Pass, return and convert to vectors list of lists over JNI

c++, java, java-native-interface, list, stdvector

Solution

I solved this problem with standard tools.

- Create in Java class as objects (O) container (C)

- Pass array of objects (O) from Java code to native part

- Create from array vector in C++ code

- Calculate new vectors

- Build array of containers (C) and insert into objects (O)

- Return array of containers (C)

Code implement:

On java part:

1 - Create array from list of points

On c++ part:

2 - build input vector

std::vector<CurvePoint> src_line;

jclass java_points_cls = env->FindClass("myPointClass");
jmethodID java_mid = env->GetMethodID(java_points_cls, "<init>", "(II)V");    
jfieldID fidX = env->GetFieldID(java_points_cls, "x", "I");
jfieldID fidY = env->GetFieldID(java_points_cls, "y", "I");

int srcCount = env->GetArrayLength(srcLines);

for (int i=0; i < srcCount; i++) 
{
    jobject cur_pnt =  env->GetObjectArrayElement(srcLines, i); 

    LinePoint src_point;        

    src_point.x = env->GetIntField(cur_pnt, fidX); 
    src_point.y = env->GetIntField(cur_pnt, fidY);    

    src_line.push_back(src_point);
}

3 - calculation `lines`

4 - build output array

jclass java_line_cls = env->FindClass("myLinesClass");

jmethodID java_line_add = env->GetMethodID(java_line_cls, "addPoint", "(II)V");  
jmethodID java_line_init = env->GetMethodID(java_line_cls, "<init>", "()V");

jobjectArray resLines = (jobjectArray) env->NewObjectArray(lines.size(),     java_line_cls, 0); 

for(int i = 0; i < lines.size(); ++i)
{
    jobject cur_line =  env->NewObject(java_line_cls, java_line_init);
    for(int j = 0; j < lines[i].size(); ++j)
        env->CallVoidMethod(cur_line, java_line_add, 
                                lines[i][j].x,
                                lines[i][j].y);
    env->SetObjectArrayElement(resLines, i, cur_line);
}

return resLines;

Java part

5 - Create list of lines from returned array

Problem

I need to pass from Java ``` List< List<MyPoint> > points; ``` over jni to C++ and convert to ``` std::vector< std::vector<MyPoint> > ``` Process this vectors and return ``` List< List<MyPoint> > ``` - How correct pass and return list of lists? - How convert list of lists of objects in vector of vectors of objects and backward?

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