Question

I’m testing some functions with Android, JNI and NDK.

I have the following JAVA class:

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.TextView;

public class JNITest extends Activity {
    private int contador;
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        contador = 0;
        TextView label = (TextView)findViewById(R.id.Text);
        label.setText(Integer.toString(contador));
    }

    public void addClick(View addButton) {
        nativeAdd(1);
        TextView label = (TextView)findViewById(R.id.Text);
        label.setText(Integer.toString(contador));
    }

    private static native void nativeAdd(int value);

    static {
        System.loadLibrary("JNITest01");
    }
}

I have used javah -jni to generate header file:

#include <jni.h>
/* Header for class es_xxxxx_pruebas_JNITest */

#ifndef _JNITestNative
#define _JNITestNative
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_xxxxx_tests_JNITest
 * Method:    nativeAdd
 * Signature: (I)V
 */
JNIEXPORT void JNICALL Java_com_xxxxx_tests_JNITest_nativeAdd
(JNIEnv *, jclass, jint);

#ifdef __cplusplus
}
#endif
#endif

As you can see, second paramter is jclass type.

I’m wondering how I can change jclass for a jobject parameter.

I need a jobject paramter to obtain a value from a field of the class is calling this native function.

How can I change method signature? or how can I obtain jobject from jclass paremeter?

Thanks.

Answer 1

Static methods do not have access to an Object (the implicit this parameter), only access to other static methods/properties of that Class. That is why your native method has a jclass instead of a jobject.

So change your Java method to be nonstatic and regenerate your header file.

As an aside you can create Java objects from JNI but in this case I think you want to be able to change the value of the member variable contador so that wouldnt help you.