在使用 JNI 将 c++ unordered_map 返回到 java 之前将其转换为 java hashMap

Question

I have a very large c++ unordered_map on android platform I would like to return this as a hashMap in java using JNI.

extern "C" JNIEXPORT jobject JNICALL Java_quantum_QUANTUM_getMessage(JNIEnv *env, jobject thisObj, jdoubleArray bbox) {

    unordered_map<int, Foo> Map;
    Map = getData(filename, bbox);

    // convert Map to java object
    // return java object
}

I am quite new to JNI. I haven't found any helpful material on how to do this.

I tried the approach in the link below but I am getting a lot of compilation errors on this.

how to correctly send a std::map<> from C++ to Java through JNI?

Another suggestions was to pass an empty javaHashmap over JNI and populate it. But I am not sure on how to do that. what is the better solution?

Answer 1

It will require some tweaking. You can do it by creating Map object inside C++ or by passing Map from Java to C++ and filling it there.

Basically, what you need is to:

1. create HashMap

  jclass mapClass = env->FindClass("java/util/HashMap");
  if(mapClass == NULL) {
    return NULL;                  // alternatively, throw exception (recipeNo019)
  }

  jobject hashMap = env->NewObject(mapClass, mapConstructorID);
  if(hashMap == NULL) {
    return NULL;                  // as above
  }

2. making sure to embed your primitive types inside object (eg you can embed int inside Integer )

  jclass integerClass = env->FindClass("java/lang/Integer");
  if(integerClass == NULL) {
    return NULL;                  // alternatively, throw exception (recipeNo019)
  }

  jmethodID integerConstructorID = env->GetMethodID(integerClass, "<init>", "(I)V");
  if(integerConstructorID == NULL) {
    return NULL;
  }

  ...
  ...

  jobject key = env->NewObject(integerClass, integerConstructorID, pair.first);

3. filling HashMap with data

  for (auto const& pair: map) {
    jobject key = env->NewObject(integerClass, integerConstructorID, pair.first);
    jobject value = env->NewObject(integerClass, integerConstructorID, pair.second);
    if(key == NULL || value == NULL) {
      return NULL;
    }
    env->CallObjectMethod(hashMap, putMethodID, key, value);
  }

That's it. You can find full sample code here:

https://github.com/mkowsiak/jnicookbook/tree/master/recipes/recipeNo047 https://github.com/mkowsiak/jnicookbook/tree/master/recipes/recipeNo048