JNI 中的 UnsatisfiedLinkError

Question

I'm working on a project for school using JNI. In a nutshell, I have gotten the first part working using the shared library and JNI, but the second part (which refers to the same library) gives an UnsatisfiedLinkError every time I try to run the code. My professor and TAs are all unable to figure out this problem and I have been trying to debug it for over a week now.

The first part is a .java program that will be executed independently, given a file of integers and a frequency, it should call native code in C to compute a direct fourier transform and return a value given a frequency. In java, I stored these ints into an array and passed the array to JNI to compute the DFT in C. This returns a single int value and works like a charm.

For the second part, we have created a gui interface which loads the JNI library and takes in a .wav file or a file of our own format. This gui is required to calculate the frequency of a file when selected from a file chooser. Since the .wav files (and the other type) have header information as well as binary data, rather than integer numbers as in the first part, my strategy is to pass the filename (as a string) to JNI so that my native code can dissect the int values into an array itself and then calculate the frequency of the file. My gui is complete, and the action event of selecting a file from the chooser is supposed to call the JNI code and return the frequency of the file. Interestingly, the code compiles in linux and loads the library successfully , but crashes with this link error when the file is chosen and the native code is supposed to be called using the file chooser. I am at a complete and total frustrated defeat. Any ideas?

HERE IS THE HEADER FILE GENERATED BY JNI

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class dft */

#ifndef _Included_dft
#define _Included_dft
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     dft
 * Method:    computeDFT
 * Signature: ([III)I
 */
JNIEXPORT jint JNICALL Java_dft_computeDFT
  (JNIEnv *, jobject, jintArray, jint, jint);

/*
 * Class:     dft
 * Method:    computeFileDFT
 * Signature: (Ljava/lang/String;)I
 */
JNIEXPORT jint JNICALL Java_dft_computeFileDFT
  (JNIEnv *, jobject, jstring);

#ifdef __cplusplus
}
#endif
#endif

HERE IS THE CODE THAT WORKS

public class dft
{
private native int computeDFT(int[] nums, int count, int freq);
private native int computeFileDFT(String fileName);

static
{
    /* 
     * !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
     * SYSTEM LOAD LIBRARY PATH BELOW
     * !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
     */
    System.loadLibrary("dft");
}

/**
 * @param args
 * @throws IOException 
 */
public static void main(String[] args) throws IOException
{
    File infile;
    Scanner sc = null;
    dft dft = new dft();
    int count = 0;
    int[] data = new int[4096];

    //code that reads the file and puts the content into an array called 'data'
            //the args[1] below is the specified frequency to be calculated in JNI

    int dftValue = dft.computeDFT(data, count, Integer.parseInt(args[1]));
    System.out.println(dftValue);
}

HERE IS THE PROBLEM CODE

private native int computeFileDFT(String fileName);

static
{
    /* 
     * !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
     * SYSTEM LOAD LIBRARY PATH BELOW
     * !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
     */
    System.loadLibrary("dft");
    System.out.println("loaded successfully");
}

    //imagine more code here creating a JPanel object,
    //adding a bunch of stuff to it, etc. 
    //One key item that is added is a button, 
    //which when pressed opens a JFileChooser object, 
    //below you will see my implementation of the 
    //ActionListener which listens to the button and is 
    //supposed to execute the native code when the file is chosen.

private class TunerButtonListener implements ActionListener
{
    //@Override
    public void actionPerformed(ActionEvent event)
    {
        int returnVal = dialog.showOpenDialog(fileButton);
        if(returnVal == JFileChooser.APPROVE_OPTION) 
        {
            currentFileName = dialog.getSelectedFile().getName();
            currentFilePath = dialog.getSelectedFile().getPath();
            message.setText(currentFileName);


            int val = computeFileDFT(currentFilePath);
                    //more code goes here that does more stuff

THE FULL ERROR MESSAGE AND STACK

Exception in thread "AWT-EventQueue-0" java.lang.UnsatisfiedLinkError: TunerPanel.computeFileDFT(Ljava/lang/String;)I
at TunerPanel.computeFileDFT(Native Method)
at TunerPanel$TunerButtonListener.actionPerformed(TunerPanel.java:167)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2012)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2335)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:404)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:289)
at java.awt.Component.processMouseEvent(Component.java:6389)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3268)
at java.awt.Component.processEvent(Component.java:6154)
at java.awt.Container.processEvent(Container.java:2045)
at java.awt.Component.dispatchEventImpl(Component.java:4750)
at java.awt.Container.dispatchEventImpl(Container.java:2103)
at java.awt.Component.dispatchEvent(Component.java:4576)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4633)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4297)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4227)
at java.awt.Container.dispatchEventImpl(Container.java:2089)
at java.awt.Window.dispatchEventImpl(Window.java:2518)
at java.awt.Component.dispatchEvent(Component.java:4576)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:672)
at java.awt.EventQueue.access$400(EventQueue.java:96)
at java.awt.EventQueue$2.run(EventQueue.java:631)
at java.awt.EventQueue$2.run(EventQueue.java:629)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:105)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:116)
at java.awt.EventQueue$3.run(EventQueue.java:645)
at java.awt.EventQueue$3.run(EventQueue.java:643)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:105)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:642)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:275)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:200)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:190)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:185)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:177)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:138)

Answer 1

Reading the enormous comment thread of the all omnipresent smart people of SO JNI subculture, i might be missing something, but to me the bug is obvious:

You have created your native JNI lib against class dft , hence your native functions are Java_dft_* . It indeed works when being called in scope of dft instance.

Then you create another class TunerPanel . Of course you declare the same native method signature, but you don't have any library with Java_TunerPanel_* functions! You are still using the same original library with Java_dft_* .

Make dft instance accessible from TunerPanel . You can't use the same native library for native methods from two different classes. Or make the native methods static. As far as my understanding of FFT goes, you don't need to keep any persistent data between runs of compute* .