Android Can't call Java method from native code JNI

Question

I have a native code and I want to call some Java methods from it using JNI, my main problem is FindClass method of JVM env always returns null in case of my classes although I write it correctly but if any Java class like String it returns a valid pointer, I got a sample code for loading the ClassLoader and let it loads my class but unfortunately I got NULL. this is the code:

unsigned char* pixels = new unsigned char[3 * width * height];
glReadPixels(0, 0, width, height, GL_RGB, GL_UNSIGNED_BYTE, pixels);
std::string imgURL(dataPath+"/Levels/screenshot.png");
stbi_write_png(imgURL.c_str(), width, height, 3, pixels, width * 3);
stbi_image_free(pixels);

JNIEnv *lJNIEnv;
manager.activity->vm->AttachCurrentThread(&lJNIEnv, NULL);
jobject lNativeActivity =  manager.activity->clazz;
jclass ClassNativeActivity = lJNIEnv->FindClass("android/app/NativeActivity");
jclass contextClass = lJNIEnv->FindClass("android/content/Context");
if(contextClass == 0)
    return;
jmethodID startActivityMethodId = lJNIEnv->GetMethodID(contextClass, "startActivity", "(Landroid/content/Intent;)V");
if(startActivityMethodId == 0)
    return;
jclass intentClass = lJNIEnv->FindClass("android/content/Intent");
if(intentClass == 0)
    return;
jmethodID intentConstructorMethodId = lJNIEnv->GetMethodID(intentClass, "<init>", "()V");
if(intentConstructorMethodId == 0)
    return;
jmethodID intentSetActionMethodId = lJNIEnv->GetMethodID(intentClass, "setAction", "(Ljava/lang/String;)Landroid/content/Intent;");
if(intentSetActionMethodId == 0)
    return;
jmethodID getClassLoader = lJNIEnv->GetMethodID(ClassNativeActivity,"getClassLoader", "()Ljava/lang/ClassLoader;");
if(getClassLoader == 0)
    return;
jobject cls = lJNIEnv->CallObjectMethod(lNativeActivity, getClassLoader);
if(cls == 0)
    return;
jclass classLoader = lJNIEnv->FindClass("java/lang/ClassLoader");
if(classLoader == 0)
    return;
jmethodID findClass = lJNIEnv->GetMethodID(classLoader, "loadClass", "(Ljava/lang/String;)Ljava/lang/Class;");
if(findClass == 0)
    return;
jstring intentString = lJNIEnv->NewStringUTF("com/game/Share");
if(intentString == 0)
    return;
jobject intentObject = lJNIEnv->NewObject(intentClass,intentConstructorMethodId);
if(intentObject == 0)
    return;
    //Here I got NULL although the package and class are correct
jclass marketActivityClass = (jclass)lJNIEnv->CallObjectMethod(cls, findClass, intentString);
if(marketActivityClass == 0)
    return;
lJNIEnv->CallVoidMethod(intentObject, intentSetActionMethodId,intentString);
lJNIEnv->CallVoidMethod(lNativeActivity, startActivityMethodId, intentObject);
manager.activity->vm->DetachCurrentThread();

any help?

Are you saying that the `GetMethodID()` call before your comment returns NULL, or that the `NewStringUTF()` after the comment returns NULL? — zenzelezz, Sep 03 '19 at 13:07
@zenzelezz You are right, I edited it, it is `CallObjectMethod` — Mohamed Moussa, Sep 03 '19 at 13:35
How do you know you get `NULL`? Note that you're not properly detaching the current thread if you get an error. And how are you getting the value for `manager.activity->clazz;`? Post that, too. — Andrew Henle, Sep 03 '19 at 17:03
See https://stackoverflow.com/questions/14586821/android-flurry-integration-with-ndk-app#comment27592124_14605837. Use `NewStringUTF("com.game.Share")`. — Alex Cohn, Sep 03 '19 at 17:34
Note that you must release all local references before detaching the thread. — Alex Cohn, Sep 03 '19 at 17:36

score 0 · Accepted Answer · answered Sep 04 '19 at 10:10

You have been bitten by a known Android JNI limitation:

You can get into trouble if you create a thread yourself (perhaps by calling pthread_create and then attaching it with AttachCurrentThread). Now there are no stack frames from your application. If you call FindClass from this thread, the JavaVM will start in the "system" class loader instead of the one associated with your application, so attempts to find app-specific classes will fail.

Following the advice listed in the article should get you unstuck:

Do your FindClass lookups once, in JNI_OnLoad, and cache the class references for later use. Any FindClass calls made as part of executing JNI_OnLoad will use the class loader associated with the function that called System.loadLibrary (this is a special rule, provided to make library initialization more convenient). If your app code is loading the library, FindClass will use the correct class loader.
Pass an instance of the class into the functions that need it, by declaring your native method to take a Class argument and then passing Foo.class in.
Cache a reference to the ClassLoader object somewhere handy, and issue loadClass calls directly. This requires some effort.

is `void android_main(struct android_app* state)` in the narive activity is considered as a separate thread? because I also get Null when I put the code on it. — Mohamed Moussa, Sep 04 '19 at 13:49
It is. However, you can extract a working classloader from `app->activity->clazz` and use option 3. See [this retrieveClass function](https://github.com/googlesamples/android-ndk/blob/master/teapots/common/ndk_helper/JNIHelper.cpp#L501-L517) for example (replace `activity_class` with `app->activity->clazz`, or `manager.activity->clazz` as you seem to have it). — Botje, Sep 04 '19 at 13:54

Android Can't call Java method from native code JNI

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