java数组和jni数组对应关系
kotlin jni
IntArray jintArray
DoubleArray jdoubleArray
LongArray jlongArray
CharArray jcharArray
ShortArray jshortArray
Array jobjectArray 
  //kotlin
  external fun arrayInt(array: IntArray): String
  external fun arrayDouble(array: DoubleArray): String
  external fun arrayLong(array: LongArray): String
  external fun arrayChar(array: CharArray): String
  external fun arrayShort(array: ShortArray): String
  external fun arrayString(array: Array<String>): String

//jni
extern "C"
JNIEXPORT jstring JNICALL
Java_com_example_myapplication_MainActivity_arrayInt(JNIEnv *env, jobject thiz, jintArray array) {
    // TODO: implement arrayInt()
}

extern "C"
JNIEXPORT jstring JNICALL
Java_com_example_myapplication_MainActivity_arrayDouble(JNIEnv *env, jobject thiz,
                                                        jdoubleArray array) {
    // TODO: implement arrayDouble()
}

extern "C"
JNIEXPORT jstring JNICALL
Java_com_example_myapplication_MainActivity_arrayLong(JNIEnv *env, jobject thiz, jlongArray array) {
    // TODO: implement arrayLong()
}

extern "C"
JNIEXPORT jstring JNICALL
Java_com_example_myapplication_MainActivity_arrayChar(JNIEnv *env, jobject thiz, jcharArray array) {
    // TODO: implement arrayChar()
}

extern "C"
JNIEXPORT jstring JNICALL
Java_com_example_myapplication_MainActivity_arrayShort(JNIEnv *env, jobject thiz,
                                                       jshortArray array) {
    // TODO: implement arrayShort()
}

extern "C"
JNIEXPORT jstring JNICALL
Java_com_example_myapplication_MainActivity_arrayString(JNIEnv *env, jobject thiz,
                                                        jobjectArray array) {
    // TODO: implement arrayString()
}
jobjectArray 实战

获取数组长度

class MainActivity : AppCompatActivity() {

    private lateinit var binding: ActivityMainBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        val array = arrayOf("first", "two")
        binding.sampleText.text = "长度为：" + arrayLength(array)

    }

    /**
     * A native method that is implemented by the 'native-lib' native library,
     * which is packaged with this application.
     */
    external fun arrayLength(array: Array<String>): Int

    companion object {
        // Used to load the 'native-lib' library on application startup.
        init {
            System.loadLibrary("native-lib")
        }
    }
}

native-lib.cpp

extern "C"
JNIEXPORT jint JNICALL
Java_com_example_myapplication_MainActivity_arrayLength(JNIEnv *env, jobject thiz,
                                                        jobjectArray array) {
    //获取数组长度
    int length = env->GetArrayLength(array);
    return length;
}

获取数组第一个元素

lass MainActivity : AppCompatActivity() {

    private lateinit var binding: ActivityMainBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        val array = arrayOf("first", "two")
        binding.sampleText.text = "第一个元素：" + arrayFirst(array)

    }

    /**
     * A native method that is implemented by the 'native-lib' native library,
     * which is packaged with this application.
     */
    external fun arrayFirst(array: Array<String>): String

    companion object {
        // Used to load the 'native-lib' library on application startup.
        init {
            System.loadLibrary("native-lib")
        }
    }
}

native-lib.cpp

extern "C"
JNIEXPORT jstring JNICALL
Java_com_example_myapplication_MainActivity_arrayFirst(JNIEnv *env, jobject thiz,
                                                       jobjectArray array) {
    //获取array第一个元素
    jobject first = env->GetObjectArrayElement(array, 0);
    
    //把jobject强转成jstring
    return static_cast<jstring>(first);
}
jintArray 实战

获取第一个元素

class MainActivity : AppCompatActivity() {

    private lateinit var binding: ActivityMainBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        val array = intArrayOf(1, 2, 3)
        binding.sampleText.text = "第一个元素：" + arrayInt(array)

    }

    /**
     * A native method that is implemented by the 'native-lib' native library,
     * which is packaged with this application.
     */
    external fun arrayInt(array: IntArray): Int

    companion object {
        // Used to load the 'native-lib' library on application startup.
        init {
            System.loadLibrary("native-lib")
        }
    }
}

native-lib.cpp


extern "C"
JNIEXPORT jint JNICALL
Java_com_example_myapplication_MainActivity_arrayInt(JNIEnv *env, jobject thiz, jintArray array) {
    jint *result = env->GetIntArrayElements(array, JNI_FALSE);
    //获取第一个元素
    return result[0];
}