NullPointer in invoking JNI function

Question

Hi everybody.
I have small problem. I'm trying to call dll (jnidll) function (f1) from java that calls function (f2) in another dll (that is not jni - actually is 3rd party so I cannot change it).
I have no problem calling function f1 from my java application, and I have no problem calling function f2 from another C++ test application. But I have problem calling f1 that calls f2.
I get NullPointerException to my java application.
Does anybody have an idea how to solve it?
I tried also to use jawin.dll for calling that non-jni dll but had the same (NullPointer) problem.

Answer 1

Ok, first I wanted only quick answer, if anybody had the same problem. Now I will post my code here, because I didn't solved the problem yet.
Let's suppose that the name of the jni dll I'm trying to create is myjnidll.dll. It's header file has jni.h included and e.g. two following lines:
JNIEXPORT jint JNICALL Java_JniPosApi00_MbtConnect (JNIEnv *, jobject, jstring);
JNIEXPORT jint JNICALL Java_JniPosApi00_MbtDisconnect (JNIEnv *, jobject);Source file of my dll has following definitions of that functions. Name of called 3rd party dll is 3rdparty.dll and it has functions Connect and Disconnect exported with the following names _Connect@4 and _Disconnect@0.
#include <windows.h>
#include <jni.h>
#include "myjnidll.h"
typedef long (__stdcall *INTxVOID)();
typedef long (__stdcall *INTxSTR)(LPCTSTR);
JNIEXPORT jint JNICALL Java_JniPosApi00_mConnect
  (JNIEnv * jEnv, jobject jObj, jstring jszSubSystem) {
    LPCTSTR szSubSystem = jEnv->GetStringUTFChars(jszSubSystem, 0);
    long result = 1;
    INTxSTR ProcAdd;
    HINSTANCE hinstLib = LoadLibrary("3rdparty.dll");
    if (hinstLib != NULL) {
        ProcAdd = (INTxSTR) GetProcAddress(hinstLib, "_Connect@4");
        if (ProcAdd != NULL)
            res = (ProcAdd)("SUBSYSTEM1");
    jEnv->ReleaseStringUTFChars(jszSubSystem, szSubSystem);
    FreeLibrary(hinstLib);
    return res;
JNIEXPORT jint JNICALL Java_JniPosApi00_mDisconnect
  (JNIEnv * jEnv, jobject jObj) {
    long res = 1;
    INTxVOID ProcAdd;
    HINSTANCE hinstLib =  = LoadLibrary("3rdparty.dll");
    if (hinstLib != NULL) {
        ProcAdd = (INTxVOID) GetProcAddress(hinstLib, "_Disconnect@0");
        if (ProcAdd != NULL)
            res = (ProcAdd)();
    FreeLibrary(hinstLib);
    return res;
}I'm sure, that the 3rdparty.dll library is in the system path.
Parameters of functions in 3rd party dll are (from the description that I have):
Connect(UCHAR FAR*)
Disconnect(void)Both functions return int. I think there will be no problem with parameters.
My java program that is calling myjnidll is
public class MyJavaCallingJniDll {
    public native int mConnect (String szSubSystem);
    public native int mDisconnect ();
    static {
        System.loadLibrary("myjnidll");
    public MyJavaCallingJniDll() {
    public int myConnect (String szSubSystem) {
        return mConnect (szSubSystem);
    public int myDisconnect (){
        return mDisconnect ();
}and I'm calling this from another java class, but there is no problem for sure.
So I did som tests and the result is, that when calling from java I get NullPointerException when I'm trying to call mConnect() (or mDisconnect()).
When I try to call from C++ I get "Unhandled exception, blabla Access violation" at the line where I'm trying to invoke functions from 3rd party dll.
I have no idea, where's the problem. Thanks anybody, who's trying to help me.