JNI converting jstring to char *

Question

I have passed a URL string from Java to C code as jstring data type through the use of JNI. And my library method needs a char * as url.

How can I convert jstring in char * ?

P.S.: Is there any advantage of using jcharArray in C? (i.e. Passing char [] instead of string in native method)

Have you considered consulting the documentation? – user207421 Sep 10 '19 at 02:55 — user207421, Sep 10 '19 at 02:55

score 242 · Accepted Answer · edited Feb 03 '20 at 14:45

242

Here's a a couple of useful link that I found when I started with JNI

http://en.wikipedia.org/wiki/Java_Native_Interface
http://download.oracle.com/javase/1.5.0/docs/guide/jni/spec/functions.html

concerning your problem you can use this

JNIEXPORT void JNICALL Java_ClassName_MethodName(JNIEnv *env, jobject obj, jstring javaString)   
{
   const char *nativeString = env->GetStringUTFChars(javaString, 0);

   // use your string

   env->ReleaseStringUTFChars(javaString, nativeString);
}

edited Feb 03 '20 at 14:45

Stoica Mircea

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answered Nov 15 '10 at 06:35

Jason Rogers

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is it necessary to keep `nativeString` constant? – Prasham Nov 15 '10 at 07:07
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if you check the second link, the prototype of the function GetStringUTFChars is: const jbyte* GetStringUTFChars(JNIEnv *env, jstring string, jboolean *isCopy); so you don't really have a choise – Jason Rogers Nov 15 '10 at 07:26
no problems, I'm getting to the end of my project with JNI but I know how much I struggled to start with – Jason Rogers Nov 15 '10 at 09:22
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I think it's worth noting that the technique outlined here (and on the Wikipedia page) uses modified UTF-8 encoding, which may not work in all situations. See http://developer.android.com/guide/practices/jni.html#UTF_8_and_UTF_16_strings – cqcallaw Oct 01 '12 at 01:59
I think this approach works only in the case when all characters are encoded with 1 byte. But an UTF8 string can have characters encoded with 2 and 4 bytes too. – WindRider Dec 05 '13 at 20:05
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Thanks, that work but interestingly const char* can not convert into char* :). – CoDe Dec 04 '15 at 13:10
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@CoDe Technically, it could with some magic, but that is best avoided; it is probably declared `const` for a reason. The correct response to your problem is to `strcpy` from the `const char*` into a `char*`. That should provide you with the mutable `char*` that you want. – Loduwijk Jun 20 '17 at 20:44

score 53 · Answer 2 · edited Jan 17 '19 at 19:17

53

Thanks Jason Rogers's answer first.

In Android && cpp should be this:

const char *nativeString = env->GetStringUTFChars(javaString, nullptr);

// use your string

env->ReleaseStringUTFChars(javaString, nativeString);

Can fix this errors:

1.error: base operand of '->' has non-pointer type 'JNIEnv {aka _JNIEnv}'

2.error: no matching function for call to '_JNIEnv::GetStringUTFChars(JNIEnv*&, _jstring*&, bool)'

3.error: no matching function for call to '_JNIEnv::ReleaseStringUTFChars(JNIEnv*&, _jstring*&, char const*&)'

4.add "env->DeleteLocalRef(nativeString);" at end.

edited Jan 17 '19 at 19:17

Dark

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answered Apr 17 '14 at 07:06

kangear

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You need to have (*env)-> instead of env-> – AlexanderNajafi Feb 15 '17 at 13:47
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You only need `(*env)->` if you are using C, not C++. – sixones Jul 11 '18 at 11:28
how to convert string to sha256 i am unable to do this pls help new in ndk android – Sunil Chaudhary Dec 19 '20 at 08:31

JNI converting jstring to char *

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