如何通过引用将列表从 Java 传递给 JNI C++ 作为向量？

Question

I am fresh in C++ and JNI

Flow should be like this

From Java I pass a empty List to JNI, JNI invoke method loadData(std::vector<int>) from MyClass and this method fill my list with a data.

So, question is

I have

class MyClass {
public:
MyClass();
~MyClass();

void loadData(std::vector<int> & vector);
};


void MyClass::loadData(std::vector<int> & vector)
{
const int size = 10;

vector.resize(size);

for (int i = 0; i < size; ++i) {
    vector.push_back(4);
}
}

This is my method that I wrote in pure C++ and now I need to use it from Java like this

public native void fillListWithData(List<Integer> list);

So, I wrote such method in JNI to associate them

extern "C" JNIEXPORT void JNICALL
Java_com_google_ar_core_examples_java
_helloar_HelloArActivity_fillListWithData(
    JNIEnv *env,
    jobject /* this */,
    jobject input
) {
myClass->loadData("HERE I NEED TO PASS MY " input);
}

And here how I should to invoke this method

public void TEST(){
    List<Integer> list = new ArrayList<>();
    fillListWithData(list);
    Log.e("TAG", "HERE I NEED TO HAVE A LIST WITH FILLED DATA");
}

I can't understand how to pass this list by reference by JNI to C++...

Any ideas appreciate

Answer 1

In this case, it's quite simple. All you have to do is to pass List to your native code and fill it inside JNI part using JNI based access methods

#include <vector>
#include "jni.h"
#include "recipeNo046_FillTheList.h"
using namespace std;

JNIEXPORT void JNICALL Java_recipeNo046_FillTheList_fillTheList
  (JNIEnv *env, jclass cls, jobject obj) {

  vector<int> vect { 1, 2, 3 };

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

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

  jmethodID addMethodID = env->GetMethodID(listClass, "add", "(Ljava/lang/Object;)Z");
  if(addMethodID == NULL) {
    return;                  //                 - || -
  }

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

  for(int i : vect) {
    // Now, we have object created by Integer(i)
    jobject integerValue = env->NewObject(integerClass, integerConstructorID, i);
    if(integerValue == NULL) {
      return;
    }
    env->CallBooleanMethod(obj, addMethodID, integerValue);
  }

  env->DeleteLocalRef(listClass);
  env->DeleteLocalRef(integerClass);

}

Note that you don't have to create List object inside JNI as you already have it there - inside C++ code. It is passed as argument of native method.

You can find full sample code here:

https://github.com/mkowsiak/jnicookbook/tree/master/recipes/recipeNo046

Once you run the code, you can see data from C++ being passed through the List object.

> make test
/Library/Java/JavaVirtualMachines/jdk-12.0.1.jdk/Contents/Home/bin/java -Djava.library.path=:./lib -cp target recipeNo046.FillTheList
library: :./lib
1
2
3

Answer 2

The answer mentioned by @Oo.oO may seem to work in Windows 10, But is not in the case with Windows 8.
Windows 8 does'nt support any containers from Cpp Libraries to work with.
The Reported Error would be - " %1 is not a valid Win32 Application "
This occurs while running the Java Program.
The dll file generated here seem to be the issue.