Android NDK: Calling Java functions from C++

Question

I am very new to JNI and I am trying to figure out how certain things work before I port my C++ iOS code to it. I was successful in getting one of the NDK samples working in Android studio and I can see how Java is able to call C++ functions.

I've been searching around and taking chunks of code, but I haven't been able to get it to work in my specific implementation.

Just to test how things worked I set up a simple text log function in java, and I am trying to call it from my native code but I've run into issues.

Here is my Java function:

public static void log(String s){
        Log.d("Native", s);
}

And C++:

void Log(std::string s){

    JNIEnv *env;
    g_JavaVM->GetEnv((void**)&env, JNI_VERSION_1_6);

    jstring jstr1 = env->NewStringUTF(s.c_str());

    jclass clazz = env->FindClass("com/android/gl2jni/GL2JNILib");
    jmethodID mid = env->GetStaticMethodID(clazz, "log", "(Ljava/lang/String;)V");

    jobject obj = env->CallStaticObjectMethod(clazz, mid, jstr1);
}

From what I've seen with different examples this should work, but it throws an error:

29835-29849/com.android.gl2jni A/libc﹕ Fatal signal 11 (SIGSEGV) at 0x00000000 (code=1), thread 29849 (Thread-17371)

Am I missing something?

EDIT:

I've changed it to GetStaticMethodID. But after logging the progress of the function I've found out that the line that fails is:

g_JavaVM->GetEnv((void**)&env, JNI_VERSION_1_6);

Which I figure is because g_JavaVM is set as static JavaVM* g_JavaVM = NULL; and then never touched again. I'm guessing I need to set this variable, but how?

Answer 1

Part of my problem was that I didn't initialize The JavaVM. The other part was that I was using C++, but I was trying to use the C functions.

The working code is:

Java:

public static void log(String s){
        Log.d("Native", s);
}

C++:

void Log(std::string s){

    JNIEnv *env;
    g_JavaVM->GetEnv((void**)&env, JNI_VERSION_1_6);

    jstring jstr1 = env->NewStringUTF(s.c_str());

    jclass clazz = env->FindClass("com/android/gl2jni/GL2JNILib");
    jmethodID mid = env->GetStaticMethodID(clazz, "log", "(Ljava/lang/String;)V");

    jobject obj = env->CallStaticObjectMethod(clazz, mid, jstr1);
}

//In some initialization function with Environment variable

env->GetJavaVM(&g_JavaVM);

Hopefully this can help other people with the same problem.