如何使用JNI传递和接收对象 [英] How to pass and receive objects using JNI

问题描述

我有一个 JAVA 应用程序，因为我想将对象作为参数传递给 C 代码使用 JNI 再次，我想从 C 代码中的对象收到 JAVA 使用 JNI 。

I have an JAVA application in that I want to pass the object as a parameter to the C code using JNI and again I want to receive the object from the C code to JAVA using JNI.

在JAVA方面，我只是创建一个应用程序并将其传递给该方法如下所示

In JAVA side i have simply created an application and pass it to the method as shown bellow

JlibFprint.fp_image_data fpimg = new JlibFprint.fp_image_data();   //object to be pass  
 //fp_image_data is the static inner class of the class JlibFprint

JlibFprint.fp_image_data fpimg1 = new JlibFprint.fp_image_data();   //received object

此对象传递给方法，如

fpimg1 = JlibFprint.binary_image(fpimg);

该方法的JNI代码如下所示：

And JNI code for the method is as shown bellow :

JNIEXPORT jobject JNICALL Java_jlibfprint_JlibFprint_binary_1image(JNIEnv *env, jclass jcls,jobject imgobj)
{
    struct fp_img img;
    struct fp_img *imgptr;
    imgptr = &img;
    jfp2cfp(env,imgobj,imgptr);     
    fp_init();
    imgptr = fp_img_binarize(imgptr);
    cfp2jfp(env, imgobj, imgptr);
    fp_exit();
    return imgobj;
}

void jfp2cfp(JNIEnv* env, jobject obj, fp_img *fpd)
{
    /* Determines all the fields of the object */
    jclass fpClass = env->FindClass("jlibfprint/JlibFprint$fp_image_data");
            jfieldID height;
            jfieldID width;
            jfieldID length;
            jfieldID data;
            jbyteArray dataArray;

            height = env->GetFieldID(fpClass, "height", "I");
            width = env->GetFieldID(fpClass, "width", "I");
            length = env->GetFieldID(fpClass, "length", "I");
            data = env->GetFieldID(fpClass, "data", "[B");

         /* Starts to fill fpd */
       fpd->height = env->GetIntField(obj, height);
        fpd->width = env->GetIntField(obj, width);
        fpd->length = env->GetIntField(obj, length);
        printf("\n height :%d",fpd->height);
        printf("\n width  :%d",fpd->width);
        printf("\n length :%d",fpd->length);
        dataArray = static_cast<jbyteArray>(env->GetObjectField(obj, data));
        env->GetByteArrayRegion(dataArray, 0, FP_PRINT_DATA_DATA_SIZE, (jbyte*)fpd->data);

}

void cfp2jfp(JNIEnv* env, jobject obj, fp_img* fpd)
{
    /* Determines all the fields of the object */
    jclass fpClass = env->FindClass("jlibfprint/JlibFprint$fp_image_data");
        jfieldID height;
        jfieldID width;
        jfieldID length;
        jfieldID data;

        jbyteArray dataArray;
        height = env->GetFieldID(fpClass, "height", "I");
        width = env->GetFieldID(fpClass, "width", "I");
        length = env->GetFieldID(fpClass, "length", "I");
        data = env->GetFieldID(fpClass, "data", "[B");

        /* Starts to fill the obj */
        env->SetIntField(obj, height, fpd->height);
        env->SetIntField(obj, width, fpd->width);
        env->SetIntField(obj, length, fpd->length);

        dataArray = env->NewByteArray(FP_PRINT_DATA_DATA_SIZE);
        env->SetByteArrayRegion(dataArray, 0, FP_PRINT_DATA_DATA_SIZE, (jbyte*)fpd->data);

        env->SetObjectField(obj, data, dataArray);
}

但是，从JAVA方面的JNI代码的这些函数返回后，该方法显示例外如

But after returning from these functions of JNI code at the JAVA side the method shows the exception like

java.lang.ArrayIndexOutOfBoundsException
    at jlibfprint.JlibFprint.binary_image(Native Method)
    at jlibfprint.SampleRun.main(SampleRun.java:96)

即对象不正确处理，不会从JNI层返回任何东西。
但是我没有得到JNI代码中应该改变的内容，以便它返回正确的对象。

i.e objects are not process correctly and it does not return any thing from the JNI layer. But I am not getting what should I change in the JNI code so that it will return the correct object.

请给我建议任何解决方案。 >

Please suggest me any solution.

推荐答案

GetByteArrayRegion（）和 SetByteArrayRegion（） / code>可以抛出ArrayIndexOutOfBoundsException，如果指定的区域超出数组边界。

Both GetByteArrayRegion() and SetByteArrayRegion() can throw ArrayIndexOutOfBoundsException if the specified region goes outside the bounds of the array.

您对SetByteArrayRegion（）的调用看起来是正确的 - 立即创建所需大小的数组。

Your call to SetByteArrayRegion() looks correct -- it immediately follows creation of the array of the required size.

验证由GetByteArrayRegion（）访​​问的条目数组的大小至少为FP_PRINT_DATA_DATA_SIZE。

Verify that the size of the array on entry, accessed by GetByteArrayRegion(), is at least FP_PRINT_DATA_DATA_SIZE.

顺便说一下，另一种方法是使用 GetByteArrayElements（）和 ReleaseByteArrayElements（），其中可能返回指向Java使用的相同内存的指针数组。

By the way, an alternative is to use GetByteArrayElements() and ReleaseByteArrayElements(), which may return a pointer to the same memory Java uses for the Array.

这篇关于如何使用JNI传递和接收对象的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！