change Byte[] values in JNI with android studio

Question

i want to reference a parameter, Byte[], in a JNI function and replace values of them.
The declaration of JNI is below.

public native void imageprocessing(long inputImage, long inputImage2, long outputImage, long outputImage2, Byte[] sim);

The sim is the target what i want to change.
The interface of it is below.

Java_com_example_duru_opencvtest_MainActivity_imageprocessing(JNIEnv *env, jobject instance, jlong inputImage, jlong inputImage2, jlong outputImage, jlong outputImage2, jobjectArray sim)

it uses jobjectArray type and i want to put int type values of native language into sim object.

so i my method is

        jbyteArray byte_array = env->NewByteArray(4);
        env->SetByteArrayRegion(byte_array, 0, 4, (jbyte*)tempSim);
        jobjectArray object_array = env->NewObjectArray(4, env->FindClass("java/lang/Byte"), byte_array);

        /* ERROR
        (*env).SetObjectArrayElement(sim, 0, (jobject)object_array[0]);
        (*env).SetObjectArrayElement(sim, 1, (jobject)object_array[1]);
        (*env).SetObjectArrayElement(sim, 2, (jobject)object_array[2]);
        (*env).SetObjectArrayElement(sim, 3, (jobject)object_array[3]);
        */

tempSim is 'int tempSim[4]' and Sim also has 4 length.

(*env).SetObjectArrayElement(sim, 0, (jobject)object_array[0]);

The bold part occur syntax error than the other part have no problem?

You can't access Java arrays like that from native code. In this case you need to use the GetObjectArrayElement function. Although I don't really see what the purpose of byte_array and object_array is. Why don't you just pass each of tempSim's values to Byte.valueOf instead? — Michael
Note: NewByteArray is for a primative array (Java: byte[]; JNI: jbytearray), not an object array (Java: Byte[]). — Tom Blodget

Alex Cohn Alex Cohn · Accepted Answer · 2018-11-20T08:27:33

jclass javaLangByteClass = env->FindClass("java/lang/Byte");
jmethodID javaLangByteConstructor = env->GetMethodID(javaLangByteClass , "<init>", "(B)Ljava/lang/Byte;")
for (int i=0; i<3; i++) {
   jobject nextElement = env->NewObject(javaLangByteClass, javaLangByteConstructor, (jbyte)tempSim[i]);
   env->SetObjectArrayElement(sim, i, nextElement);
   env->DeleteLocalRef(nextElement);
}

See the comment below: Byte.valueOf() may be more efficient than the constructor:

jclass javaLangByteClass = env->FindClass("java/lang/Byte");
jmethodID javaLangByteStaticValueOf = env->GetStaticMethodID(javaLangByteClass , "valueOf", "(B)Ljava/lang/Byte;")
for (int i=0; i<3; i++) {
   jobject nextElement = env->CallStaticObjectMethod(javaLangByteClass, javaLangByteStaticValueOf, (jbyte)tempSim[i]);
   env->SetObjectArrayElement(sim, i, nextElement);
   env->DeleteLocalRef(nextElement);
}

change Byte[] values in JNI with android studio

1 Answers