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Thread: JNI CallStaticVoidMethod crash when Java variables declared outside method

  1. #1
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    Post JNI CallStaticVoidMethod crash when Java variables declared outside method

    I have started learning JNI to call C/Objective C from Java.
    I have a simple java class like this where int i is declared and assigned within the setValue method:

    // --------JAVA CLASS EXAMPLE 1

    public class Test {

    private native void _start();

    private void setValue(string str) {
    int i;
    i = 6;
    System.out.println(str);
    }
    }

    The corresponding C/Objective C code does a callback to Java using CallStaticVoidMethod.

    // ----------------JNI function in Objective C

    JNIEXPORT void JNICALL Java_org_test_try__start
    (JNIEnv * env, jobject obj){


    JNF_COCOA_ENTER(env);

    jobject myObject = (*env)->NewGlobalRef(env, obj);
    jclass myClass = (*env)->GetObjectClass(env, myObject);
    jmethodID myMethod = (*env)->GetMethodID(env, myClass, "setValue", "(Ljava/lang/StringV");
    char* str = "Confirmed";
    jstring js = (*env)->NewStringUTF( env, str );
    (*env)->CallStaticVoidMethod(env, myClass, myMethod, js);

    JNF_COCOA_EXIT(env);
    }

    This all works fine.
    However, if I declare the int i in the java class and not inside the method as follows, the JVM crashes.

    // --------JAVA CLASS EXAMPLE 2

    public class Test {

    private int i;

    private native void _start();

    private void setValue(string str) {;
    i = 6;
    System.out.println(str);
    }
    }

    It SEEMS the callback can't access the int variable when it is declared at class level.
    Why is this and how can I fix that?


  2. #2
    Senile Half-Wit Freaky Chris's Avatar
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    Default Re: JNI CallStaticVoidMethod crash when Java variables declared outside method

    Hi SGARCH,

    By the looks of things you are invoking CallStaticVoidMethod on a Non-Static method. This should be erroneous.

    Change this to be CallVoidMethod, and you might have more luck. Looks like the JVM might be trying to invoke it as a static, in which case the reference to i in the second example wouldn't work, as i is not static.
    chris[at]javaprogrammingforums[dot]com

    Prifysgol Bangor University, North Wales

  3. #3
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    Default Re: JNI CallStaticVoidMethod crash when Java variables declared outside method

    Many thanks for your help - so simple now that I see it. CallVoidMethod works fine.

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