Store a c++ object instance inside JNI jobject and retrieve later

Question

I have a JNI class with methods init() work(), and cleanup(). On the C++ side I create an instance of a C++ class Foo during init(), then call some methods on it during work(), and finally delete it inside cleanup(). Right now I store instance of Foo as a global singleton on the C++ so that I can retrieve it from the different JNI calls. What I would really like to do is store a pointer to the Foo instance inside the jobject instance that gets passed to each JNI call, so that I can avoid having a global singleton and also so that I can support multiple instances of Foo. Is something like this possible?

Answer 1

You can store a pointer to your C++ object as a Java class member. For example, in Java:

class Foo
{
    public long ptr = 0;

    public native void init();
    public native void work();
    public native void cleanup();
}

And in C++:

jfieldID getPtrFieldId(JNIEnv * env, jobject obj)
{
    static jfieldID ptrFieldId = 0;

    if (!ptrFieldId)
    {
        jclass c = env->GetObjectClass(obj);
        ptrFieldId = env->GetFieldID(c, "ptr", "J");
        env->DeleteLocalRef(c);
    }

    return ptrFieldId;
}

class Foo
{
    /* ... */
};

extern "C"
{
    void Java_Foo_init(JNIEnv * env, jobject obj)
    {
        env->SetLongField(obj, getPtrFieldId(env, obj), (jlong) new Foo);
    }

    void Java_Foo_work(JNIEnv * env, jobject obj)
    {
        Foo * foo = (Foo *) env->GetLongField(obj, getPtrFieldId(env, obj));

        foo->work();
    }

    void Java_Foo_cleanup(JNIEnv * env, jobject obj)
    {
        Foo * foo = (Foo *) env->GetLongField(obj, getPtrFieldId(env, obj));

        delete foo;
    }
}