传递Java ArryList <String> JNI C函数并在C中打印列表

Question

I have the following native method in Java :

public class ConsoleIO
{

    public native static void printList(List<String> list);

   ......
}

and the corresponding C implementation with JNI is the following :

JNIEXPORT void JNICALL Java_ConsoleIO_printList(JNIEnv *env, jclass cls, jobject obj)
{



}

Now I need to pass a list of Strings from java to the printList(List list) method and loop through it from the above C implementation and print them in the console from the C function.

I know that there is no C representation of this List type, but I need to know that how I can do access this List of strings in C and print them out in C itself?

Thanks in advance!

Answer 1

Java has Type Erasure . So List<String> becomes List in the compiled code.

So what you are really looking for is to implement the below Java calls using JNI:

jint size = list.size();
for (jint i = 0; i < size; i++) {
    jobject elem = list.get(i);
    jstring str = (jstring)elem;
    ... print str ...
}

In the above C code, list.size() and list.get() must be replaced with calls of GetMethodID , CallIntMethod and CallObjectMethod . The string can be retrieved from str using GetStringChars or GetStringUTFLength . And some memory management is needed: ReleaseXXX .

Answer 2

I found the solution for this :

JNIEXPORT void JNICALL Java_ConsoleIO_printList(JNIEnv *env, jclass cls, jobject obj)
{

    jclass listClass = (*env)->GetObjectClass(env,obj);

    jmethodID sizeMethod = (*env)->(env,listClass,"size","()I");
    jmethodID getMethod = (*env)->GetMethodID(env,listClass,"get","(I)Ljava/lang/Object");

    jint size =  (*env)->CallIntMethod(env,sizeMethod);


}