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

17 Apr 2007 00:32:29 -0700
On 17 Apr., 09:23, wrote:

On 16 Apr., 19:22, Gordon Beaton <> wrote:

On 16 Apr 2007 09:57:06 -0700, wrote:

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

So, how can you be certain that the code you've posted actually
contains the error?

Since the actual String creation and method call don't appear to be
wrong, a logical conclusion is that the error is elsewhere. For
example, almost any kind of memory corruption could result in this
call subsequently failing in an erratic or unexplainable manner.

There are important things missing from your post. How are prxParams
(especially the jobj it holds) and the JNIEnv initialized? Since the
native method is presumably called from a separate thread started from
native code, you need to call AttachCurrentThread() somewhere or the
JNIEnv won't be valid here.

You should be cleaning up every object you create in this method.
either with DeleteLocalRef(), or by wrapping the method in calls to

Try to post a compilable example.



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.

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,

  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,

        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);
                writeLog("Is Null\n");

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

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


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

                closeNotify = false;

                if(notifyThread == null){
                        notifyThread = new Thread(this);


                return 0;

        public void run() {

                VectorXLDriverLib.setNotifier(portHandle, queueLevel, this,

                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 "+;
                        ;//System.out.println("null call received");

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

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


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);


         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


Instead of calling

 continueListening = (*env)->CallBooleanMethod(env, prxParams.caller,

jmid, eventObj, jStr);

i had to call

 continueListening = (*env)->CallBooleanMethod(env, prxParams.caller,

jmid, eventObj.ref, jStr);

because eventObj is a structur, which contains the real obj:

struct javaObject
        jobject ref; //reference to the java object
        jclass jcls; //class id of the java obj
}typedef JAVAOBJECT;

OMG. What a stupid mistake....this costs me at least one day!!!
Sometimes stupidity really hurts ;)

Thanks gordon, without your hints i properly would continue searching
the whole week;)

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