Question

I’m currently using both C++ and Java in a project and I’d like to be able to send an object which is contained in C++ to my Java interface in order to modify it via a GUI and then send the modification back in C++.

So far I’ve been returning either nothing, an int or a boolean to Java via the JNI interface. This time I have to send an object through the interface. I have made similar class definition available both in C++ and in Java. I’d like to know how I’d go about creating the object so that I can use it in Java.

In C++ I have:

JNIEXPORT MyObject JNICALL Java_ca_X_Y_Z_C_1getMyObject(JNIEnv* env, jclass, jint number);

This function would get called by Java in order to get the object from the C++ side (the object is contained in a singleton, easily accessible).

On the Java end, I do a simple call to this method,

MyObject anObject = C_getMyObject(3);

which should return me the newly created object.

Java currently returns me a UnsatisfiedLinkError when I do the actual call. What is wrong?

Answer 1

Here’s the solution I opted to use:

First, I would create a similar object in Java. Then, from C++ I would instanciate it and pass it all the values.

(C++)
clazz = env->FindClass("java/lang/Integer");
jmethodID method = env->GetMethodID(clazz, "<init>", "(I)V");
return env->NewObject(clazz, method, (jint)anInteger);

But then I realised this wasn’t very portable and was a bit too complicated.

Instead, I decided to return a string that Java would parse and use to initialize the object on its side.

(JAVA)
String aString = "valuesoftheobject";
MyObject myObject(aString);

MyObject would have a constructor which takes a string. I believe the solution is simple and effective.