How do I convert jstring to wchar_t *

Question

Let's say that on the C++ side my function takes a variable of type jstring named myString. I can convert it to an ANSI string as follows:

const char* ansiString = env->GetStringUTFChars(myString, 0);

is there a way of getting

const wchar_t* unicodeString = ...

Answer 1

Here is how I converted jstring to LPWSTR.

const char* nativeString = env->GetStringUTFChars(javaString, 0);
size_t size = strlen(nativeString) + 1;
LPWSTR lpwstr = new wchar_t[size];
size_t outSize;
mbstowcs_s(&outSize, lpwstr, size, nativeString, size - 1);

Answer 2

I try to jstring->char->wchar_t

char* js2c(JNIEnv* env, jstring jstr)
{
    char* rtn = NULL;
    jclass clsstring = env->FindClass("java/lang/String");
    jstring strencode = env->NewStringUTF("utf-8");
    jmethodID mid = env->GetMethodID(clsstring, "getBytes", "(Ljava/lang/String;)[B");
    jbyteArray barr = (jbyteArray)env->CallObjectMethod(jstr, mid, strencode);
    jsize alen = env->GetArrayLength(barr);
    jbyte* ba = env->GetByteArrayElements(barr, JNI_FALSE);
    if (alen > 0)
    {
        rtn = (char*)malloc(alen + 1);
        memcpy(rtn, ba, alen);
        rtn[alen] = 0;
    }
    env->ReleaseByteArrayElements(barr, ba, 0);
    return rtn;
}

jstring c2js(JNIEnv* env, const char* str) {
    jstring rtn = 0;
    int slen = strlen(str);
    unsigned short * buffer = 0;
    if (slen == 0)
        rtn = (env)->NewStringUTF(str);
    else {
        int length = MultiByteToWideChar(CP_ACP, 0, (LPCSTR)str, slen, NULL, 0);
        buffer = (unsigned short *)malloc(length * 2 + 1);
        if (MultiByteToWideChar(CP_ACP, 0, (LPCSTR)str, slen, (LPWSTR)buffer, length) > 0)
            rtn = (env)->NewString((jchar*)buffer, length);
        free(buffer);
    }
    return rtn;
}



jstring w2js(JNIEnv *env, wchar_t *src)
{
    size_t len = wcslen(src) + 1;
    size_t converted = 0;
    char *dest;
    dest = (char*)malloc(len * sizeof(char));
    wcstombs_s(&converted, dest, len, src, _TRUNCATE);

    jstring dst = c2js(env, dest);
    return dst;
}

wchar_t *js2w(JNIEnv *env, jstring src) {

    char *dest = js2c(env, src);
    size_t len = strlen(dest) + 1;
    size_t converted = 0;
    wchar_t *dst;
    dst = (wchar_t*)malloc(len * sizeof(wchar_t));
    mbstowcs_s(&converted, dst, len, dest, _TRUNCATE);
    return dst;
}

Answer 3

If this helps someone... I've used this function for an Android project:

std::wstring Java_To_WStr(JNIEnv *env, jstring string)
{
    std::wstring value;

    const jchar *raw = env->GetStringChars(string, 0);
    jsize len = env->GetStringLength(string);
    const jchar *temp = raw;
    while (len > 0)
    {
        value += *(temp++);
        len--;
    }
    env->ReleaseStringChars(string, raw);

    return value;
}

An improved solution could be (Thanks for the feedback):

std::wstring Java_To_WStr(JNIEnv *env, jstring string)
{
    std::wstring value;

    const jchar *raw = env->GetStringChars(string, 0);
    jsize len = env->GetStringLength(string);

    value.assign(raw, raw + len);

    env->ReleaseStringChars(string, raw);

    return value;
}

Answer 4

I know this was asked a year ago, but I don't like the other answers so I'm going to answer anyway. Here's how we do it in our source:

wchar_t * JavaToWSZ(JNIEnv* env, jstring string)
{
    if (string == NULL)
        return NULL;
    int len = env->GetStringLength(string);
    const jchar* raw = env->GetStringChars(string, NULL);
    if (raw == NULL)
        return NULL;

    wchar_t* wsz = new wchar_t[len+1];
    memcpy(wsz, raw, len*2);
    wsz[len] = 0;

    env->ReleaseStringChars(string, raw);

    return wsz;
}

EDIT: This solution works well on platforms where wchar_t is 2 bytes, some platforms have a 4 byte wchar_t in which case this solution will not work.

Answer 5

Rather simple. But do not forget to free the memory by ReleaseStringChars

JNIEXPORT jboolean JNICALL Java_TestClass_test(JNIEnv * env, jobject, jstring string)
{
    const wchar_t * utf16 = (wchar_t *)env->GetStringChars(string, NULL);
    ...
    env->ReleaseStringChars(string, utf16);
}

Answer 6

And who frees wsz? I would recommend STL!

std::wstring JavaToWSZ(JNIEnv* env, jstring string)
{
    std::wstring value;
    if (string == NULL) {
        return value; // empty string
    }
    const jchar* raw = env->GetStringChars(string, NULL);
    if (raw != NULL) {
        jsize len = env->GetStringLength(string);
        value.assign(raw, len);
        env->ReleaseStringChars(string, raw);
    }
    return value;
}

Answer 7

Just use env->GetStringChars(myString, 0); Java pass Unicode by it's nature

Answer 8

JNI has a GetStringChars() function as well. The return type is const jchar*, jchar is 16-bit on win32 so in a way that would be compatible with wchar_t. Not sure if it's real UTF-16 or something else...

Answer 9

A portable and robust solution is to use iconv, with the understanding that you have to know what encoding your system wchar_t uses (UTF-16 on Windows, UTF-32 on many Unix systems, for example).

If you want to minimise your dependency on third-party code, you can also hand-roll your own UTF-8 converter. This is easy if converting to UTF-32, somewhat harder with UTF-16 because you have to handle surrogate pairs too. :-P Also, you must be careful to reject non-shortest forms, or it can open up security bugs in some cases.

Answer 10

If we are not interested in cross platform-ability, in windows you can use the MultiByteToWideChar function, or the helpful macros A2W (ref. example).