Re: JNI: Calling Java Method with String as Parameter from C

On 16 Apr., 19:22, Gordon Beaton <n...@for.email> wrote:


Thx gordon. Your totally right with your advise. I added now all
neccessary code parts.. I did some testing this morining and what i
found out is pretty interesting.

C:
struct RxParams
{
        XLhandle eventHandle;
        long portHandle;
        const char *callBackName;
        jobject caller;
        JNIEnv *env;

}typedef RXPARAMS;

JNIEXPORT jlong JNICALL Java_VectorXLDriverLib_setNotifier(JNIEnv
*env, jclass jcls, jobject portHandle, jint queueLevel , jobject
caller, jstring callBackName)
{
         [...]

         RXPARAMS prxParams;

        prxParams.eventHandle = eventHandle;
        prxParams.portHandle = ph;
        prxParams.caller = caller;
        prxParams.env = env;
        prxParams.callBackName = (*env)->GetStringUTFChars(env,callBackName,
0);

        listenCan(prxParams);
  }
  return xlStatus;

}

void listenCan(RXPARAMS par)
{
        writeLog("In RxThread\n");
        unsigned int msgsrx = RECEIVE_EVENT_SIZE;
        jsize count;
        jmethodID jmid;
        jclass icls;
        jboolean continueListening = 1;

        char string[50];
        JAVAOBJECT eventObj; //object used for callBack to java

        RXPARAMS prxParams = par;

        JNIEnv *env = par.env;

        writeLog("Before get Structure XLCANEvent\n");
        // Klasse des aufrufenden Java Objektes ermitteln:
         icls = (*env)->GetObjectClass(env, prxParams.caller);

        writeLog("before getMID: icls: %02x\n", icls);
        jmid = (*env)->GetMethodID(env, icls,
prxParams.callBackName,"(LXLCanEvent;Ljava/lang/String;)Z");

        writeLog("after getMID\n");
        if (jmid == 0){
                writeLog("jmid == NULL\n");
        }
        eventObj = CreatejObject(env, "XLCanEvent");

        writeLog("Before Get Field Ids\n");
         jfieldID jFldTransId = (*env)->GetFieldID(env, eventObj.jcls,
"transid", "I");
        jfieldID jfldPortHandle = (*env)->GetFieldID(env,
eventObj.jcls, "portHandle", "I");
         jfieldID jfldFlag = (*env)->GetFieldID(env, eventObj.jcls,
"flags", "I");
         jfieldID jfldCanId = (*env)->GetFieldID(env, eventObj.jcls,
"id", "J");

        jint test = 5;

        writeLog("CallBooleanMethod with Data\n");
        // Methode "intern" des aufrufenden Java Objektes aufrufen:
        (*env)->SetIntField(env, eventObj.ref, jFldTransId, test);
        (*env)->SetIntField(env, eventObj.ref, jfldPortHandle, test );
        (*env)->SetIntField(env, eventObj.ref, jfldFlag, test );
        (*env)->SetLongField(env, eventObj.ref, jfldCanId, test );

        const char *pWelt = "Hallo Welt";
        jstring jStr = NULL;
        jStr = (*env)->NewStringUTF(env, pWelt);

        if (jStr != NULL){
                jsize size = (*env)->GetStringUTFLength(env,jStr);
                writeLog("Is Not Null. Size: %d \n", size);
        }
        else
                writeLog("Is Null\n");

         continueListening = (*env)->CallBooleanMethod(env, prxParams.caller,
jmid, eventObj, jStr);

#ifdef DEBUG
    writeLog("Listener has been terminated");
#endif

}

JAVAOBJECT CreatejObject(JNIEnv *env , char * className)
{
        jclass jcls;
  jfieldID jfid;
  jmethodID jmid;
  jobject jobj;

        JAVAOBJECT javaObj;

        javaObj.ref = NULL;
        javaObj.jcls = 0;

   javaObj.jcls = (*env)->FindClass(env, className);
   if (javaObj.jcls == NULL)
   {
      writeLog("Error FindClass\n");
      return javaObj;
    }

        //Get constructor id
   jmid = (*env)->GetMethodID(env, javaObj.jcls, "<init>","()V");
   if (jmid == NULL)
   {
      writeLog("Error GetMethodID\n");
      return javaObj;
   }

        //create new object
   javaObj.ref = (*env)->NewObject(env, javaObj.jcls, jmid);
   if (jobj == NULL)
   {
      writeLog("Error NewObject\n");
      return javaObj;
   }

   return javaObj;

}

Java Code:

public class VectorCanLib implements Runnable{

        //Returns the Id of the gen
        public synchronized long registerListener(VectorXlLibraryNotifier
callBack)
        {

                closeNotify = false;

                if(notifyThread == null){
                        notifyThread = new Thread(this);
                        notifyThread.start();
                }

                listeners.add(callBack);

                return 0;
        }

        public void run() {

                VectorXLDriverLib.setNotifier(portHandle, queueLevel, this,
"internalNotifyCallback");

                System.out.println("Internal notifier closed");
        }

        private synchronized boolean internalNotifyCallback(XLCanEvent e,
String obj)
        {
                //TODO: Alle abgespeicherten callbacks nacheinander benachrichtigen

                if(e != null){
                        System.out.println("Received: TransID: " + e.transid + " PH "+
e.portHandle +" CANID "+ e.id);
                }
                else{
                        ;//System.out.println("null call received");
                }

                if(obj != null)
                {
                        System.out.println(obj.length());
                        System.out.println("object is not null : " + obj.toString());
                }

                if(closeNotify){
                        System.out.println("CloseNotify is " + closeNotify);
                }
                return !closeNotify;

        }

}

public abstract class VectorXLDriverLib{

        [.....]

        public static native long setNotifier(XLPortHandle portHandle, int
queueLevel, Object caller, String callBackName);

        static {
        try{
                System.load(System.getProperty("user.dir") + "\\" + "MyDll.dll");
        }
        catch(Exception e) {
                e.printStackTrace();
        }

}

public class XLCanEvent {
        public char channelIndex;
        public int transid; //internal use only
        public int portHandle; //internal use only
        public long id;/
        public int flags;

}

Now the example should contain everything neccessary. What i found out
is that if i replace in the function listenCan()

         continueListening = (*env)->CallBooleanMethod(env, prxParams.caller,
jmid, eventObj, jStr);

with

         continueListening = (*env)->CallBooleanMethod(env, prxParams.caller,
jmid, NULL, jStr);

than the string obj will be correctyl given to java! Could it be that
the eventObj overrides the string reference??? I changed nothing else
in the code expect of these line!

Thanks for your help in advance

jmid, eventObj, jStr);

jmid, eventObj.ref, jStr);