if the root cause is that the JVM isn't properly loading the .so file (in the same directory, and I've tried -Djava.library.path=. ) or if it's successfully loading it and it's not finding the method correctly.

This error message gives so little to go on, is there a way to get more info about what exactly is causing it?

I'm not opposed to posting the source code if it would help, though I'd have to do some editing before I can upload it so I'll wait to see if you guys think it might be helpful.

Talker.java:

public class Talker {
    /**
     * ROS Native methods
     *
     * Simple passthrough to the C++ native methods in the ROS layer
     */
    private static native void rosAdvertise();
    private static native void rosPublish();
    private static native void rosSpinOnce();

    {
        System.loadLibrary("ros-test-native-talker");
    }

    public static void main(String[] args) {
        rosAdvertise();

        while (true) {
            rosPublish();

            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            rosSpinOnce();
        }
    }
}

ros-test-native-talker.cpp:

#include "test_rostest_Talker.h"
#include "ros/ros.h"
#include "std_msgs/Time.h"

ros::Publisher outbound;


JNIEXPORT void JNICALL Java_test_rostest_Talker_rosAdvertise
    (JNIEnv *, jclass) {
    int argc = 0;
    char **argv;
    ros::init(argc, argv, "ros-native-timing-tester");

    ros::NodeHandle n;
    outbound = n.advertise<std_msgs::Time>("chatter", 1000);
}


JNIEXPORT void JNICALL Java_test_rostest_Talker_rosPublish
    (JNIEnv *, jclass) {
    ros::Time tx_timestamp = ros::Time::now();

    ROS_INFO("Sending message at %d.%d", tx_timestamp.sec, tx_timestamp.nsec);

    std_msgs::Time msg;
    msg.data = tx_timestamp;

    outbound.publish(msg);
}


JNIEXPORT void JNICALL Java_test_rostest_Talker_rosSpinOnce
    (JNIEnv *, jclass) {
    ros::spinOnce();
}

and the output:

rush@lubuntu64vm:~/javarostest$ java -Djava.library.path=. -cp ros-test-native-1.0-SNAPSHOT.jar test.rostest.Talker                                                                                            
Exception in thread "main" java.lang.UnsatisfiedLinkError: test.rostest.Talker.rosAdvertise()V
        at test.rostest.Talker.rosAdvertise(Native Method)
        at test.rostest.Talker.main(Talker.java:21)

Answer 1

I have no clue why but refactoring the above code slightly caused it to work. If I take the native methods out of the main classand put them in a separate class (thus removing the static modifier on the native methods) which is invoked by the main class, it all links and works fine. I'm not certain nor do I even have a clue why, but I think the static modifier on those methods was causing some issues.