jstring return in JNI program

Question

This is JNI code.

Java code:

public class Sample1 {
 
    public native String stringMethod(String text);
    
    public static void main(String[] args)
    {
       System.loadLibrary("Sample1");
       Sample1 sample = new Sample1();
    
       String  text   = sample.stringMethod("world");
    
       System.out.println("stringMethod: " + text);    
   }
}

Cpp Method for stringMethod function:

JNIEXPORT jstring JNICALL Java_Sample1_stringMethod
   (JNIEnv *env, jobject obj, jstring string) {
   
 const char *name = env->GetStringUTFChars(string, NULL);//Java String to C Style string
 char msg[60] = "Hello ";
 jstring result;

 strcat(msg, name);
 env->ReleaseStringUTFChars(string, name);
 puts(msg);
 result = env->NewStringUTF(msg); // C style string to Java String
 return result;    
 }

When running my java code. I got the result below.

stringMethod: world

But I appended the string "world" with "Hello ". I'm also returning here the appended string. But why I'm getting only "world" not "Hello World". Really I confused with this code. What should I do to get the result with appended string?

The problem is elsewhere, your concatenation is perfectly valid: codepad.org/WWR4LzfV — Pavel Zdenek
– Pavel Zdenek, Commented Dec 11, 2012 at 22:26

user4302170 · Accepted Answer · 2021-03-15 14:47:56Z

23

JNIEXPORT jstring JNICALL Java_Sample1_stringMethod
    (JNIEnv *env, jobject obj, jstring string) 
{    
    const char *name = (*env)->GetStringUTFChars(env,string, NULL);
    char msg[60] = "Hello ";
    jstring result;
    
    strcat(msg, name);  
    (*env)->ReleaseStringUTFChars(env,string, name);   
    puts(msg);            
    result = (*env)->NewStringUTF(env,msg); 
    return result;        
}

edited Mar 15, 2021 at 14:47

user3453226

answered Nov 28, 2014 at 2:44

user4302170

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3 Comments

gokan Over a year ago

Well, the only difference that I see between your code and the author's code is that he is using C++ syntax for JNI and you C syntax for JNI...

nn0p Over a year ago

Should (*env)->NewStringUTF(...) be env->NewStringUTF(...)?

agastalver Over a year ago

@nn0p When using env functions in c, the statement is (*env)->Function(env, parameters). However in c++, this turns to: env->Function(parameters).

PunyCode · Accepted Answer · 2020-01-08 11:54:32Z

0

There are several ways but the best I got by converting const char * to c++ string and then to jbyteArray, and its easy to conversion of byteArray to UTF-8 on java side.

C++ side:

const char* string = propertyValue;
std::string str = string;

jbyteArray array = env->NewByteArray(str.length());
env->SetByteArrayRegion(array,0,str.length(),(jbyte*)str.c_str());


return array;

Java/kotlin side:

String((array), Charset.defaultCharset()))

edited Jan 8, 2020 at 11:54

PunyCode

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answered Dec 16, 2019 at 6:06

vijaycaimi

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