JNI C++ wrapper for Android: classes and callbacks

JNI Wrapper for C++ Classes in Android: Creation and Callbacks

Integrating object-oriented C++ code into an Android app via JNI requires converting it into C functions while storing a pointer to the object. The C++ object's address is saved as a long in a Java class, enabling lifecycle management and multiple instances.

Consider a basic C++ class:

class NativeClass {
public:
    NativeClass() {}
    ~NativeClass() {}
    void fun() {
        __android_log_write(ANDROID_LOG_ERROR, log_tag, "Hello from native");
    }
};

JNI functions for working with the class:

• Initialization: jlong Java_com_myprog_example_Native_initNativeClass(JNIEnv* env, jobject thiz) returns (jlong) new NativeClass();
• Method call: void Java_com_myprog_example_Native_NativeClassFun(JNIEnv env, jobject thiz, jlong cppClass) calls ((NativeClass) cppClass)->fun();
• Destruction: void Java_com_myprog_example_Native_destroyNativeClass(JNIEnv env, jobject thiz, jlong cppClass) executes delete (NativeClass) cppClass;

Java wrapper:

public class Native { 
    static { System.loadLibrary("Native"); } 
    public static native long initNativeClass();
    public static native void destroyNativeClass(long cppClass);
    public static native void NativeClassFun(long cppClass);
}

public class NativeClass {
    long cppClass;
    NativeClass() {
        this.cppClass = Native.initNativeClass();
    }
    @Override protected void finalize() {
        free();
    }
    private void free() {
        Native.destroyNativeClass(cppClass);
    }
    public void fun() {
        Native.NativeClassFun(cppClass);
    }
}

Implementing Callbacks from Native Code

For asynchronous scenarios, such as network scanning, native code must call Java methods during operation. A Listener interface in C++ and a ThreadPool are used to serialize calls in a single thread.

Extended C++ class with callback support:

class NativeClass {
public:
    class Listener {
    public:
        virtual void print(const char* str) = 0;
        virtual ~Listener() {}
    };
private:
    bool started = false;
    Listener* listener = nullptr;
    static void worker(NativeClass* nc) {
        while(nc->started) {
            if(nc->listener != nullptr) {
                nc->listener->print("From native");
            }
            sleep(1);
        }
    }
public:
    NativeClass() {}
    ~NativeClass() {
        if(listener != nullptr) delete listener;
    }
    void setListener(Listener* listener) {
        this->listener = listener;
    }
    void scan() {
        started = true;
        std::thread thread = std::thread(worker, this);
        thread.join();
        started = false;
    }
    void stop() {
        started = false;
    }
};

Java interface and wrapper:

public interface NativeListener {
    void print(String str);
}

public class NativeClass {
    long cppClass;
    // constructor, finalize, free as above
    public void scan() { Native.NativeClassScan(cppClass); }
    public void stop() { Native.NativeClassStop(cppClass); }
    public void setListener(NativeListener listener) {
        Native.NativeClassSetListener(cppClass, listener);
    }
}

Key component — implementing Listener in C++ with JNI:

class Listener : public NativeClass::Listener {
private:
    JavaVM* jvm;
    JNIEnv* env;
    jweak listenerRef;
    jmethodID printMethod;
public:
    Listener(JavaVM* jvm, JNIEnv* env, jobject listener) {
        this->jvm = jvm;
        listenerRef = env->NewWeakGlobalRef(listener);
        ThreadPool::execute({
            jvm->AttachCurrentThread(&env, NULL);
            if(env != NULL) {
                jobject localRef = env->NewLocalRef(listenerRef);
                if(localRef != NULL) {
                    jclass clazz = env->GetObjectClass(localRef);
                    printMethod = env->GetMethodID(clazz, "print", "(Ljava/lang/String;)V");
                    env->DeleteLocalRef(localRef);
                }
            }
        });
    }
    ~Listener() {
        ThreadPool::execute({
            env->DeleteWeakGlobalRef(listenerRef);
            jvm->DetachCurrentThread();
        });
    }
    void print(const char* msg) {
        ThreadPool::execute({
            jobject localRef = env->NewLocalRef(listenerRef);
            if(localRef != NULL) {
                jstring jMsg = env->NewStringUTF(msg);
                env->CallVoidMethod(localRef, printMethod, jMsg);
                env->DeleteLocalRef(jMsg);
                env->DeleteLocalRef(localRef);
            }
        });
    }
};

JNI function for setting the listener:

JNIEXPORT void JNICALL Java_com_myprog_example_Native_NativeClassSetListener(
    JNIEnv* env, jobject thiz, jlong cppClass, jobject listener) {
    ((NativeClass*) cppClass)->setListener(new Listener(jvm, env, listener));
}

Implementation Recommendations

• Single-threaded ThreadPool: Ensures sequential JNI calls from a fixed thread.
• Weak references (jweak): Prevent memory leaks; strong local references are created only for the duration of the call.
• Lifecycle management: The Listener destructor cleans up resources via the ThreadPool.

List of key practices:

• Always check for NULL on local references before use.
• Use NewWeakGlobalRef for long-lived references and delete them timely.
• Call AttachCurrentThread/DetachCurrentThread only within the ThreadPool.
• For synchronous callbacks, add condition variables.
• Avoid the UI thread for blocking operations like scan().

Key Takeaways

• Storing a C++ object pointer as a long in Java provides full object semantics.
• A single-threaded ThreadPool is critical for stable JNI calls from native threads.
• Weak global references (jweak) + local refs minimize memory leak risks.
• Java wrapper finalization automatically cleans up native resources.
• Asynchronous callbacks are suitable for real-time tasks like network scanning.

— Editorial Team