How to call Java method from C++ (java returns String[])?

Question

As far as I know I can't call method from C++ that will return from Java string array, so only solution for this is loop call method that will return array elements on by one, but how can I store them in C++ array?

In Java I have method:

public static String getData(int index){ return arry[index]; }

and in C++:

char * args[10];
for (int i = 0; i < arrayLength; i ++) {
    jmethodID mid = env->GetStaticMethodID(INF.cls_JSox, "getData",(I)Ljava/lang/String;");
    jobject result = env->CallStaticObjectMethod(cls, mid, num, i);
    const char *nativeString = env->GetStringUTFChars( (jstring) result, 0);
    const size_t len = strlen(nativeString);
    cout << "copying... \n";
    strncpy(args[i], nativeString, len);
    env->ReleaseStringUTFChars( (jstring)result, nativeString);
}

but when i = 1 I'm getting memory violation error in Java. How can I correctly copy data from received strings to the char * args[10]? The length of every string passed from Java is about 3-5 chars.

It's absolutely possible to use JNI to call a Java method that returns a string array. — Stuart Cook
– Stuart Cook, Commented Sep 24, 2011 at 14:57
Does 'INF.cls_JSox' in 'GetStaticMethodID' refer to the same class as 'cls' in CallStaticObjectMethod ? — bobby
– bobby, Commented Sep 26, 2011 at 6:11

Andrey Starodubtsev · Accepted Answer · 2018-10-28 10:15:31Z

Assuming that the C++ code snippet you posted is complete, you are getting an access violation because you need to allocate args[i] before copying a value into it - a args[i] = new char[ len + 1 ] would do.

You can actually call a method from C++ that returns a Java string array, assume your method was:

public static String[] getData() { return array; }

Then on the native side you should be able to do something like:

jmethodID method = env->GetStaticMethodID( cls, "getData", "()[Ljava/lang/String;" );
jarray    data   = (jarray) env->CallStaticObjectMethod( cls, method );

// assumption: the result of getData() is never null
jsize const length = env->GetArrayLength( data );

// assumption: the String[] is always of length > 0
char** args = new char*[ length ];

for( jsize index(0); index < length; ++index )
{
    jstring element = (jstring) env->GetObjectArrayElement( data, index );

    // assumption: there are no null strings in the array
    char  const* nativeString = env->GetStringUTFChars( element, 0 );
    jsize const  nativeLength = strlen( nativeString );

    args[index] = new char[ nativeLength + 1 ];
    strncpy( args[index], nativeString, nativeLength );

    env->ReleaseStringUTFChars( element, nativeString );
    env->DeleteLocalRef( element );
}

I haven't tried to compile the above snippet so there might be an error or two, but it should be an ok starting point. I've left the code using a char* array and native strings, so at some point the code is going to have to call delete[] on each member of the array and on the array itself. It may end up ultimately simpler using std::vector and std::string for memory management depending on how the supplied strings are going to be used.

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