Updating jint variable in JNI

Question

When below JNI function is called dataByteArray (of jbyteArray type) is correctly received by the Application (Android/java) calling the function. But sizeDataByteArray (of jint type) is not received by the application. Please let me know what is wrong in below code.

JNIEXPORT jboolean JNICALL Java_com_example_helloworld_Tester_getData
(JNIEnv* env, jclass clasz, jbyteArray dataByteArray, jint sizeDataByteArray) {

    // Some code 
    l_data = “01:02:03:AB:CD:EF”;
    l_data_size = strlen(l_data);
    env->SetByteArrayRegion(dataByteArray, 0, strlen(l_data), (jbyte *) l_data);
    sizeDataByteArray = l_data_size;

    // Some code
    return true; 
}

Answer 1

You can create class for return multiple values from native code:

public class ReturnValue {
    public final boolean success;
    public final byte[] result;

    public ReturnValue(boolean success, byte[] result) {
        this.success = success;
        this.result = result;
    }
}

return this object from jni you can implement this way:

JNIEXPORT jobject JNICALL Java_com_example_helloworld_Tester_getData
          (JNIEnv* env, jclass clasz, jbyteArray dataByteArray, jint sizeDataByteArray) {
    const char* cls_name = "com/example/helloworld/ReturnValue";
    // try to obtain ReturnValue class
    jclass cls = env->FindClass(cls_name);
    // if exception occurred return control to java 
    if (env->ExceptionOccurred())
        return NULL;

    const char* constructor_signature = "(Z[B)V";
    // try to obtain ReturnValue constructor
    jmethodID constructorId = env->GetMethodID(cls, "<init>", constructor_signature);
    // if exception occurred return control to java 
    if (env->ExceptionOccurred())
            return NULL;

    jboolean success = true;
    jbyteArray data = env->NewByteArray(0);

    // fill data array...

    // create ReturnValue object. If exception occurred, 
    // control will be returned java automatically in this case
    return env->NewObject(cls, constructorId, success, data);
}

Read additional information about jni method calling and method's signatures in this article.