JNI with C++ structures

[John Coleman]

Hi,

I want to call a function in C++ that returns a pointer to a structure array, like this

BOOL __stdcall GetPositionBlotter(int* iCount, void* RecArray);

Where RecArray is elements like this

typedef struct _POSITIONREC {
char SymIdent[20];
char SymExchange[15];
char SymSecType[20];
char SymExpiry[30];
char SymRight[30];
char SymCurrency[20];
DWORD Quantity;
double MarketPrice;
double MarketValue;
double AverageCost;
double UnrealizedPNL;
double RealizedPNL;
} POSITIONREC, *LPPOSITIONREC;
;

How do I get that C++ structure back to Java? I suppose I need an array of a class? I guess somehow I need to instantiate those classes in C++ and load the fields in a loop and pass them back as the result.

Can anyone provide simple example code of how I do that please?

John

[Jean-Francois Briere]

Very simple but complete working example. Study it thoroughly:

// 
// Main.java
// 
public class Main {
    static {
        System.loadLibrary("Main");
    }
    
    public static void main(String[] args) {
        new Main().exec();
    }
    
    void exec() {
        PositionRec[] posRecArray = getPositionBlotter();
        
        if (posRecArray != null) {
            for (int i = 0; i < posRecArray.length; i++)
                System.out.println(posRecArray[i]);
        }
    }
    
    native PositionRec[] getPositionBlotter();
}
 
// 
// PositionRec.java
// 
public class PositionRec {
    public String symIdent;
    public int quantity;
    public double marketPrice;
    public String toString() {
        return "[" + symIdent + "," + quantity + "," + marketPrice + "]";
    }
}
 
// 
// Main.h
// 
#ifndef _Included_Main
#define _Included_Main
 
#include <jni.h>
 
#ifdef __cplusplus
extern "C" {
#endif
 
/*
 * Class:     Main
 * Method:    getPositionBlotter
 * Signature: ()[LPositionRec;
 */
JNIEXPORT jobjectArray JNICALL Java_Main_getPositionBlotter(JNIEnv * env, jobject obj);
 
#ifdef __cplusplus
}
#endif
 
#endif /* _Included_Main */
 
// 
// Main.cpp
// 
#include <windows.h>
#include "Main.h"
 
typedef struct _POSITIONREC {
    char SymIdent[20];
    DWORD Quantity;
    double MarketPrice;
} POSITIONREC, * LPPOSITIONREC;
 
BOOL __stdcall GetPositionBlotter(int * iCount, void * recArray) {
    LPPOSITIONREC pPosRec = new POSITIONREC[2];
    strcpy(pPosRec[0].SymIdent, "Hello");
    pPosRec[0].Quantity = 1;
    pPosRec[0].MarketPrice = 2.0;
    strcpy(pPosRec[1].SymIdent, "World!");
    pPosRec[1].Quantity = 3;
    pPosRec[1].MarketPrice = 4.0;
    *iCount = 2;
    *((LPPOSITIONREC *)recArray) = pPosRec;
    return TRUE;
}
 
typedef struct _JNI_POSREC {
    jclass cls;
    jmethodID ctorID;
    jfieldID symIdentID;
    jfieldID quantityID;
    jfieldID marketPriceID;
} JNI_POSREC;
 
JNI_POSREC * jniPosRec = NULL;
 
void loadJniPosRec(JNIEnv * env) {
    if (jniPosRec != NULL)
        return;
    jniPosRec = new JNI_POSREC;
    jniPosRec->cls = env->FindClass("PositionRec");
    jniPosRec->ctorID = env->GetMethodID(jniPosRec->cls, "<init>", "()V");
    jniPosRec->symIdentID = env->GetFieldID(jniPosRec->cls, "symIdent", "Ljava/lang/String;");
    jniPosRec->quantityID = env->GetFieldID(jniPosRec->cls, "quantity", "I");
    jniPosRec->marketPriceID = env->GetFieldID(jniPosRec->cls, "marketPrice", "D");
}
 
void fillJavaPosRecValues(JNIEnv * env, jobject jPosRec, POSITIONREC cPosRec) {
    env->SetObjectField(jPosRec, jniPosRec->symIdentID, env->NewStringUTF(cPosRec.SymIdent));
    env->SetIntField(jPosRec, jniPosRec->quantityID, cPosRec.Quantity);
    env->SetDoubleField(jPosRec, jniPosRec->marketPriceID, cPosRec.MarketPrice);
}
 
/*
 * Class:     Main
 * Method:    getPositionBlotter
 * Signature: ()[LPositionRec;
 */
JNIEXPORT jobjectArray JNICALL Java_Main_getPositionBlotter(JNIEnv * env, jobject obj) {
    loadJniPosRec(env);
    
    LPPOSITIONREC pPosRec;
    int count;
    
    if (!GetPositionBlotter(&count, (void *)&pPosRec))
        return NULL;
    
    jobjectArray jPosRecArray = env->NewObjectArray(count, jniPosRec->cls, NULL);
    
    for (int i = 0; i < count; i++) {
        jobject jPosRec = env->NewObject(jniPosRec->cls, jniPosRec->ctorID);
        
        fillJavaPosRecValues(env, jPosRec, pPosRec[i]);
        env->SetObjectArrayElement(jPosRecArray, i, jPosRec);
    }
    
    return jPosRecArray;
}

Regards