JNI on Android: How to retrieve a string from Java code?

Question

I read a lot of examples to retrieve a java string in C/C++ code, but it seems that I miss something. this simple code doesn't work..

In ActivityTest (android java code) I've:

public static native void nativeInit(String stringfromjava);

In TestActivity I've:

ActivityTest.nativeInit("test");

and in my test-jni.c:

JNIEXPORT void JNICALL  Java_com_test_jni_ActivityTest_nativeInit(JNIEnv* env, jclass cls, jobject obj, jstring stringfromjava){

__android_log_print(ANDROID_LOG_INFO, "TESTJNI","Native Init started");

const char* w_buf = (*env)->GetStringUTFChars(env, stringfromjava, 0);

if(w_buf == NULL) {
    __android_log_print(ANDROID_LOG_INFO, "TESTJNI","file path recv nothing");
}

else {
        __android_log_print(ANDROID_LOG_INFO, "TESTJNI","String: %s", w_buf);
}

(*env)->ReleaseStringUTFChars(env, stringfromjava, w_buf);

}

But in my logcat I get only:

I/TESTJNI (18921): Native Init started
I/TESTJNI (18921): String: 

Where I'm wrong...?

FIXED Thanks to Mario, removing "jobject obj" from the signature fixed my issue!

Answer 1

Only wrote one short test so far (similar to your program), but my function had a bit different signature (might depend on SDK/NDK/JDK version? took it from some tutorial code I found):

extern "C" void Java_com_whatever_Activity_method(JNIEnv* env, jobject jthis, jstring param);

Obviously you won't need extern "C" if you're not writing C++.

The Java signature would be:

native void method(String param);

Edit:

To obtain the string (not sure if this is 100% correct, but it works):

const char *cparam = env->GetStringUTFChars(param, 0);
// .. do something with it
env->ReleaseStringUTFChars(param, cparam);

Open to suggestions in case there's something wrong in there. It works fine, but might still be some issue, so feel free to comment.