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

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;}

jboolean isCopy;const char *convertedValue = (env)->GetStringUTFChars(yourJStringParam, &isCopy);std::string string = std::string(convertedValue, length)

This works just fine. Give it a try.

explanation is here:

http://www.club.cc.cmu.edu/~cmccabe/blog_jni_flaws.htmlhttps://developer.android.com/training/articles/perf-jni (Region calls)

jsizestring_j2c(JNIEnv *env, jstring p, char *buffer) {    if (buffer != NULL) {        // Returns the length (the count of Unicode characters) of a        // Java string.        const jsize len = (*env).GetStringLength(p);        // Translates 'len' number of Unicode characters into modified        // UTF-8 encoding and place the result in the given buffer.        (*env).GetStringUTFRegion(p, 0, len, buffer);        // Returns JNI_TRUE when there is a pending exception;        // otherwise, returns JNI_FALSE.        const jboolean is_error = (*env).ExceptionCheck();        if (is_error == JNI_TRUE) {            return -1;        }    }    // Returns the length in bytes of the modified UTF-8    // representation of a string.    const jsize len = (*env).GetStringUTFLength(p);    return len;}const jsize len = string_j2c(env, p, NULL);char buffer[len];const jsize ret = string_j2c(env, p, buffer);if (ret == -1) { // error}else {    __android_log_print(ANDROID_LOG_DEBUG, "Native", "%s", buffer);}