jstring(JNI) to std::string(c++) with utf8 characters

Question

How to convert jstring (JNI) to std::string (c++) with utf8 characters?

this is my code. it worked with non-utf8 characters, but it is wrong with utf8 characters.

std::string jstring2string(JNIEnv *env, jstring jStr){
    const char *cstr = env->GetStringUTFChars(jStr, NULL);
    std::string str = std::string(cstr);
    env->ReleaseStringUTFChars(jStr, str);
    return str;
}

Answer 1

After a lot time to find solution. i was found a way:

In java, a unicode char will be encoded using 2 bytes (utf16). so jstring will container characters utf16. std::string in c++ is essentially a string of bytes, not characters, so if we want to pass jstring from JNI to c++, we have convert utf16 to bytes.

in document JNI functions, we have 2 functions to get string from jstring:

// Returns a pointer to the array of Unicode characters of the string. 
// This pointer is valid until ReleaseStringchars() is called.
const jchar * GetStringChars(JNIEnv *env, jstring string, jboolean *isCopy);


// Returns a pointer to an array of bytes representing the string 
// in modified UTF-8 encoding. This array is valid until it is released 
// by ReleaseStringUTFChars().
const char * GetStringUTFChars(JNIEnv *env, jstring string, jboolean *isCopy);

GetStringUTFChars, it will return a modified utf8.

GetStringChars will return jbyte *, we will read char code from jbytes and convert it to char in c++

this is my solution (worked well with ascii and utf8 characters):

std::string jstring2string(JNIEnv *env, jstring jStr) {
    if (!jStr)
        return "";

    const jclass stringClass = env->GetObjectClass(jStr);
    const jmethodID getBytes = env->GetMethodID(stringClass, "getBytes", "(Ljava/lang/String;)[B");
    const jbyteArray stringJbytes = (jbyteArray) env->CallObjectMethod(jStr, getBytes, env->NewStringUTF("UTF-8"));

    size_t length = (size_t) env->GetArrayLength(stringJbytes);
    jbyte* pBytes = env->GetByteArrayElements(stringJbytes, NULL);

    std::string ret = std::string((char *)pBytes, length);
    env->ReleaseByteArrayElements(stringJbytes, pBytes, JNI_ABORT);

    env->DeleteLocalRef(stringJbytes);
    env->DeleteLocalRef(stringClass);
    return ret;
}