How to return string array to java JNI

Question

I want to return finalResult array to java. Any help Appreciated.

extern "C"
JNIEXPORT void JNICALL
Java_com_testjniproject_MainActivity_AnalyseImageDisplay(JNIEnv *env, jobject instance, jlong sourceImageArray)
{
    Mat &srcInputImg = *(Mat*)sourceImageArray;
    vector<tuple<string, string, int>> result = AnalyseImage(srcInputImg);
    enter code here
    string finalResult[3];
    for (int i=0; i < result.size(); i++)
    {
        cout << get<0>(result[i]) << " : " << get<1>(result[i]) << " With confidence " << get<2>(result[i]) << "%" << endl;
        finalResult[i] = get<1>(result[i]);
    }
}

Bruno · Accepted Answer · 2018-09-28 12:06:41Z

6

You can do this :

extern "C" {
JNIEXPORT jobjectArray JNICALL Java_com_testjniproject_MainActivity_AnalyseImageDisplay(
        JNIEnv *env,
        jobject instance,
        jlong sourceImageArray) {
    jobjectArray result;
    result = (jobjectArray)env->NewObjectArray(3,env->FindClass("java/lang/String"),env->NewStringUTF(""));
    for(int i=0; i<3; i++) {
         env->SetObjectArrayElement(result,i,env->NewStringUTF(finalResult[i].c_str()));
    }
    return result;
}
}

edited Sep 28, 2018 at 12:06

answered Sep 28, 2018 at 11:38

Bruno

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10 Comments

Botje Over a year ago

You could pass nullptr or NULL as initial value of the NewObjectArray call instead of creating a dummy string

Krishna Over a year ago

@Bruno Am getting some error as follows - error: no viable conversion from 'std::__ndk1::string' (aka 'basic_string<char, char_traits<char>, allocator<char> >') to 'const char *' env->SetObjectArrayElement(fresult,i,env->NewStringUTF(finalResult[i]))

Botje Over a year ago

use the c_str() method of std::string. Also make sure that your string is actually UTF-8 or ASCII.

Bruno Over a year ago

My bad, I wrote my example to quickly. As @Botje said, use the c_str() method. I've edited my answer

Krishna Over a year ago

My JNI not returning any result though i can run successfully. @Botje How do i convert string array to UTF- 8?

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