将字符串数组从Java传递到C ++

Question

so I have String resultNative[] in Java and char result[][] in C++, how can I pass array from java to c++ using JNI? I have tried the following code but error. thank you.

JNIEXPORT void JNICALL Java_com_qualcomm_QCARSamples_ImageTargets_GetTargetData_returnResult(JNIEnv *env, jobject obj) {
int size = env->GetArrayLength(resultNative);

for (int i=0; i < size; ++i)
{
    jstring string = env->GetObjectArrayElement(resultNative, i);
    result[i] = env->GetStringUTFChars(string, i);
    env->ReleaseStringUTFChars(string, resultNative);
    env->DeleteLocalRef(string);
}

}

Answer 1

I think you should make a copy of what GetStringUtf returns before releasing it. Otherwise you store a pointer to a freed memory and perhaps access it later on. Using std:string is the right way to do it in c++, but you could use strdup if you really insist.

If result is an array of std:string, this is as simple as result[i] = std:string(env->GetStringUTFChars(string, i));

Answer 2

Your code is missing a parameter. Also, you're releasing the string before you even use it. Assigning it to result[i] just makes a shallow copy (copy of the pointer, not the contents). After releasing it, result[i] becomes invalid.

JNIEXPORT void JNICALL Java_com_qualcomm_QCARSamples_ImageTargets_GetTargetData_returnResult(JNIEnv *env, jobject obj, jobject stringArray)
{
    int length = env->GetArrayLength(stringArray);

    for (int i = 0; i < length; ++i)
    {
        jstring jstr = (jstring) env->GetObjectArrayElement(stringArray, i);
        const char* cpp_string = env->GetStringUTFChars(jstr, 0);

        std::cout<<cpp_string<<"\n";

        env->ReleaseStringUTFChars(jstr, cpp_string);
        env->DeleteLocalRef(jstr);
    }
}

OR if you want to save the string then:

JNIEXPORT void JNICALL Java_com_qualcomm_QCARSamples_ImageTargets_GetTargetData_returnResult(JNIEnv *env, jobject obj, jobject stringArray)
{
    int length = env->GetArrayLength(stringArray);

    std::vector<std::string> array_of_str(length);

    for (int i = 0; i < length; ++i)
    {
        jstring jstr = (jstring) env->GetObjectArrayElement(stringArray, i);
        const char* cpp_string = env->GetStringUTFChars(jstr, 0);

        array_of_str.emplace_back(cpp_string);

        env->ReleaseStringUTFChars(jstr, cpp_string);
        env->DeleteLocalRef(jstr);
    }
}