Question

我正在尝试使用android NDK。

有没有办法将在JNI中创建的数组（在我的情况下为int[]）返回给Java？如果是这样，请提供一个JNI函数的快速示例。

-Thanks

Answer 1

如果您已经检查了文档，但仍然有问题应该是您的初始问题的一部分。在这种情况下，示例中的JNI函数会创建许多数组。外部数组由使用JNI函数NewObjectArray()创建的“对象”数组组成。从JNI的角度来看，这是一个二维数组，是一个包含许多其他内部数组的对象数组。

以下for循环使用JNI函数NewIntArray()创建int []类型的内部数组。如果您只想返回一维的单维数组，那么NewIntArray()函数就是您用来创建返回值的函数。如果您想创建一个字符串的单维数组，那么您将使用NewObjectArray()函数，但该类具有不同的参数。

因为你想返回一个int数组，所以你的代码看起来像这样：

JNIEXPORT jintArray JNICALL Java_ArrayTest_initIntArray(JNIEnv *env, jclass cls, int size)
{
 jintArray result;
 result = (*env)->NewIntArray(env, size);
 if (result == NULL) {
     return NULL; /* out of memory error thrown */
 }
 int i;
 // fill a temp structure to use to populate the java int array
 jint fill[size];
 for (i = 0; i < size; i++) {
     fill[i] = 0; // put whatever logic you want to populate the values here.
 }
 // move from the temp structure to the java structure
 (*env)->SetIntArrayRegion(env, result, 0, size, fill);
 return result;
}

Answer 2

如果有人想知道如何返回String []数组：

java代码

private native String[] data();

原生出口

JNIEXPORT jobjectArray JNICALL Java_example_data() (JNIEnv *, jobject);

原生代码

  JNIEXPORT jobjectArray JNICALL   
               Java_example_data  
  (JNIEnv *env, jobject jobj){  

    jobjectArray ret;  
    int i;  

    char *message[5]= {"first",   
                       "second",   
                       "third",   
                       "fourth",   
                       "fifth"};  

    ret= (jobjectArray)env->NewObjectArray(5,  
         env->FindClass("java/lang/String"),  
         env->NewStringUTF(""));  

    for(i=0;i<5;i++) {  
        env->SetObjectArrayElement(  
        ret,i,env->NewStringUTF(message[i]));  
    }  
    return(ret);  
  }

来自链接： http://www.coderanch.com/t/326467/java/java/Returning-String-array-program-Java

Answer 3

基于提出的问题，这已经在第一个答案中得到了解释，即我们如何通过jobjectArray传递int []。但是这是一个示例，我们如何返回包含数据列表的jobjectArray。例如，这在某些情况下很有用：当某人需要返回2D格式的数据以绘制带有x和y点的线时。以下示例显示jobjectArray如何以以下格式返回数据：

JNI的Java输入：
数组[{Arraylist个x个浮点数] [Arraylist个y个浮点数]

JNI输出到Java：
x个浮点中的jobjectArray [Arraylist个，y个浮点中的[Arraylist个]

    extern "C" JNIEXPORT jobjectArray JNICALL
        _MainActivity_callOpenCVFn(
                JNIEnv *env, jobject /* this */,
                jobjectArray list) {

         //Finding arrayList class and float class(2 lists , one x and another is y)
            static jclass arrayListCls = static_cast<jclass>(env->NewGlobalRef(env->FindClass("java/util/ArrayList")));
            jclass floatCls = env->FindClass("java/lang/Float");
         //env initialization of list object and float
            static jmethodID listConstructor = env->GetMethodID(arrayListCls, "<init>", "(I)V");
            jmethodID alGetId  = env->GetMethodID(arrayListCls, "get", "(I)Ljava/lang/Object;");
            jmethodID alSizeId = env->GetMethodID(arrayListCls, "size", "()I");
            static jmethodID addElementToList = env->GetMethodID(arrayListCls, "add", "(Ljava/lang/Object;)Z");

            jmethodID floatConstructor = env->GetMethodID( floatCls, "<init>", "(F)V");
            jmethodID floatId = env->GetMethodID(floatCls,"floatValue", "()F");


        //null check(if null then return)
        if (arrayListCls == nullptr || floatCls == nullptr) {
            return 0;
        }

    //     Get the value of each Float list object in the array
        jsize length = env->GetArrayLength(list);

        //If empty
        if (length < 1) {
            env->DeleteLocalRef(arrayListCls);
            env->DeleteLocalRef(floatCls);
            return 0;
        }

// Creating an output jObjectArray
    jobjectArray outJNIArray = env->NewObjectArray(length, arrayListCls, 0);

        //taking list of X and Y points object at the time of return
        jobject  xPoint,yPoint,xReturnObject,yReturnObject;

            //getting the xList,yList object from the array
            jobject xObjFloatList = env->GetObjectArrayElement(list, 0);
            jobject yObjFloatList = env->GetObjectArrayElement(list, 1);


     // number of elements present in the array object
        int xPointCounts = static_cast<int>(env->CallIntMethod(xObjFloatList, alSizeId));

        static jfloat xReturn, yReturn;
                jobject xReturnArrayList = env->NewObject(arrayListCls,listConstructor,0);
        jobject yReturnArrayList = env->NewObject(arrayListCls,listConstructor,0);

    for (int j = 0; j < xPointCounts; j++) {
            //Getting the x points from the x object list in the array
            xPoint = env->CallObjectMethod(xObjFloatList, alGetId, j);
            //Getting the y points from the y object list in the array
            yPoint = env->CallObjectMethod(yObjFloatList, alGetId, j);

//Returning jobjectArray(Here I am returning the same x and points I am receiving from java side, just to show how to make the returning `jobjectArray`)  

            //float x and y values
            xReturn =static_cast<jfloat >(env->CallFloatMethod(xPoint, floatId,j));
            yReturn =static_cast<jfloat >(env->CallFloatMethod(yPoint, floatId,j));


            xReturnObject = env->NewObject(floatCls,floatConstructor,xReturn);
             yReturnObject = env->NewObject(floatCls,floatConstructor,yReturn);

            env->CallBooleanMethod(xReturnArrayList,addElementToList,xReturnObject);


            env->CallBooleanMethod(yReturnArrayList,addElementToList,yReturnObject);
            env->SetObjectArrayElement(outJNIArray,0,xReturnArrayList);
            env->SetObjectArrayElement(outJNIArray,1,yReturnArrayList);
        __android_log_print(ANDROID_LOG_ERROR, "List of X and Y are saved in the array","%d", 3);

    }

    return outJNIArray;

Answer 4

简单的解决方案是将数组数据从C中写入文件，然后从Java中访问文件

如何将数组从JNI返回到Java？

4 个答案: