Question

I am using the Java Native Interface to access several native methods within a DLL I created in C++. I can load the library without any problems, but I can only call the methods when they are located in the Java application’s Main Class. In other words everything works perfectly as long as I declare the methods and place the loadLibrary command in the Main Class.

However, if I attempt to place the loadLibrary command, and declare the native methods in a separate class, I get the following error:

Exception in thread "main" java.lang.UnsatisfiedLinkError: sdr_api.SDR_API_Interface.SetDebugFlag_Native(Z)V
at sdr_api.SDR_API_Interface.SetDebugFlag_Native(Native Method)
at sdr_api.SDR_API_Interface.SetDebugFlag(SDR_API_Interface.java:74)
at sdr_api.SDR_API.main(SDR_API.java:183)

I’ve tried looking through the JNI documentation to check whether there is a requirement that all native methods must be declared in the main class, but I can’t find anything. I would like to move these methods into a separate class, as I need to use them in a separate thread (I only need one instance of the class, so they could be declared static if necessary).

My best guess is that declaring the methods in a particular class somehow changes the manner in which they are called within the DLL, but I don’t know how.

Any help would be greatly appreciated!

EDITED: Added relevant code samples

SDR_API Class:

package sdr_api;

public class SDR_API 
{       
    public static void main(String[] args) 
    {

        try
        {
            double Rate = 2e5;
            double TxRxFreq = 5e9;
            double RxGain = 0;
            double TxGain = 0;

            SDR_API_Interface mySDRAPI = new SDR_API_Interface();
            mySDRAPI.SetDebugFlag(true);                              

            mySDRAPI.Setup_BS_Config("addr=192.168.10.2",Rate,Rate,TxRxFreq,TxRxFreq,RxGain,TxGain);
            mySDRAPI.Setup_MT_Config("addr=192.168.20.2",Rate,Rate,TxRxFreq,TxRxFreq,RxGain,TxGain);

         }
         catch(Exception e)
         {
        System.err.println("Exception thrown in main program: "+e);
         }
    }
}

SDR_API_Interface Class:

package sdr_api;

public class SDR_API_Interface 
{
    private native boolean Setup_MT_Config_Native(String _IP_Address, 
                                                  double _Rx_Rate, 
                                                  double _Tx_Rate,
                                                  double _Rx_Freq, 
                                                  double _Tx_Freq, 
                                                  double _Rx_Gain, 
                                                  double _Tx_Gain);
    private native boolean Setup_BS_Config_Native(String _IP_Address, 
                                                  double _Rx_Rate, 
                                                  double _Tx_Rate,
                                                  double _Rx_Freq, 
                                                  double _Tx_Freq, 
                                                  double _Rx_Gain, 
                                                  double _Tx_Gain);

    private native void SetDebugFlag_Native(boolean _DebugFlag);

    private native double[] TransmitUL_Native(double[] _TxWaveform);
    private native double[] TransmitDL_Native(double[] _TxWaveform);

    private boolean Debug_Flag = false;

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

    public boolean Setup_MT_Config(String _IP_Address, 
                                    double _Rx_Rate, 
                                    double _Tx_Rate,
                                    double _Rx_Freq, 
                                    double _Tx_Freq, 
                                    double _Rx_Gain, 
                                    double _Tx_Gain)
    {
        return Setup_MT_Config_Native(_IP_Address, 
                                      _Rx_Rate, 
                                      _Tx_Rate,
                                      _Rx_Freq, 
                                      _Tx_Freq, 
                                      _Rx_Gain, 
                                      _Tx_Gain);        
    }

    public boolean Setup_BS_Config(String _IP_Address, 
                                    double _Rx_Rate, 
                                    double _Tx_Rate,
                                    double _Rx_Freq, 
                                    double _Tx_Freq, 
                                    double _Rx_Gain, 
                                    double _Tx_Gain)
    {
        return Setup_BS_Config_Native(_IP_Address, 
                                      _Rx_Rate, 
                                      _Tx_Rate,
                                      _Rx_Freq, 
                                      _Tx_Freq, 
                                      _Rx_Gain, 
                                      _Tx_Gain);         
    }

    public void SetDebugFlag(boolean _DebugFlag)
    {
        Debug_Flag = _DebugFlag;
        SetDebugFlag_Native(Debug_Flag);   
    }

    public boolean GetDebugFlag()
    {
        return Debug_Flag;
    }
}

JNI Machine Generated header file:

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

#ifndef _Included_sdr_api_SDR_API
#define _Included_sdr_api_SDR_API
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     sdr_api_SDR_API
 * Method:    Setup_MT_Config_Native
 * Signature: (Ljava/lang/String;DDDDDD)Z
 */
JNIEXPORT jboolean JNICALL Java_sdr_1api_SDR_1API_Setup_1MT_1Config_1Native
  (JNIEnv *, jobject, jstring, jdouble, jdouble, jdouble, jdouble, jdouble, jdouble);

/*
 * Class:     sdr_api_SDR_API
 * Method:    Setup_BS_Config_Native
 * Signature: (Ljava/lang/String;DDDDDD)Z
 */
JNIEXPORT jboolean JNICALL Java_sdr_1api_SDR_1API_Setup_1BS_1Config_1Native
  (JNIEnv *, jobject, jstring, jdouble, jdouble, jdouble, jdouble, jdouble, jdouble);

/*
 * Class:     sdr_api_SDR_API
 * Method:    SetDebugFlag_Native
 * Signature: (Z)V
 */
JNIEXPORT void JNICALL Java_sdr_1api_SDR_1API_SetDebugFlag_1Native
  (JNIEnv *, jobject, jboolean);

/*
 * Class:     sdr_api_SDR_API
 * Method:    TransmitUL_Native
 * Signature: ([D)[D
 */
JNIEXPORT jdoubleArray JNICALL Java_sdr_1api_SDR_1API_TransmitUL_1Native
  (JNIEnv *, jobject, jdoubleArray);

/*
 * Class:     sdr_api_SDR_API
 * Method:    TransmitDL_Native
 * Signature: ([D)[D
 */
JNIEXPORT jdoubleArray JNICALL Java_sdr_1api_SDR_1API_TransmitDL_1Native
  (JNIEnv *, jobject, jdoubleArray);

#ifdef __cplusplus
}
#endif
#endif

Answer 1

@Ryan
On carefully analyzing the header file you can see the name of native method is
sdr_api_SDR_API_SetDebugFlag_Native

and the call from SDR_API_Interface is-
sdr_api.SDR_API_Interface.SetDebugFlag_Native

Probably you have created the header file with a different class
i.e SDR_API which is under the package sdr_api.

which should be actually created with the Interface class-
sdr_api.SDR_API_Interface.SetDebugFlag_Native

All you need to do is compile the class SDR_API_Interface, create the header file and then create the new dll to use it.

The dll you are using is still referencing a call to native method from the SDR_API class.
It should be SDR_API_Interface class.

Let me know if you still have doubts.